Yeast Respiration and Fermentation TEACHER REFERENCE PAGES

GC Instructions

INTRODUCTION     Most organisms, including yeasts, use oxygen in a process called cellular respiration.  Cellular respiration is the controlled breakdown of carbohydrate to carbon dioxide and water with capture of some of the energy in the form of ATP. The rest of the energy is lost in the form of heat.  The first stage of the breakdown is called glycolysis and the second stage is called the Krebs Cycle.  During this process, electrons are transferred from the carbohydrates to oxygen in the process called electron transport and water is formed as the final product of electron transport.  Electron transport produces a chemosmotic gradient of protons (H+) and positive charges across a membrane and this gradient can drive the formation of ATP.  Cellular respiration produces approximately 38 ATP molecules from each molecule of the sugar glucose that is broken down.  The carbon that was in the carbohydrate is fully oxidized to form CO2 during respiration. For glucose, the 6 carbons become 6 CO2 molecules.

Table 1.  Comparison of respiration and fermentation of glucose in yeast.

PROCESS CONDITIONS PRODUCTS FROM

GLUCOSEAMOUNT OF

ATP

RESPIRATION AEROBIC 6 CO2 + 6 H2 38

FERMENTATION ANAEROBIC 2 CO2 + 2 C2H6O 2

    Fermentation, a process that can occur in the absence of oxygen, partially breaks down carbohydrate by glycolysis to capture a small amount of energy in the form of ATP.  The initial reactions of fermentation and respiration are the same, but fermentation stops after glycolysis whereas respiration continues into the Krebs Cycle.  The carbohydrate leftovers are different depending upon the organism that performs the fermentation; usually one product is more oxidized (electron-poor) than the starting molecule and the other is more reduced (electron-rich).  In the case of yeast fermentation, the products from one glucose (C6H12O6) molecule are two molecules of ethanol (C2H5OH) and two molecules of CO2.  Human anaerobic (oxygen-free) muscle produces two molecules of lactic acid (C3H6O3).  Even though the products are different, each fermentation results in a limited, anaerobic breakdown of carbohydrate with energy release.  Since the process does not completely break down the carbohydrate, it does not release much energy that can be captured in the form of ATP.  In yeast fermentation, there are 2 ATP molecules produced for each glucose molecule that is fermented.  This is a low yield compared to that of respiration, but the ability to perform fermentation allows the yeast to survive and grow in environments where no oxygen exists (see Table 1).

Gas Chromatography     The major technique that is used to determine the type of organic molecules produced during fermentation is gas chromatography.  Gas chromatography (GC) is the separation of compounds in the gas phase, depending on their relative ability to adsorb onto the

column packing and their volatility into the gas phase at the temperature used.  The gas chromatograph is a simple, sensitive instrument which can be used to separate and identify about 60% of all known organic compounds.     The compounds to be separated are injected into a gas stream which passes through a column at a preset speed.  Under a constant set of conditions in terms of temperature, gas flow rate, and column packing and size, repeated injections of a compound elute from (come out of) the column at a nearly constant time from injection.  Different compounds elute at different times.  One factor which affects elution time is the molecular weight of the compound; heavier compounds move more slowly through the column.  Elution time is also affected by polarity and other factors.  The column is first injected with known compounds called standards, and their retention times are determined.  Then, unknown mixtures of compounds can be injected, and if the known compounds are in the mixture, their peaks can be recognized by their characteristic retention times.     A gas chromatograph detects the presence of a compound in its eluate (exiting stream) by means of some property of the compound.  One common method used by GC detectors is to compare the conductivity of a heated filament which is placed within a stream of pure reference gas (helium in our lab) to a heated filament placed in a stream of gas containing our sample molecules.  When molecules from our sample pass the detector filament, the changes in conductivity caused by temperature changes are converted into electrical signals which appear as peaks on a computer data screen.  Peaks seen in the eluate are plotted on a chart, and the integrated peak area is proportional to the concentration of the compound.  Many GCs report the integral area of each of the peaks, following the plotted graph of the peaks.  An approximate proportionality between peak height and concentration can also be seen (see sample printouts).  See Instructions for the Gas Chromatograph.

Plant hormones are chemical messengers that are produced in one part of the plant and have a physiological effect on a target tissue that may be distant from the site of production. When hormones reach the target tissue they can: (1) have a direct effect on the target tissue causing a rapid metabolic response; (2) involve the use of a second messenger within target cells; and/or (3) affect transcription of nuclear deoxyribonucleic acid (DNA). Unlike animals, plants have no specialized organs designed solely for hormone synthesis and secretion . Leaves, stem tips, root tips, flowers, seeds, and fruits all produce hormones. Most plant hormones are functional at very low concentrations.

Auxins, cytokinins, gibberellins, abscisic acid, and ethylene are the best known plant hormones. All are in some way involved in regulating plant growth and development. Some promote growth by stimulating cell enlargement or division while others inhibit growth by inducing dormancy or promoting senescence. Recently brassinolides, jasmolates, and salicylic acid have been shown to have hormonal function.

Principles of Hormone Function

Often two or more hormones work synergistically. In a classic 1957 experiment, Skoog and Miller provided evidence that auxins and cytokinins work together in the differentiation of plant organs. Using tobacco tissue culture, they showed that when a

tissue culture medium contains low concentrations of auxin and optimal cytokinin levels, then formation of shoots is favored. In contrast, when the culture medium is supplied with optimal concentrations of auxin combined with low concentrations of cytokinins, root formation is favored.

Hormones sometimes work antagonistically. Apical dominance is a process in which lateral buds of stems remain dormant as long as the stem apex remains intact. It has been shown that auxin produced in the stem apex is responsible for maintaining lateral bud dormancy by causing cells in the lateral buds to produce another hormone, ethylene, which is a growth inhibitor. During early spring, rapidly growing root tips will generate a high concentration of cytokinin that counteracts the effect of ethylene on the lateral buds of the stem. The lateral buds released from dormancy by cytokinins can then begin growth on their own.

Auxins

Auxins were the first class of plant hormones to be identified. Many auxins, both natural and synthetic, are now known and all have similar effects on plant growth and development. The most widely studied naturally occurring auxin is indol-3-acetic acid (IAA), which is chemically related to the amino acid tryptophan. IAA can be synthesized from tryptophan in intact cells but other synthetic pathways are available. Because auxins can have an effect in very low concentrations, plants regulate synthesis and disassembly of auxin very precisely. Auxins are produced in young shoots and always travel downward in the plant from shoot to root. This polar movement of auxin is not well understood but requires calcium ions (Ca 2 ) and most likely involves special carriers in cell membranes. Naturally occurring auxins promote cell enlargement, are important in tropisms, prevent abscission , promote fruit development, and are involved in apical dominance. Synthetic auxins such as naphthalene acetic acid are used as rooting hormones. Other synthetic auxins include 2,4-D (2,4-dichlorophenoxyacetic acid) and 2,4,5-T (2,4,5-trichlorophenoxyacetic acid) that are used as weed killers.

The effect of IAA on cell enlargement has been well studied. IAA stimulates special pumps in the cell membrane of target cells to release H ions into the cell wall, resulting in a pH drop to approximately 5.0 in the cell wall. Enzymes that are pH-dependent then break down important structural bonds between cellulose microfibrils causing an increase in cell wall plasticity . As the cell wall becomes more plastic, water is able to flow in and the cell enlarges. Auxin also may have an effect on transcription of nuclear DNA that can contribute to cell enlargement.

Calcium acts as a second messenger in processes involving auxin. Auxin stimulates the release of Ca2 from the vacuole and endoplasmic reticulum in target tissues which affects Ca-dependent enzymes, including kinases , phophatases, and phospholipases.

Plant Hormones: Roles

Hormone Role

Auxins Involved in differentiation of vascular tissue, control cellular elongation, prevention of abscission, involved in apical dominance and various tropisms, stimulate the release of ethylene, enhance fruit development

Cytokinins Affect cell division, delay senescence, activate dormant buds

Gibberellins Initiate mobilization of storage materials in seeds during germination, cause elongation of stems, stimulate bolting in biennials, stimulate pollen tube growth

Abscisic Acid Maintains dormancy in seeds and buds, stimulates the closing of stomata

Ethylene Causes ripening of climacteric fruits, promotes abscission, causes formation of aerenchyma tissue in submerged stems, determines sex in cucurbits

Jasmonates Involved in response to environmental stresses, control germination of seeds

Brassinolides Promote of elongation, stimulate flowering, promote cell division, can affect tropic curvature

Salicylic Acid Activates genes involved with plant's defense mechanisms

Auxins are involved in tropisms, which are growth responses to directional environmental stimuli such as light, gravity, and touch. In phototropism, unidirectional light will cause auxin to move toward the darkened side of the organ and stimulate enlargement of cells on the darkened side. This causes the organ to bend toward the light. This effect is often seen in potted plants growing in windowsills.

Other Plant Hormones

Cytokinins (for example, zeatin, isopentenyl adenine) have an effect on cell division. As previously mentioned, cytokinins work synergistically with auxin in the control of tissue and organ differentiation. Cytokinins are produced in root tips and may be transported in the xylem toward the shoot.

Gibberellins are a very large class of compounds, all with a similar chemical makeup. There have been as many as eighty-four gibberellins identified (named GA1 through GA84), but GA3, called gibberellic acid, has been the most studied. Gibberellins promote cell elongation, overcome genetic dwarfism, stimulate bolting in biennials, and are involved in seed germination. During the germination of grass seeds the imbibition (intake) of water stimulates the production of gibberellins by the embryo that diffuse throughout the seed. A protein -rich layer just internal to the seed coat, the aleurone layer, responds to gibberellins by synthesizing hydrolytic enzymes that aid in mobilization of stored food in the endosperm for use by the embryo.

Abscisic acid (ABA), is a growth inhibitor that, despite its name, is probably not involved in leaf or fruit abscission. One role of ABA is the stimulation of stomatal closure. When ABA binds to receptors on guard cell membranes, chloride ion channels open, letting chloride ions move out of the guard cells . The resulting depolarization of the membrane stimulates the movement of potassium ions (K ) ions out of guard cells, which then lose water, causing the stomata to close.

Ethylene is the only plant hormone that is a gas. Ethylene is also considered a growth inhibitor as it may have a role in causing bud dormancy, and it is involved with leaf abscission, causes fruit ripening, may determine sex in cucurbits (melon family), and stimulates formation of aerenchyma (gas transport tissue) in submerged roots and stems.

Brassinolides are plant steroids (many animal hormones are steroids) that may be involved in the light-induced expression of genes.

SEE ALSO C ELL W ALL ; M ERISTEMS ; P LANT D EVELOPMENT ; S ENESCENCE

George Wittler

Bibliography

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants, 6th ed. New York: W. H. Freeman and Company, 1999.

Taiz, Lincoln, and Eduardo Zeiger. Plant Physiology, 2nd ed. Sunderland, MA: Sinauer Associates, Inc., 1998.

Read more: Hormones, Plant - Biology Encyclopedia - cells, body, function, process, animal, DNA, organs, used, membrane http://www.biologyreference.com/Ho-La/Hormones-Plant.html#ixzz1ZLC7plEx

Cytokinins

The discovery of cytokinins

Cell division is a fundamental process of living meristematic tissues. Growing cells in sterile culture was of interest to early cell biologists so that one might study cellular processes without the influences of tissues, organs, and so on. The idea that the cell cycle could be regulated by chemicals was inspired by the relationship between Agrobacterium tumefaciens (a bacterium) and its host plants. An infection with this bacterium caused a rapidly growing tumor to develop in just about any tissue of suitable host species. This fact indicated that the cells of higher plants are totipotent (capable of becoming meristematic...changing its developmental fate). This tumor was a lump of undifferentiated (having no particular fate) cells. Once infected, the lump could be cured of its infection by either heat shock or by antibiotics and the tumor would continue to grow in a tumor form.

It was soon discovered that auxins could initiate the formation of callus in plant tissues too. You observed some callus at the stumps where you applied the auxin, IBA, at 5000 ppm on stems of kidney bean plants. Under good circumstances this can also initiate root formation, demonstrating that dedifferentiated callus tissue can be hormonally induced to differentiate along the line leading to root formation. You also have done a rooting project with Mung beans in which the hypocotyl increased in diameter with callus development, and then nearly burst-open with root development along the vascular traces in the stem.

People trying to get tissues to grow in vitro, knew the importance of minerals and vitamins in the medium. They tried various additives to get tissues to grow optimally. Single tissues could grow for at least a limited time, but getting a whole plant to develop from the culture was not possible. One additive that seemed the help a lot was the addition of coconut milk (liquid endosperm). With this one addition and some small amount of auxin, one could regenerate an entire plant from just about any tissue.

The search was on to find out what the "magic" of the coconut milk was. Just recently (2001) Folke Skoog died...he and his colleague Carlos Miller tried many possible substitutes for coconut milk. The goal was to find out what chemical could stimulate cell division and be far more reliable that the batch-to-batch inconsistency observed with coconut milk.

The nitrogenous base, adenine, had at least some activity in this regard. Fresh herring sperm DNA was totally inactive, but autoclaved herring sperm DNA stimulated the cell cultures. They purified the fractions from this sample until they had a chemical that worked...they named it kinetin...and determined its chemical structure in 1955.

Indeed you can see that this is an adenine derivative (6-furfurylaminopurine). In spite of this, kinetin is not a natural cytokinin. Many nitrogenous bases are modified in various ways in the DNA of organisms, but this particular chemical has never been found to occur without the autoclaving manipulation.

This discovery led researchers to start looking for other compounds that would be active in cell division. The concept was based on the idea of structure-activity relationship. Obviously the natural cytokinin must have structure similar to kinetin.

Simultaneously the pharmaceutical industry started screening synthetic compounds that would be even more effective than kinetin. The results of those studies are shown above.

As you might recall we have been using kinetin and benzyl adenine in various projects in lab. I think you recall from the tissue culture project that we got excellent shoot development in media with some BA and could get excellent callus with a different ratio

of BA:NAA. Indeed the synthetic cytokinin, BA, is used routinely in tissue culture as it is stable to autoclaving and therefore is easy to use in such work.

The antagonist discovered has been thought to be a competitive inhibitor for the receptor for cytokinins in plant cells...another structure-activity relationship.

The natural cytokinins

Indeed as the decades passed, the natural cytokinins were found in plant extracts...

Again you will notice how these are adenine derivatives and these can be part of a nucleotide (with added ribose sometimes with the phosphate too). The most common natural cytokinin in plants is trans-zeatin. A graph showing the dose responses of tobacco callus cultures to zeatin and kinetin shows that the callus tissue is far more sensitive to the natural zeatin than it is to the synthetic kinetin...you did this project in class...were your results the same or different?

The cytokinin synthesis pathway

The precursor for the side-chains of adenine in cytokinins is generally isoprene, so the terpenoid biosynthesis pathways are partially shared with gibberellins. The material made in this case is isopentenyl pyrophosphate:

You will notice above that the first steps of the pathway are known only vaguely in plants! But as the genome of Arabidopsis has been searched, more of the enzymes are being found by comparison with bacterial genes. The pathway continues, ultimately producing IPA and Zeatin.

I am assuming here that you know how plants produce five-carbon sugars or five-carbon sugar phosphates to do this. That IS a valid assumption, right?!

Cytokinins occur in both free and conjugate forms

As you can see in the diagram below, the natural cytokinin ribosides can be attached to sugars to form glycosides that have reduced or no cytokinin activity. The plant also can produce enzymes to cleave the sugar and restore full cytokinin activity. Thus conjugation with sugars and retrieval from these bound forms is a possible pathway in plant cells.

However, it is also true that the ribosides themselves are a form of conjugation. All studies to date seem to indicate that the free-base has to be cleaved from the ribose too before the compound has any true activity. Plants carry out this reaction easily and rapidly, so ribosides appear to have activity on their own, but this is an artifact. Cells in culture require the cytokinin to be free...these sometimes lack the enzymes to cleave the ribose, so supplied ribosides are inactive and the free-base must be supplied in the medium.

The native cytokinins also occur as modified bases in RNA and DNA strands. In fact cis-zeatin (a less active form) is found in many tRNA molecules in almost all living cells of all species! The extent to which the free cytokinin pool is altered by conjugation with other nucleotides or released from nucleotide polymers is not clear.

Cytokinins can be degraded

In addition to synthesis and conjugation, the pools of cytokinins can be altered by degradation. Below is how one natural cytokinin is made inactive:

Transport of cytokinins is acropetal

The primary site of cytokinin synthesis in a plant is most likely the root tip. The apical bud of plants, young primordial leaves and flowers, and developing seeds inside fruits are also known to produce cytokinins. The root-produced cytokinins are transported acropetally to the shoot tip.

The transported cytokinins can be recovered in xylem sap that exudes from cut stems and this has been found to be in the form of zeatin ribosides.

Hopefully the discussion above is leading you to thinking about a diagram we have seen before in connection with auxins and gibberellins. Indeed this diagram represents the homeostasis of cytokinin pools.

So what's going on with Agrobacterium tumefaciens?

As you recall Agrobacterium infection can cause a cytokinin-induced tumor to develop and the plant can be "cured" of its bacteria by holding it at 42° C. This idea is shown below.

The Agrobacterium injects a plasmid (naked circular DNA) into the host (in this case tomato) cells. This plasmid is called the Ti (Tumor inducing) plasmid. This piece of prokaryotic DNA has two segments of DNA called the "left border" and the "right border" with genes in between. These "borders" permit recombination of the genes into the host genome. The genes turn on cytokinin synthesis! The structure of the T-DNA in the Ti plasmid is linearized below...

This, in turn results in the development of a "crown gall" tumor on the plant. The fact that the plant can be cured of the bacteria later and the tumor continues to thrive, simply demonstrates that the Ti DNA has become a permanent addition to the host genome. The bacteria may be killed, but the genes remain in the cells.

This realization of course provided the smart researcher with a useful tool for transforming plant cells with foreign DNA. Anytime a scientist wants to insert an engineered gene into a plant cell, the gene simply has to be put between the borders of the Ti plasmid (probably along with an antibiotic resistance gene for selection purposes) and let the Agrobacterium inject the gene into the cells for incorporation in the genome. Of course we are interested in making a transgenic whole plant, so we cut out the cytokinin synthesis gene and replace it with the gene of our research interest. Since the cytokinin over-production genes are absent, the cells can develop into whole transgenic plants (thanks to totipotency of plant cells!). This process is briefly outlined below.

The relative amounts of cytokinins and auxins can regulate the differentiation that can occur from transformed cells. So when it is time to regenerate whole plants from engineered cells, the relative ratio of these two hormones regulates what develops. Here you can see an array of cytokinin concentrations and auxin concentrations on callus growth. The no hormone control is in the lower left corner.

A close-up of the four most-interesting of those plates is presented below. Here you can see the combinations of hormones that control differentiation. Explants just increase in size but maintain leaf morphology in the control. With lots of both cytokinin and auxin, callus proliferates as in the normal Crown Gall form as caused by the normal Ti-plasmid from Agrobacterium. With low cytokinin but lots of auxin you get rooty explants which is what you noticed in our Mung-bean lab exercise. With low auxin but lots of cytokinin the explants form callus and then produce small shoots. Moving transformed cells among these four hormone concentrations, it is possible to regenerate whole cells from them.

Cytokinins have many roles in plants

Cytokinins are known from cytokinin-overproducing mutants to produce additional leaves and branches on the stem. The stems and leaves produce additional chlorophyll. Wounding often produces a new branch. Leaf senescence is delayed. Apical dominance is released. Cuttings produce adventitious roots slowly and require additional auxin to reliably root. Tumors may form at nodes.

The results of applied cytokinins could include release of apical dominance as you demonstrated in kidney beans in laboratory.

Cytokinins regulate the cell cycle as we learned early in the semester.

Cytokinins delay senescence. You carried out this project at home with isolated wheat primary leaf tips in various solutions of plant hormones. Those treated with cytokinins

should have demonstrated delayed senescence (stayed green longer). Of course, remember the auxin dose response...there is such a thing as too much of a good thing!

Cytokinins cause nutrient diversion. Cytokinin-treated leaves become "sinks" for nutrients such as amino acids. This is shown in a classic experiment in plant physiology below.

Here you can see in seedling B that the cytokinin-treated leaf on the left attracted the radio-tagged amino acid from the untreated leaf on the right. In seedling C, when the tagged leaf is also treated with cytokinin, there is not even the small amount of leakage to the other leaf observed in the control (seedling A).

Cytokinin Mode of Action

We are only beginning to understand cytokinin modes of action, but here is one idea of where we think this kind of research will lead us...

Initially the cytokinin signal binds to a receptor's CHASE domain. This triggers a cascade of phosphorylations of proteins, ultimately ending in phosphorylation of a shuttle protein, AHP.

The phosphorylated AHP protein enters the nucleus, phosphorylates type B ARR proteins, that turn on the synthesis of type A ARR proteins. When these gene products are, in turn, phosporylated, they influence other effectors that result in cytokinin responses. There is also a negative feedback loop here to shut down the system when enough phosphorylated ARR is present.

Hormones and Pathways

Hormone changes are no less important but not quite as obvious as food and water balance. The two most important for us are cytokinens, produced by the roots, and auxins, produced by the leaves and buds. These two hormones are in constant communication via the plants vascular pathways. Woody plants typically show strong growth at the branch tip (terminal bud) and the root tip. Strong terminal buds or terminal shoot growth (early in the season) produce a strong auxin signal that does two things. It suppresses bud break at all the buds behind it on the branch and stem. It also travels down the pathways to the root tip where it serves as a powerful growth regulator for the root tip. There it is destroyed. The strongly growing root tip produces cytokinen which follows the same pathway back to the terminal bud or shoot where it serves as a strong growth regulator. As you can see, this is a self reinforcing cycle. Unaltered, this cycle produces a plant that grows strongly at the branch tips and at the ends of the roots.

Terminal BudSub Topics

Classification of Buds Terminal Bud Facts about Terminal Buds Significance of Terminal Bud

Plant buds are small incompletely developed part of the plant,which occurs in the axil of a leaf or at the tip of the stem.

A bud remains dormant in unfavorable conditions but is consists of cells which are capable of division when the conditions are favorable.

The buds are covered with modified leaves that is the tough scales to protect these tender and delicate structures.

Classification of Buds

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Plant buds can be classified as: 

 

Terminal bud:Buds located at the tip of stem and increases the length of the plant  Lateral buds: Buds at sides of the stem,increases the girth of the stem   Axillary bud: the buds formed in the angle which leaf forms with the stem and

produce branches.

 

Plant buds can also be classified according to their internal structures:

 

Floral buds producing blossoms. a bud from which leaves (but not flowers) develop are called leaf buds, while those buds yielding both leaves and flowers in the earliest stages of development are termed mixed buds

Terminal Bud

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A  bud that developes at the apex of the stem,and is the primary growing point of the stem.The terminal bud is dominant as it represses the growth of the lateral/axillary bud below it and this phenomenon is known as Apical Dominance.The terminal bud produces hormones 'auxin' which inhibits the growth of axillary buds, thereby contributing to increase in length of the plant.In some ornamental plants to make a plant bushier or shorter gardeners removes the apical tip and hence suppresses the effect of hormone,which allows the lower dormant lateral buds to develop and permits the side shoots to develop.

In plants,the cells which posses the capability to divide and grow are known as the meristematic cells.Apical meristems found at the tip of stems and root which contributes to increase in length.The terminal buds have this tissue called the apical meristem. 

Facts about Terminal Buds

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Plant buds are small incompletely developed part of the plant,which occurs in the axil of a leaf or at the tip of the stem.A bud remains dormant in unfavorable conditions but is consists of cells which are capable of division when the conditions are favorable.The buds are covered with modified leaves that is the tough scales to protect these tender and delicate structures.

Significance of Terminal Bud

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The terminal bud helps in the lengthwise growth of the plant.The age of the plant can be calculated by counting the scars over the plant,which are formed by the scales covering the terminal bud.If we remove the terminal bud the flow of would be redirected to the axillary buds below and force them into growth giving more shoots but delaying flowering.

Plant Tissue Culture - The conventional breeding methods are the most widely used for crop improvement. But in certain situations, these methods have to be supplemented with plant tissue culture techniques either to increase their efficiency or to be able to achieve the objective, which is not possible through the conventional methods.The term tissue culture is commonly used in a very wide sense to include in vitro culture of plant cells, tissues as well as organs. But in a strict sense, tissue culture denotes the in vitro cultivation of plant cells in an unorganised mass, e.g., callus cultures.

 

Another term, cell culture is used for in vitro culture of single or relatively small groups of plant cells, e.g., suspension cultures. But in general, the term tissue culture is applied to both callus and suspension cultures, and cell culture is often used for callus culture as well. When organised structures like root tips, shoot tips, embryos, etc. are cultured in vitro to obtain their development as organised structures, it is called organ cultures. In this book, the term tissue culture is used in its broad sense to denote aseptic culture of plant cells, tissues, and organs.

 

 plant tissue culture or cell culture:-it is the technique of in vitro, culture in which isolated plant cells, tissues, organs or even entire plant are subjected to grow in nutrient media in glass containers(tubes,conical flask,petri dishes) under aseptic conditions. Aseptic culture of plant may be of the following types as:-

(a) ORGAN CULTURE:- These are cultures of isolated plant organs including cultures derived from root tips,stem tips,leaf primordia or immature part of flowers and immature fruits.

(b) EMBRYO CULTURE:- These are culture of isolated immature or mature embryos.

(c) CALLUS OR TISSUE CULTURE:- These are culture of tissue arising from disorganized proliferation of cells from segment of plant organs.tissue or callus culture are , generally grown on solid medium as amass of cells.

(d) SUSPENSION CULTURE:- These are often called cell cultures, as they represent a lower level of organisation than tissue or callus culture. suspension culture are in vitro cultures of isolated cells and very small cell groups remaining dispersed as they grow in excited liquid media.

Read more: http://wiki.answers.com/Q/What_are_different_types_of_plant_tissue_culture#ixzz1ZLErQDUw

Interaction between Hormones, Light, and Nutrition on Extension of Lateral Buds in Phaseolus vulgaris L.

1. TIN SHEIN 1 and 2. D. I. JACKSON

+ Author Affiliations

1. Lincoln College Canterbury, New Zealand Received December 21, 1972.

Abstract

Gibberellic acid (GA3), kinetin, and indol-3yl-acetic acid (IAA) contained in lanolin were applied in various combinations and concentrations to decapitated stems and petioles and to buds of Phaseolus vulgaris L. GA3 applied alone usually promoted growth of main stems and laterals but this was by no means consistent and occasionally it acted in the opposite way. IAA applied alone reduced lateral bud extension slightly, but not consistently; however, when applied with GA3 or GA3 plus kinetin, it often markedly inhibited the promotion caused by these compounds. On occasions, however, GA3 and IAA acted synergistically to promote and sometimes to inhibit lateral shoot growth. Kinetin alone showed few significant effects on lateral shoot growth but applied with GA3 it often dramatically increased GA3-induced growth of main stems and laterals. The diversity of these results, which parallels that found in the literature, was shown to be partly dependent on the point of hormone application and age of the plant or bud, on concentration of hormone and on light intensity or nutrition. However, no meaningful relationships were found and it is concluded that growth of laterals and main stems is dependent on a hormone balance which can be critically modified by a wide range of internal and external factors the nature of which is still to be determined.

Water pollution and plants

Water pollution and plantsThe water bodies of the earth are being continuously polluted by a variety of sources. The pollution is occurring in all types of water bodies; both freshwater bodies like ponds, lakes and rivers as well as marine bodies like coastal and deep-water seas. Major causes of water pollution are deposition of acid, organic sewage, detergents, agricultural chemicals, industrial effluents, silt, oil and heat into the water bodies.

Effects of acid depositionVarious acid gases, aerosols and other acidic substances released into the atmosphere from the industrial or domestic sources of combustion of fossil fuels eventually come down to the ground. These substances are deposited directly on the water bodies. In addition, these substances also reach the water bodies along with run-off rainwater from the polluted soil. Deposition of acidic substances causes acidification of water by lowering its pH below 6.0. The sulphates, nitrates and chlorides have been reported to

make water bodies like lakes, rivers and ponds acidic in many countries.

Nutrient deficiency in aquatic ecosystem: The decomposing bacteria and fungi decrease in acidified water. This reduces the rate of decomposition of organic matter and, therefore, the nutrient cycling in the aquatic ecosystem. Thus, low pH causes nutrient deficiency and consequent general reduction in abundance of aquatic plants in the affected water body. Decrease of species diversity: Critical pH for most of the aquatic species is 6.0. The number and variety of aquatic species in the water body generally decreases below this pH. Change in species composition: The number and abundance of acid tolerant species increases while that of sensitive species decreases. In the initial phase of water acidification, filamentous algae grow very fast and form thick mats. However, most of the diatoms and green algae disappear below the pH 5.8. Diatoms and small siliceous phytoplankton populations are highly sensitive to pH changes and species composition of their communities shows highly specific changes with pH change of the water body. Among green algae, Cladophora is highly acid tolerant species and becomes abundant in highly acidic freshwater bodies. Euglena and some other unicellular algae are found up to pH of 1.6 while Chlamydomonas acidophila is found in water up to pH of 1.0. Macrophytes are generally absent in extremely acidic water. Potamogeton pectinalis is only aquatic macrophyte found in heavily acdified water. At pH lower than 4.0, angiosperm species using dissolved carbon dioxide e.g. Juncus bulbosus, Juncus effusus, Sparganium emersum, Gyceria fluitans, Eleocharis acicularis, Typha latifolia and bryophytes like Polytrichum, Anisothecium, Fontanalis, Catharina become the only survivors. The roots of macrophytes are generally affected adversely in acidic water and result in poor plant growth. Plants with deep roots and rhizomes are less affected while plants with short root systems are severely affected. Yellowing of plants is common in polluted water. Effects of organic matter depositionLarge amounts of dead and decaying animal and plant material, fecal material and other organic material is deposited directly from sewage discharges or is washed along with rainwater into the water bodies. Most important consequnces of such organic matter deposition are as following.

Increase in decomposer microbes: Increased addition of organic matter into the water body results in rapid multiplication and increase in decomposer aerobic and anaerobic bacteria. The rapid decomposition of organic matter by these increases nutrient availability in the water. Eutrophication: Addition of organic matter and its rapid decomposition resulting in increased nutrient supply causes much nutrient enrichment (eutrophication) of water body. In such a condition, planktonic green and blue-green algae grow very rapidly causing water blooms. In addition to these many types of hydrophytes like Salvinia, Azolla, Eicchhornia etc. also become abundant. All this rapid growth of planktonic and free-floating hydrophytes reduces light penetration into deeper layers of water body and submerged flora gradually declines. Abundant flora after death further increases supply

of organic matter in the water body. Oxygen depletion: Rapid decomposition of organic matter by aerobic bacteria during eutrophication phase consumes much water-dissolved oxygen. On the other hand, gradual decrease of submerged aquatic flora results in reduced oxygenation of water. Both these phenomena together result in increase in biological oxygen demand (B.O.D.) of the water. Biological oxygen demand (B.O.B.) of water is defined as the amount of oxygen needed by a unit volume (usually one litre) of water sample to completely decompose the organic matter present in it by microbial activity, measured at 20oC and tested at least five days after sampling. The B.O.D. value of fresh, unpolluted water is usually below 1 ppm while in organic matter polluted water, it may be more than 400 mg/litre. Effects of detergent depositionVarious detergents from domestic or industrial use directly released or washed down into the water bodies cause serious effects of plants.

Most of the domestic and industrial detergents contain high (up to 40%) phosphate content. Addition of such detergents into water results in phosphate-enrichment of water. Most of the detergents that are toxic, enter the plants through roots or surface absorption. Common effects of detergents on plants are as follows. Retardation of plant growth, root elongation, carbon dioxide fixation, photosynthesis, cation uptake, pollen germination and growth of pollen tubes. Destruction of the chlorophylls and cell membranes. Alteration of the absorption maxima of chlorophylls. Binding of membrane lipids and proteins. Denaturation of proteins and thus causing enzyme inhibition in various metabolic processes. Non-degradable alkyl benzene sulphonates and phosphate-rich detergents interfere with gaseous exchange even in very low concentrations. Cation-active compounds hinder algal growth between 0.1 and 10.0 ppm while non-ionic compounds hinder algal growth between <1.0 and around 10,000 ppm concentrations depending upon the species and the compound. Macrophytes are most sensitive to damage by anionic surfactants. Effects of agricultural chemical depositionMany chemical fertilizers, pesticides, insecticides, herbicides etc. are applied to crops far in excess. These excess chemicals are washed away with rainwater, first into the soil then finally into the water bodies.

Chemical fertilizers entering the water bodies result in eutrophication by enriching the water with major plant nutrients. Many of these fertilizers are acidic in nature e.g. ammonium. These cause acidification of water. Pesticides, herbicides and insecticides also cause pH changes in the water bodies. The effects of these plants on aquatic plants are similar to those of their overdose in foliar application. The herbicides act directly on aquatic flora but insecticides act indirectly by allowing algal blooms to develop in the water body. Different substances have different patterns of their toxic action, decomposition pathways and environmental persistance. Most common effect of these substances is reduced photosynthesis. Some may uncouple oxidative phosphorylation or inhibit nitrate reductase enzyme. The uptake and

bioaccumulation of these substances in aquatic plants is great due to their low solubility in water. Effects of industrial effluent depositionVarious inorganic and organic waste products from industries and mining activities are directly deposited into the water bodies. A large amount of these substances deposited on the soil, comes into water bodies indirectly along with surface run-off. Fly-ash, various organic/liquid effluents and heavy metals e.g. Hg, Cu, Cd, Pb, Zn, Ni, Ti, Se etc. are important industrial pollutants of water.

Fly ash forms thick, floating covering over the water surface. This reduces the penetration of light into deeper layers of water body. Fly-ash increases the alkalinity of water and thus, causes reduced uptake of essential bases. All these phenomena cause death of aquatic plants. Organic/liquid effluents disturb the pH of water and depending upon their chemical composition, cause specific toxicity effects on the aquatic plants. Change in the floristic composition of the water body is most obvious and direct effect of pollution by such effluents. Heavy metals usually occur together and with many other types of pollutants so the effects of single metal are usually difficult to interpret. There may be synergistic, additive or antagonistic interactions between metals regarding their effects on plants. Impact of metals is reduced in hard, well-buffered freshwater systems. For example, Cd-uptake by Nitella and Elodea is less in hard water, Zn-toxicity is less with high Ca for Stigeoclonium and Hormidium while is less with high pH for Hormidium. Bioaccumulation of metals is more in mosses than in angiosperms and is usually more in lower plant parts. Chelators decrease while methylated forms increase the metal toxicity to aquatic plants. Reduced oxygen and low temperature also increase metal toxicity. While bryophytes appear to be highly resistant to heavy metal toxicity, in all classes of algae, strains vary in tolerance to metals. Photosynthesis and growth in most of the algae is inhibited at 1-2 ppm of Cu++. Cholrella is more sensitive to Cu++ than Scendensmus. Cholrella is retarded more by Ag than by Cd, Hg or Ni while cell division in the genus is reduced more by Cd than by Cu or Hg at 0.32 ppm. Lemanea is quite resistant to Zn and Pb. Efects of silt depositionThe top soil removed due to erosion is carried with rainwater or flood water and deposited into the water bodies causing silt deposition in them.

The deposition of silt increases the turbidity of water and reduces light-penetration deep into the water, causing decline in submerged flora. Silt deposition, in general, inhibits growth of aquatic plants. Phytoplankton is particularly affected by silt deposition due to reduction in surface exchange of gases and nutrients. Species tolerant to turbidity (e.g. Ceratophyllum demersum, Lemna minor agg., Nuphar lutea, Polygonum amphibium, Sagittaria sagittifolia, Scirpus lacustris) becomes highest followed by species of intermediate tolerance (e.g. Callitriche spp., Myriophyllum spicatum, Potamogeton natans, P. pectinatus, Sparganium emersum, S. erectum) while least tolerant species (e.g. Elodea canadensis, Potamogeton perfoliatus, Ranunculus spp., Mosses) are much reduced.

Effects of oil depositionWashing of oil tankers and storage containers in many rivers, large lakes and particularly near sea coasts causes deposition of oil slicks on water. Oil spills in water are also common during normal transport operations or during accidents involving oil tankers.

Oil pollution of water body prevents oxygenation of water. Oil depletes oxygen of the water body by consuming dissolved oxygen in oil degradation.

Oil inhibits planktonic growth and photosynthesis in aquatic macrophytes. Oil may even cause destruction of aquatic flora if it catches fire. Effects of waste heat depositionMany industries, particularly thermal power plants, take water from rivers, lakes or sea to cool the heat-producing boilers and equipment. The heated water is then returned to the water body. The deposition of waste heat into the water body has many consequences for the plants in it.The solubility of oxygen in water is reduced at higher temperature. Thus, the reduced oxygenation of water adversely affects the aquatic flora. Reduced oxygenation and high temperature of water causes reduction in the activity of aerobic decomposers. The reduced decomposers result in decreased organic matter decomposition and consequently, reduced nutrient availability in the water body. In high temperature, aquatic plants show increased respiration, reduced photosynthesis and general inhibition of enzyme activity with increasing temperature. The aquatic flora and primary productivity of the aquatic ecosystem declines with increasing temperature. Green algae are mostly replaced by blue-green algae, which have comparatively less primary productivity. With increase in temperature, species diversity of the water body declines and heat-tolerant species gradually become dominant. Posted by garg at 6:14 AM Labels: plants, WAter pollution

Correlative Inhibition of Lateral Bud Growth in Pisum sativum L. and Phaseolus vulgaris L.: Studies of the Role of Abscisic Acid

1. J. C. WHITE and 2. T. A. MANSFIELD

+ Author Affiliations

1. Department of Biological Sciences, University of Lancaster Bailrigg, Lancaster LA1 4YQ

Received December 15, 1976.

Abstract

The possibility has been investigated that abscisic acid (ABA) might act as a correlative inhibitor of lateral bud growth in Pisum sativum and Phaseolus vulgaris. Application of ABA in small quantities (2μg) to axillary buds on decapitated plants of P. sativum caused appreciable inhibition of their growth, and induced a compensatory growth of the bud on an adjacent node. Application of this same quantity of ABA to axillary buds on decapitated plants of Phaseolus vulgaris was without effect, but a high concentration in lanolin (1 mg g−1) did substantially reduce bud outgrowth. Endogenous ABA-like substances in Phaseolus vulgaris, detected by bioassay and electron capture g.l.c., were present in similar concentrations in shoot tips, lateral buds on intact plants and lateral buds on plants decapitated 24 h earlier.

The effects of applied ABA suggested that it might be involved in the mechanism of correlative inhibition in Pisum sativum, but it was not possible to test this hypothesis by determining endogenous ABA levels in axillary buds because of their small size. The evidence presented here suggests that ABA is not a correlative inhibitor in Phaseolus vulgaris even though at high concentration it can inhibit the growth of axillary buds.

Alterations of the biochemical pathways of plants by the air pollutant ozone: which are the true gauges of injury?Heath RL.

Source

Department of Botany and Plant Sciences, University of California, Riverside, CA, USA. heath@ucr.edu

Abstract

Plant strategies to survive ozone stress include exclusion or tolerance of ozone. If these processes fail, past observations of ozone injury have indicated many physiological and metabolic changes then occur; most of these changes are likely to have been initiated at the level of gene expression, suggesting signal transduction. In the last decade considerable understanding of the biochemical process within plants has been developed. Currently there are several hypotheses regarding a response of plants to ozone fumigation: [1] membrane dysfunction and alteration of purpose; [2] stress ethylene interactions; [3] impairment of photosynthesis via changes in Rubisco levels and the guard cells so that the stomata do not track correctly the environment; [4] antioxidant protection through metabolites and enzyme systems to reduce the oxidant load; and [5] general impairment or disruption of metabolic pathways. Many believe that free radicals and other oxidative products, formed in plant leaves under ozone exposure, are responsible for much of the spread of the biochemical alterations. There are obvious chemicals that may account for the changes that are observed, such as hydrogen peroxide. Once the ozone enters the tissue, evidence suggests the first line of defense is a range of antioxidants, such as ascorbate, glutathione peroxidase, superoxide dismutase, and catalase. If overwhelmed, subsequent events occur which are highly suggestive of

systemic acquired resistance. Furthermore, other defensive indicators, such as salicylic acid and jasmonic acid, tend to increase, but more slowly than ethylene, and spread their signaling effects more widely in the plant. The primary set of metabolic reactions that ozone triggers is thought to be "wounding" responses with a secondary response of senescence. The dramatic strides in understanding the genetic make-up of plants, gene control, and signal transduction/control over the last few years will only accelerate in the future. We need now to have an understanding of those events that can be translated into more detailed schemes of how ozone alters much of the basic metabolism of plants and how plants counteract or cope with ozone. What is now known about how varied biochemicals and their pathways are changed upon ozone exposure will be discussed.

Plant Diseases Development and Management

EB-31 (Revised), February 2001

Marcia P. McMullen and H. Arthur LameyExtension Plant Pathologists

The interactions between plants and disease organisms are complex, and commercial growers and home gardeners alike may have difficulty understanding plant diseases. Confusion can be reduced by learning a few basic concepts and principles of how diseases develop and how they are managed. This publication presents these concepts and is dedicated to commercial growers, commercial applicators, home gardeners and others who want more information on how plant diseases develop and are managed. To serve these diverse groups, examples of common North Dakota diseases were selected from both the commercial farm and from the home garden. Various circulars available at offices of the NDSU Extension Service provide detailed information on many of these diseases.

HOW DISEASE DEVELOPS

WHAT IS A DISEASE?

A disease is any abnormal condition that damages a plant and reduces its productivity or usefulness to man. Under this definition, air pollution can cause disease, as can many fungi and other infectious living organisms. This illustrates the first important concept: there are two basic types of diseases, non-infectious (abiotic) and infectious (biotic).

TYPES OF DISEASES

NON-INFECTIOUS (Abiotic)

Non-infectious diseases are caused by some environmental factor that produces an abnormal plant (Table 1); that is, one that has an abnormal appearance. Non-infectious diseases are not caused by a living, parasitic organism (an organism that gets its food by attacking other organisms), but are abiotic in nature.

NutritionNutrition is a frequent cause of non-infectious disease. Either too much (excess) or too little (deficiency) can cause problems. For example, plants that are deficient in nitrogen develop a general yellowing, beginning with the lower leaves and progressing upward. Trace element deficiencies such as iron chlorosis, caused by iron deficiency, are common. Iron chlorosis occurs in many North Dakota trees and shrubs, especially silver maple, oak, and spirea. Iron chlorosis is recognized by progressively smaller leaves on the new growth; these leaves are yellow with green veins. When iron chlorosis is severe, leaves may turn brown and become brittle as well. Lime-induced chlorosis is common in our alkaline soils because the iron in the soil is not readily available to plants. Iron chlorosis also is common on certain soybean varieties. Zinc deficiency is common on dry beans and fairly common on flax, causing yellow leaves and stunted growth. Excess trace elements may also cause growth problems, but these are rare in North Dakota.

MoistureDeficient or excessive moisture (water) can cause disease. Moisture deficiency produces stunted, stressed or wilted plants. In addition, this stress may predispose (weaken) plants to infection by infectious organisms or increase the effects of infectious disease. For example, some tree canker organisms commonly infect trees stressed by drought or extreme cold. The effects of stem rust and root rot on small grains are greater when plants are moisture stressed (deficient in water). Excess moisture also has adverse effects, such as suffocation of roots due to lack of oxygen or predisposing plants to water mold infections.

TemperatureFrost is a common problem in spring and fall, affecting tender farm crops and garden vegetables. Extremely high temperatures in summer can also cause problems. For example, heat sterility in small grains is common in North Dakota. In oats, this is referred to as "blast."

Other Meteorological ConditionsHigh soil temperatures early in the season may injure or kill plant tissues at the soil surface, resulting in a constricted stem; this is called heat canker. Bright

sun, high temperatures, and strong dry winds may suddenly desiccate (dry) leaves of crops and garden plants, resulting in sunscald. When lightning strikes the ground it may kill plants in somewhat circular patches up to 50 feet in diameter.

Toxic ChemicalsToxic chemicals injure plants. Salt may damage or kill farm crops growing in saline seeps; road salt may severely damage boulevard trees and other vegetation. Air pollution also damages vegetation. Bronzing of beans caused by ozone is common in the state. The source of the ozone is not known.

INFECTIOUS (Biotic)

Infectious diseases are caused by organisms that attack plants and get their nutrition from them. The plant attacked is called the host plant. The organism causing the disease is called a pathogen. The pathogen can spread from a diseased plant to a healthy plant. There are five common groups of pathogens (Table 1). A few other kinds of micro-organisms may cause plant disease but are not common in North Dakota.

FungiFungi are the most common pathogens in North Dakota. They produce tiny thread-like filaments called hyphae. Most pathogenic fungi produce spores which serve to reproduce and disseminate them. Spores function similarly to the seeds of higher plants. Some spores are formed in masses, like the orange pustules of rust fungi. Other spores develop in specialized fruiting structures. These structures are called signs of the pathogen and are useful in field identification of disease. Symptoms are also useful in identification of a disease. Symptoms are visible abnormalities such as wilts, rots, and other types of tissue death, stunting, excessive growth, or abnormal color.

Examples of common pathogenic fungal diseases in North Dakota include: rusts of small grains, sunflower and dry beans; cereal smuts and head scab of small grains; Cercospora leafspot of sugarbeet; white mold or Sclerotinia of dry beans, sunflowers, and canola; early blight and late blight of potatoes; root rots of small grains, sugarbeets and dry beans; apple scab; anthracnose of muskmelon; tree cankers; Septoria leafspot of tomato; peony blight; powdery mildews of ornamentals, and plum pockets.

Most fungi that cause plant diseases are parasites, organisms that get their food from other living organisms. However, not all fungi are parasites. Many live on dead or decaying organic matter and are called saprophytes. Mushrooms that spring up in lawns are among the most spectacular saprophytic fungi. There are also many inconspicuous ones that rot organic matter. The sooty molds seen on

wheat heads at harvest also are saprophytic, living on the already ripe or senescing glumes and awns.

BacteriaBacteria are tiny one-celled organisms that multiply by cell division. They can be seen only with a microscope. Most are saprophytes, but there are a few common and serious bacterial pathogens that attack North Dakota plants. Examples of common bacterial diseases include bacterial blights of dry beans, bacterial blights and black chaff of wheat and barley, ring rot and blackleg of potato, fireblight of apples and related plants, bacterial wilt of cucumber and muskmelon, angular leafspot of cucumber, and bacterial speck and spot of tomato.

VirusesViruses are 1,000 times smaller than the tiniest living cell. Most viruses have a core of nucleic acid, the basic unit of heredity, and have a protein coat covering the core. Viruses are usually in the form of rods or spheres and alter the activities of the host to manufacture more virus. Some viruses are transmitted mechanically (by contact with another plant, or contaminated workers' hands or tools); others are transmitted (carried) by insects and by eriophyid mites. Examples of virus diseases that can cause serious losses are wheat streak mosaic, barley yellow dwarf, bean common mosaic, potato virus diseases, tobacco mosaic, cucumber mosaic, and squash mosaic.

PhytoplasmasPhytoplasmas lack a rigid cell wall, have no defined shape and can only be seen with an electron microscope. They are usually systemic in the host (distributed internally throughout the host) and are transmitted by leafhoppers. Phytoplasmas cause growth abnormalities such as witches' brooms (a broom-like mass of plant branches) or excessive tillering (stooling of small grains). Our most common phytoplasma is aster yellows. Aster yellows produces witches' brooms and greenish flowers on marigold; carrots develop yellow tops and hairy roots. The disease produces greenish flowers on flax, "purple top" on potato and tomato, and bladder-like pods on canola. Purple top of potato is often accompanied by the formation of small aerial tubers in the leaf axils (the point where the leaf joins the stem).

NematodesNematodes are tiny roundworms. Most can be seen only with a microscope, but a few can be seen with the naked eye. Reproduction is by formation of eggs. Some parasitic forms attack plant roots and can cause severe damage. Nematode problems are common in warm climates but are rare in North Dakota. Two potentially serious nematode diseases occur in neighboring states but are not yet confirmed (2000) in North Dakota. They are the soybean cyst nematode and the pine wilt nematode. The pine wilt nematode is unusual in that it invades the vascular (water-conducting) tissues of pine trees instead of attacking the roots.

DISEASE DEVELOPMENT

This section discusses how plant pathogens produce disease: how they infect the host, how they reproduce, how they are disseminated (spread), and how they survive between crops.

WHAT DOES IT TAKE TO PRODUCE DISEASE?

Three factors interact to produce disease; the host, the pathogen, and the environment (Figure 1). If any one of these three factors is unfavorable or missing, disease will not develop. For example, the flax rust pathogn attacks only flax as a host, and different races of flax rust attack different varieties of flax. Flax rust develops only when a suitable combination of rust race (pathogen) and variety (host) interact under environmental conditions favoring this disease. The environment often limits disease in North Dakota and is the reason that some diseases rarely occur and others occur sporadically. Many diseases are favored by humid or rainy weather and may be more common and severe in years favoring good crop production.

Figure 1. The disease triangle — disease develops only when all three factors are favorable. (4KB illustration)

INFECTION BY PATHOGENS

Pathogens can infect plants in several ways (Figure 2). Fungi and bacteria may penetrate (enter) through natural openings such as the stomata — these are tiny "breathing" pores in the leaf that allow the exchange of gases (Figure 3). Viruses, fungi, and bacteria also enter through wounds. Bacteria frequently enter through hail wounds. Many fungi can penetrate the leaf surface directly without wounds or stomata.

Figure 2. Methods of infection by pathogens. Some typical pathogens are illustrated, but they are not shown on the same size scale. For relative sizes, see Table 1. (14KB illustration)

Figure 3. Diagram illustrating some of the ways pathogens can infect a leaf. (Shown in cross section — edge of cut open leaf in foreground.) (30KB illustration)

Some viruses and most phytoplasmas are carried by insects, especially sucking insects such as aphids and leafhoppers. Insects that carry and transmit plant disease organisms by their feeding are called vectors. The aster leafhopper, for example, is the vector of both the aster yellows phytoplasma and the oat blue dwarf virus.

Some viruses are transmitted mechanically by the rubbing together of leaves or by humans touching diseased and then healthy leaves. Tobacco mosaic virus and potato virus X are common examples. Many fruit tree viruses are transmitted by grafting.

Many pathogens, especially foliar (leaf) pathogens, need a film of water on the plant to begin growth, penetrate the host, and establish infection. This is why wet or humid weather is so important in the development of many fungal and bacterial diseases.

DISSEMINATION OF PATHOGENS

Pathogens are disseminated (spread) by wind, insects, water, man, animals and birds.

WindWind disseminates fungus spores from plant to plant in a field or across fields. Pathogens such as the wheat leaf rust, wheat stem rust, and barley stem rust pathogens are spread long distances by the wind (Figure 4). Leaf rust is wind-borne from the major winter wheat areas of Kansas, Nebraska and Oklahoma (Occasionally it survives the winter in North or South Dakota). Stem rust is wind-borne from Mexico and Southern Plains states to wheat and barley crops in the Northern Great Plains.

Figure 4. Long distance spread of wheat stem rust and wheat leaf rust spores from their overwintering areas. (8KB illustration)

InsectsInsects are important in carrying viruses and phytoplasmas from southern areas. Common insect vectors include aphids for the barley yellow dwarf virus and the aster leafhopper for the aster yellows phytoplasma. Insects also spread these pathogens from plant to plant. Bacterial wilt of cucumber and muskmelon is disseminated by both the striped and spotted cucumber beetles; only the striped cucumber beetle is common in North Dakota.

WaterWater can carry pathogens from field to field. Rain and splashing water can disseminate many fungi and bacteria. Septoria leafspot of tomato and the bacterial blights of dry beans are common examples. Rain and wind form numerous tiny airborne water droplets called aerosols. Many bacteria are disseminated long distances in wind-driven aerosols. Water flowing over the surface of fields spreads disease organisms such as Sclerotinia (white mold), Verticillium, and downy mildew.

ManMan spreads pathogens and weeds over long distances. International plant quarantines attempt to prevent this dissemination by authorizing inspection of planes, ships, cars, and luggage for prohibited pests at ports of entry. The Mediterranean fruit fly as well as many weeds and pathogens are frequently intercepted at ports of entry. The gardener who brings fruit in his or her luggage and the commercial grower who brings seed of a high performance crop in his pocket may introduce a new pathogen or other pest.

Man can also disseminate pathogens and other pests locally. Dutch elm disease was originally introduced into the United States on elm logs; much of its local spread and buildup has been on elm firewood collected from diseased trees. The soybean cyst nematode, not yet reported (2000) in North Dakota, could be introduced on contaminated farm implements from infested areas in neighboring states. Man can locally disseminate bacterial blights, rust and anthracnose of dry beans by cultivating the crop when it is wet.

Animals and BirdsAnimals and birds also may disseminate pathogens. For example, the soybean cyst nematode can be disseminated in the feces of birds; animals may spread water-borne pathogens by walking through an infected crop when the plants are wet.

SURVIVAL OF PATHOGENS

In North Dakota we are primarily concerned about pathogen survival between crop seasons. When this is over the winter, it is called overwintering.

SoilMany pathogens form resistant structures that survive long periods of time in the soil. For example, the Aphanomyces root rot pathogen of sugarbeets survives for over 20 years as resistant spores in the soil, and the sunflower downy mildew pathogen survives for at least 14 years as resistant spores in the soil. The white mold pathogen of dry beans and sunflower survives for six to eight years or more as resistant bodies in the soil. The Verticillium wilt pathogen of potatoes, tomatoes and other crops survives for at least several years in the soil.

Plant PartsPathogens may survive on crop refuse. These include the leafspot pathogens of wheat and barley which survive on stubble, and the tomato Septoria leafspot pathogen which survives on the old dead vines. Destroying or burying this crop refuse reduces next year's disease potential.

Seed and Vegetative Plant PartsThe smuts of small grains survive on or in the seed and survive in storage as long as the seeds remain viable (can germinate). Vegetative plant parts such as tubers, roots and corms provide a mode of survival for many pathogens. Many potato pathogens are carried on or within the tubers.

Insects and MitesThe bacterial wilt pathogen of cucumber and muskmelon is suspected to overwinter in the digestive tract of cucumber beetles. The wheat streak mosaic virus overinters in wheat curl mites that survive on winter wheat and some perennial grasses.

Mild ClimatesThe cereal rust pathogens usually do not overwinter in North Dakota but survive year-round in the southern United States and Mexico and are carried north each year by wind.

HOW DISEASES ARE MANAGED

This section focuses on management of infectious diseases. Non-infectious diseases are not discussed as their management involves a remedy of the physical factors that induce them.

The four basic methods of infectious disease management are: exclusion, eradication, host resistance, and protection (including the use of fungicides). These four methods reduce pathogen populations or slow their development. Protectant fungicides and resistant varieties slow down the development of pathogen populations. Cultural practices also can reduce the pathogen's population. In short, management practices prevent or delay the introduction of pathogens or reduce initial pathogen populations and retard their subsequent increase.

Sound management is based on correct diagnosis. This is essential to distinguish infectious from similar appearing non-infectious diseases as well as to correctly identify the pathogen involved in infectious disease.

Correct identification of the pathogen is essential to know the pathogen's life cycle and how it relates to the cycle of disease development. This information is needed to develop a management program that attacks the pathogen at the weakest point in its life cycle. When fungicides are used, the type and the timing are important. For example, in the case of dry bean diseases three different types of fungicides are used for management of each of three major diseases: rust, bacterial blights, and white mold. In each case timing is important and application must be started before the disease is widespread. For head scab of wheat and barley, timing of fungicide application is critical as well, at early heading for barley and early flowering for wheat.

EXCLUSION

Exclusion means exclusion of pathogens. Pathogens can be excluded (or kept away) from hosts by quarantines that prevent their introduction, and by use of seed stocks certified to be pathogen-free or within certain prescribed tolerances for low levels of pathogens.

QUARANTINE

International quarantines are familiar to anyone who has traveled overseas. Planes, cars, trucks and luggage are checked at ports of entry to prevent the introduction of pathogens and other pests into areas where they do not occur. Many disease organisms would flourish in our state if they were introduced. State and local quarantines are used to keep black wart of potato and the golden

nematode restricted to a few localized areas of the eastern United States. International quarantine has prevented new introductions of pathogens from other countries.

SEED CERTIFICATION

Seed certification is used to certify that potato seed tubers and seed of dry beans have low levels of pathogens or are pathogen-free in the case of potato ring rot. The crop is grown from seed produced under carefully controlled conditions. Sometimes the seed crop is grown in an isolated area to reduce disease potential. For example, foundation seed potatoes are grown in Golden Valley County, a western county isolated from the rest of the potato production in North Dakota. Crops are field inspected and must meet certain tolerances to be certified. Certification may be done in conjunction with indexing. Many seed potatoes are now produced in greenhouses.

INDEXING

Indexing involves laboratory or greenhouse tests to determine infection by pathogens in vegetatively propagated plants such as potatoes and fruit trees. Only the healthy materials are saved for further increase.

CulturingPlant parts to be used for increase are laboratory cultured to determine if they are infected with pathogenic fungi or bacteria. Plant parts found free of pathogens are used for further vegetative increase and propagation. This technique is used on chrysanthemums, carnations and potatoes.

Indicator HostsIndicator hosts are plants that produce rapid and distinctive symptoms when inoculated with a virus. They are used to detect specific viruses in individual plants. Fruit trees and potatoes are commonly indexed for viruses. Only those trees free of virus are used as sources of budwood for graft propagation. Indexed nursery stock is commonly used for orchard plantings, as this is the only practical way to control fruit tree virus disease.

SerologySerology is the use of specific antibodies present in the antiserum of warm-blooded mammals. These antibodies are produced in the blood of rabbits (usually) that have been immunized with a specific pathogen. This procedure is used to index barley and potato seed stocks for specific viruses. It is also used to

detect some bacteria. Many highly specific and sophisticated tests are available now.

Embryo Test For Barley Loose Smut DetectionThe embryo test determines if loose smut is present and, if it is, the percentage of infection. If the loose smut infection exceeds 1 or 2 percent the seed lot should be treated with an effective systemic fungicide prior to planting. The embryo test is not available for use on wheat seed.

ERADICATION

Eradication means elimination of the pathogen. In actual practice, this term may be used when the pathogen is not completely eliminated but the populations are greatly reduced.

CROP ROTATION

Crop rotation involves growing different crops in the same field or plot in succeeding years. Pathogens such as the fungi that cause tan spot of wheat, dry bean rust, Cercospora leafspot of sugarbeet, and Septoria leafspot of tomato attack only one host, and populations of the pathogen increase when the same host is grown repeatedly on the same land. Crop rotation helps keep populations of these pathogens at low levels. There is one precaution, however: the grower must consider nearby areas as well. Disease organisms may spread from nearby fields or garden plots if disease was present in those areas the previous year. Disease can be expected to develop first in the area next to last year's crop. If weather favors disease development, the entire field or garden plot may eventually become diseased.

Crop rotation is an effective tool for reducing many pathogen populations. However, some pathogens survive many years in the soil and are not affected much by normal crop rotations. Long rotations may be necessary but often are impractical. The sunflower downy mildew pathogen, the sugarbeet Aphanomyces pathogen and the white mold organism survive many years in the soil. The pathogen that causes Verticillium wilt of tomato survives several years in the soil and also attacks many other garden vegetables, so it is difficult to eliminate by rotation. In the case of Verticillium wilt in tomato, use of resistant host varieties is the practical solution. In the case of white mold, some navy, kidney and black beans show partial resistance to white mold. In the case of sunflower downy mildew, only a few hybrids are resistant to all races of the mildew fungus.

ERADICATE ALTERNATE HOSTS

Many rust fungi require two hosts to complete their life cycle. The second host, called the alternate host, is essential to overwintering of many rusts in northern climates. Some of these same rust fungi reproduce indefinitely without the alternate host in warm climates. Examples of alternate hosts include common barberry (not ornamental barberry) for wheat and barley stem rust, buckthorn for oat crown rust, and juniper for apple rust. The rust fungi's sexual phase occurs on these alternate hosts.

It was hoped that eradication of barberry from the Upper Midwest in the 1930s would break the pathogen cycle and eliminate the stem rust fungus. After most barberry had been eradicated, stem rust still occurred. It was determined that stem rust survives year-round in Mexio and the Gulf Coast in the summer spore stage, without requiring the barberry for the overwintering stage. These spores are wind-blown thousands of miles north to the Upper Midwest every year (Figure 4).

Nevertheless, the barberry eradication program had two very important accomplishments: 1) stem rust infections started later in the season, and 2) the sexual phase of the fungus was eliminated, which slowed down the development of new rust races. Elimination of buckthorn near oat fields produces similar results.

To manage apple rust, commercial growers try to remove all junipers within two miles of their orchards. This is not feasible for the homeowner, who must use fungicides or resistant varieties for management of apple rust when weather is wet in spring and early summer.

SANITATION

Sanitation is the removal of crop refuse. Tillage is sometimes used to bury the refuse. The quantity of a pathogen available to produce infection is called the inoculum. Burial by tillage reduces the inoculum of the wheat tan spot pathogen, the barley spot blotch pathogen, and many garden pathogens.

All diseased tomato vines and refuse should be removed, and apple leaves that had apple scab should be raked up and detroyed by burning, burying or sending to the landfill. Diseased leaves and vines also can be composted if the compost is allowed to heat sufficiently, as described in Extension Circular PP-737 Rev., "Home Garden Disease Control Begins This Fall," or PP-469 Rev., "Plant Disease Control in the Home Garden."

When Septoria leafspot of tomato is severe, picking off badly diseased leaves (sanitation) before spraying with a fungicide helps reduce the inoculum and improves fungicidal control.

HOST RESISTANCE

Resistance is the ability of a host to resist infection by a pathogen. Resistant varieties are favored by commercial growers and gardeners when they are available.

Resistance has been the best and most cost-effective method of managing stem rust and leaf rust of wheat. Homeowners who have the Verticillium wilt pathogen in their garden soil must grow a resistant tomato variety to manage the disease.

Some foliar (leaf) pathogens may rapidly develop new races quite capable of attacking certain types of host resistance. Many foliar pathogens are extremely variable and produce billions of spores that are disseminated great distances by the wind. Consequently, new races of some foliar pathogens may become widespread in a short period of time. This results in disease outbreaks and a continued need for plant breeding programs.

Soil-borne pathogens (root and vascular wilt pathogens such as Verticillium) are also variable, but new races may not become widespread as quickly.

There are two types of host plant resistance: race specific resistance and general resistance.

Race specific resistance usually provides a high level of resistance, but it fails when new races of the pathogen develop. In the late 1990s new races of wheat leaf rust developed that attacked some of the previously resistant wheat cultivars.

General resistance is usually a stable type of resistance that is effective against all races of the pathogen. General resistance usually does not exhibit as high a level of resistance as race specific resistance. However, general resistance slows down disease development compared to that on a susceptible variety.

PROTECTION

Protection means protecting plants from infection. Storing potatoes and other vegetables in cold storage protects against infection because it is too cold for many pathogens to develop, or development is greatly slowed down. Seed

potatoes are grown in isolated areas where aphid populations are low (exclusion) and thus easily managed (protection); this minimizes aphid-borne virus infection.

CULTURAL PRACTICES

Time of planting may help plants escape infection. Winter wheat is planted in September after the destruction of volunteer wheat. Destruction of volunteers prior to winter wheat planting destroys the green bridge that wheat curl mites survive on between the summer and fall crops. The wheat curl mite is the vector of the wheat streak mosaic virus. Delayed planting of winter wheat also reduces the risk of survival and buildup of the mite in the fall of the year and exposure of the wheat crop to high wheat curl mite populations.

Many dry bean and garden bean pathogens are disseminated in water, so beans should not be cultivated when they are wet.

A plastic mulch used on tomatoes greatly reduces blossom end rot, a non-infectious disease that develops under conditions of drought or fluctuating soil moistures. The mulch produces a more uniform soil moisture.

Most powdery mildews are favored by high humidity. Powdery mildew is a common problem on alpine current, lilac, roses and shaded lawns. Pruning shrubs and trees to allow better air circulation and sunlight penetration may help reduce powdery mildew in shady locations.

HANDLING PRACTICES

Development of potato late blight in storage can be minimized by proper handling practices during the growing season. This includes hilling the soil around the plants to reduce the chances of late blight spores coming into contact with the tubers. The vines should be killed several weeks before harvest by using approved vine killers or chopping the vines off at ground level. The late blight fungus on the tops will be minimal at harvest and tubers will be mature. Mature tubers are less prone to infection.

MANAGING INSECT VECTORS

Many insects carry disease organisms such as viruses and bacteria. Managing these insect vectors may reduce the chance of disease. A prime example is the cucumber beetle, vector of the bacterial wilt pathogen of cucumber and muskmelon. The bacterium is carried from plant to plant by the beetle and

overwinters in the beetle. A good program for managing the cucumber beetle, started as soon as the plants emerge, will prevent serious losses from the bacterial wilt disease. Similarly, good aphid management is essential for raising virus-free seed potatoes.

In addition to successful management of insect vectors, weeds and other hosts that can serve as a reservoir for both vectors and pathogens must be managed. Weed management is essential around plantings of potatoes, tomatoes, cucumbers, melons, peppers and many other commercial and garden crops.

FUNGICIDES

Protectant fungicides act on the plant surface to protect against infection, and systemic fungicides are taken up by the plant tissues and then function to prevent infections. Some new fungicides have limited therapeutic (curative) properties.

Current fungicide recommendations are given in Extension Circular PP-622, "Field Crop Fungicide Guide," and Circular F-1192, "Insect and Disease Management Guide for Woody Plants in North Dakota."

Protectant FungicidesProtectant fungicides work on the plant surface to prevent spore germination or kill developing fungus hyphae before the host plant is penetrated and infection becomes established.

Protectant fungicides commonly used on North Dakota farms include maneb (Maneb 80, Maneb 75 DF), mancozeb (Dithane DF, Manzate 200 DF, Penncozeb DF), triphenyl tin hydroxide (Super Tin, Agri Tin), chlorothalonil (Bravo), and copper fungicides. Protectant fungicides commonly used on North Dakota home gardens or ornamentals include captan (Orthocide), chlorothalonil (Daconil, Ortho Multi Purpose Fungicide), maneb, mancozeb, thiram, and copper fungicides. The dicarboximide fungicides iprodione (Rovral) and vinclozolin (Ronilan) also are protectant fungicides that provide good control of Sclerotinia (white mold) on certain field crops.

Most protectant fungicides cannot stop development of a pathogen once infection occurs. Leafspots become visible a number of days after infection: four to five days for tan spot of wheat, potato late blight and sugarbeet Cercospora leafspot, and seven to 14 days for many other pathogens. If a protectant fungicide is applied after infection has occurred but before leafspots show, the disease is not cured but continues developing and symptoms appear. Then frustrated growers

and gardeners may claim that the fungicide "didn't work." Protectant fungicides do not cure infections and will not work unless the application is timely (early enough). Waiting too long usually results in failure. A few examples will illustrate this.

Gardeners sometimes begin spraying their tomato plants with a protectant fungicide such as Ortho Multi-Purpose Fungicide when all the lower leaves on their tomato plants are dead and the middle leaves already are heavily spotted with Septoria leaf spot. The disease continues to develop on the middle leaves resulting in defoliation (loss of leaves). Since some upper leaves were already infected before spraying started, symptoms may develop on these leaves, too. Only two or three tip leaves were uninfected at the first spraying. These leaves were all that was protected against infection and are all that will be left after disease has defoliated the rest of the plant. If the gardener had sprayed when the first few spots showed on the lower leaves, some lower leaves might have been lost to Septoria, but the rest of the plant would have remained healthy.

Timely application of protectant fungicides also is critical for crop diseases, such as rust of dry beans or leaf rust of winter wheat. Rust diseases have a tremendous capacity to reproduce in a short period of time. Rapid development of these diseases is favored by very susceptible varieties and rainy, humid weather. If rust is detected on these susceptible crops in a given area, if environmental conditions are favorable, and if those crops have good yield potential, growers should apply protectant fungicides to the crop prior to rust formation on the healthy leaves. Applying too late will mean that rust infections will already have occurred and new spore formation will not be stopped.

Protectant fungicides are often combined with a spreader-sticker which reduces wash-off of the fungicide during rainfall. These compounds help to wet the leaf and to bind the fungicide to the leaf. Protectant fungicides must be applied so that thorough coverage of the foliage is obtained. Protectant fungicides are redistributed on the plant by dew and rain, but uniform coverage is still essential. A fungicide should be applied before a rainy period to provide protection during the rainy period. The application must be made early enough so that the spray droplets have dried before the rain begins. If necessary, another application can be made after the rains are over (Figure 5).

Figure 5. Effect of spraying a protectant fungicide before or after an infection period (disease-favoring weather). Note that spraying after infection even though symptoms are not yet evident, will not prevent disease development. Spraying before an infection period protects against infection. (15KB illustration)

Protectant fungicides are used both as foliar fungicides and as seed treatments. Seed treatment is discussed in extension Circular PP-447, "Seed Treatment for Disease Control." Seed treatments are used to control both seed-borne diseases and soil-borne diseases that can cause death of seedlings.

Sulfur is a protectant fungicide that effectively reduces powdery mildew on a number of crops, including sugarbeets, garden and dry peas and ornamentals. Sulfur also has some eradicant action against established powdery mildew infections. This therapeutic effect of sulfur is an exception to the general rule that protectant fungicides do not cure infections.

Systemic fungicidesSystemic fungicides are taken up (absorbed) by the plant. They include some seed treatment fungicides such as carboxin (Vitavax), the benzimidazole fungicides, the sterol inhibitors, the phenylamides and the strobilurins. Benzimidazoles are commonly used as foliar fungicides and include benomyl (Benlate), thiabendazole (Mertect, TBZ, Arbotect), and thiophanate methyl (Topsin M). These fungicides move upward in the plant but cannot move down the leaf or stem. They function as locally systemic fungicides and usually do not move into new foliage. Most sterol inhibitors and strobilurins are locally systemic.

Locally systemic fungicides have several advantages: they protect both sides of the leaf even if only one side was sprayed, they are not washed off by rain or decomposed by sunlight, the interval between sprays may be longer than that with the protectant fungicides, and some have a therapeutic effect. The therapeutic effect ranges from 24-36 hours for the benzimidazoles and up to four days for some sterol inhibitors.

Benzimidazole fungicidesBenomyl (Benlate) and thiophanate methyl (Topsin M) are currently registered for control of white mold (Sclerotinia) on dry beans and give good control if properly applied at the right time. The white mold fungus infects dry beans by means of airborne spores which must begin growth on dead plant tissue, and then growth spreads to green tissue. The site of this dead tissue is the dead blossoms or dead lower leaves. Since these fungicides cannot move down the plant, they are effective only if there is complete coverage of the entire plant. Canopy penetration is essential to good control.

Phenylamide (acylalanine) fungicidesMetalaxyl (Allegiance) and mefenoxam (Ridomil Gold, Apron XL) are systemic fungicides with activity against water molds and related fungi such as Pythium, Phytophthora (potato late blight pathogen) and the downy mildew organisms. Oxadixyl (Anchor) is a systemic seed treatment fungicide with similar mode of action to metalaxyl and mefenoxam.

Sterol inhibitor (ergosterol biosynthesis inhibitor) fungicidesThe sterol inhibitor fungicides inhibit the formation of ergosterol in the higher fungi, but not the water molds. This prevents cell wall formation and stops growth of the fungus. Sterol-inhibiting fungicides currently available in North Dakota (2000) include imazalil (several brand names), triforine (Funginex, for home garden use), difenoconazole (Dividend seed treatment), triadimefon (Bayleton) triadimenol (Baytan), propiconazole(Tilt), tebuconazole (registered Raxil seed treatments, registration pending for Folicur foliar treatments), and tetraconazole (registration pending for Eminent). They have therapeutic activity, many for up to four days after infection has initiated, may suppress spore formation in established infections, most are taken up by the plant (some are only locally systemic), and they are used at very low dosages.

Strobilurin fungicidesThe strobilurin fungicides were developed recently. This is a methoxyacrylate class of chemistry which is related to naturally occurring products found in a group of forest mushrooms called "pine-cone mushrooms." Some are locally systemic and many have an extremely broad spectrum of activity against all four classes of fungi, something that is extremely rare with most classes of fungicides. These fungicides are environmentally friendly. Azoxystrobin (Quadris) and kresoxim-methyl (Sovran, Cygnus) were the first of this class. Other strobilurins include trifloxystrobin (Flint), BAS 500 (Headline, expected to be registered for the 2002 season) and a product with a related mode of action called famoxadone (Famoxate, registration expected in several years). Some strobilurins such as azoxystrobin are locally systemic, moving into and up the leaf toward the tip. Other strobilurins such as kresoxim-methyl and trifloxystrobin have only slight uptake by the leaf and are redistributed on the plant through vapor action. Most fungicides in this class are strongly bound to the cuticle (leaf surface) and so are not easily washed off the leaf. They inhibit fungal respiration, which is a different mode of action from that of other fungicides.

New Classes of FungicidesSeveral other fungicides have been developed recently, each of which is in a unique class of chemistry. Hymexazol (Tachigaren) is an isoxazole class of chemistry that is environmentally friendly and has excellent activity against Aphanomyces and also Pythium. It is registered for use as a seed pelleting for sugarbeets for early season control of Aphanomyces seedling disease.

Another new chemistry is fludioxanil (Maxim), a phenylpyrrole class of chemistry that is an environmentally friendly product registered for seed treatment on many crops and also as a potato seed piece treatment. It has excellent activity against silver scurf of potato, as well as Rhizoctonia black scurf of potato and Fusarium dry rot.

Fluazinam (Omega) is a pyridinamide class of chemistry that is a contact fungicide with a broad spectrum of activity, including activity against Sclerotinia (white mold).

Fenhexamid (Elevate) is a hydroxyanilide class of chemistry that is used as a protectant fungicide.

Some fungicides that are in varying classes not listed above include the registered fungicides cymoxanil (Curzate), dimethomorph (Acrobat) and propamocarb (Tattoo or Previcur) and the registration pending product zoxamide (Gavel). All four have good activity against the potato late blight pathogen and the downy mildews. Cymoxanil is an acetimide fungicide that is locally systemic, has a short residual and has both curative and protectant properties. Dimethomorph is a cinnamic acid derivative that is locally systemic and has curative and preventive activity. Propamocarb is a carbamate fungicide that is systemic and is translocated into new foliage. Zoxamide is an amide fungicide with a different mode of action from other potato late blight fungicides.

Some products such as acibenzolar (Actigard) and several others act to enhance the natural defenses of the plant. Currently there are no uses registered for North Dakota crops.

Resistance to FungicidesFungi may develop resistance to fungicides. Repeated use of the same fungicide or fungicides with the same mode of action can lead to development of resistance.

Fungi are less likely to develop resistance to the older protectant fungicides that have multiple sites of action against fungi. Nevertheless, the Cercospora leafspot pathogen of sugarbeet has developed tolerance to triphenyltin hydroxide. This tolerance is expressed as a reduced sensitivity to the fungicide, requiring higher application rates to control Cercospora leafspot.

Fungi are more likely to develop resistance to systemic fungicides with a single specific mode of action. Resistance to the benzimidazoles and the phenylamides can occur rapidly, with resistant strains of fungi showing insensitivity to high rates of the fungicide. Resistance also can develop rapidly to the dicarboximides, the sterol inhibitors, and the strobilurins.

The Cercospora leafspot pathogen of sugarbeet is resistant to benzimidazole fungicides in many areas of the world, including many fields in Michigan, Minnesota and North Dakota. The Fusarium dry rot pathogen and the silver scurf pathogen of potato have developed resistance to this class of fungicide across much of the United States and Canada. These three fungicides are closely related, and a pathogen that develops resistance to one of the benzimidazole fungicides will be resistant to all three. This is called cross resistance.

The potato late blight fungus can develop resistance rapidly to the phenylamides metalaxyl and mefenoxam. Certain genotypes of the fungus have resistance, including the US8 genotype now present in North Dakota and many other parts of the United States and Canada. The presence of this new genotype has seriously limited the usefulness of this class of fungicide for potato late blight management.

The gray mold fungus has rapidly developed resistance to the dicarboximide fungicides iprodione (Rovral) and vinclozolin (Ronilan) under greenhouse conditions. Certain grape pathogens have rapidly developed resistance to this class of fungicide in California.

Cases of resistance to sterol inhibitors are fairly common, and resistance management practices are recommended for this class of fungicide. Although the strobilurin fungicides have been used but a few years, it is recognized that resistance can occur to this class of fungicide, and cases of resistance are already being reported world-wide.

Resistance Management

There are several ways to retard the development of resistance. These include:

Tank mix with a fungicide with a different mode of action. Mancozeb or chlorothalonil can be tank mixed effectively with benzimidazole or phenylamide fungicides.

Alternate applications between or among two or more classes of fungicides with different modes of action. This is a good strategy for resistance management of triphenyltin hydroxide, the sterol inhibitors, the dicarboximides and the strobilurins. Although the sterol inhibitors and the phenylamides have some post-infection activity, they are best used in a preventive manner, which reduces the likelihood that resistance will develop.

Apply a limited number of applications in a block at a critical period in the pathogen disease cycle. A different mode of action should be used at other less critical times in the disease cycle, so as to minimize the exposure of the "at risk" fungicide. This has been recommended with some of the strobilurins.

Limit the number of applications of an "at risk" fungicide per year. This has been done with the phenylamide fungicides, the sterol inhibitors, and the strobilurins. Use of these fungicides may be restricted to the most critical parts of the season.

Avoid reduced rates of fungicides. These reduced rates may facilitate the development of resistance in fungi.

Do not use phenylamides as soil treatments against airborne pathogens.

Use of FungicidesAll pesticides are poisons, and they should be used with care. Most commonly used fungicides have a relatively low level of toxicity, but triphenyltin hydroxide (Super Tin, Agri Tin) is an exception in that it is quite toxic. Many fungicides have information on the label restricting the number of days before harvest that the last application can be made to food crops. Most have a 24-hour period during which re-entry in a field should be done only if wearing protective clothing (long pants, long-sleeved shirt, shoes, chemical-resistant gloves).

Most fungicides can be stored successfully from year to year, but a dry storage area is essential, especially for the maneb and mancozeb fungicides. Care should be taken in storing liquid or flowable fungicides as some will not stand freezing. Check the label for information on use and storage.

INTEGRATED DISEASE MANAGEMENT

Effective, practical disease management usually involves several techniques. Management programs based on only one or two techniques can be effective in the short term but may become ineffective if used frequently. The development of fungicide resistance is a good example; the epidemics of rust caused by newly prevalent races of the fungus is another. Growers need to integrate as many different management tools as possible for long term success.

Stem rust of wheat is managed by host resistance, the elimination of barberry (this prevents the sexual phase that produces new rust races), and by constant monitoring of the rust races by plant pathologists. Monitoring provides early warning for plant breeders of the buildup of new rust races and the need for alternative management for the grower.

The main tools for management of dry bean rust are sanitation, crop rotation, tolerant varieties and fungicides. All of these procedures must be done well and in a timely fashion to produce effective management.

Tan spot of wheat can be reduced by sanitation, crop rotation, and protection (fungicides); researchers are developing varieties with improved tan spot resistance. When wheat is planted directly into standing wheat stubble, both sanitation and rotation are eliminated as management tools and growers must rely primarily on variety choice and fungicides for tan spot reduction.

Homeowners should make every effort to remove diseased plant debris from the yard and garden in the fall - sanitation is the first step to disease management for

next spring. Sanitation should be followed by seed treatment, rotation, use of resistant varieties, and timely fungicide applications.

Whenever possible, all the principles of management, eradication, exclusion, host resistance, and protection, should be practiced. The use of these combined practices usually produces the most reliable and stable plant disease management.

Finally, all growers should keep close watch on the weather forecasts, as this helps to anticipate disease-favoring weather and allows more timely preventive measures. Some Disease Forecasting models for major crop diseases have been developed and are now available on the Internet or through toll-free telephone numbers. The disease forecasting information aids in predicting disease outbreaks and the need for fungicides.

Table 1. Common causes of diseases.How They Can Be Seen

Type Cause Description

How TheyReproduce

Equipment 

MagnificationRequired*

Examples

Non-Infectious(Abiotic)

Nutrition  Deficiency or excess of essential elements 

--Visual (no special equipment needed)

--Nitrogen deficiency, iron deficiency, zinc deficiency

Moisture  Deficiency or excess of water 

--Visual 

--Moisture stress or wilting, suffocation of roots in wet soil

Temperature 

Cold or heat  --

Visual --

Frost, heat sterility

Other Meterological Conditions

Sun, wind, etc. --

Visual --

Heat canker, sunscald, lightning injury

Toxic Chemicals 

Salt, air pollutants, etc. 

--Visual 

--Salt injury (road salt, saline seeps), ozone injury

Infectious (Biotic) 

Fungi  Grow as tiny threadlike filaments; large fruiting structures may develop from these filaments. 

Spores, Cell division 

Microscope (occasionally a hand lens) 

20-250  Rusts, smuts, leafspots, white mold (canola, dry beans and sunflower), powdery mildew, tree cankers, apple scab, wheat and barley scab.

Bacteria  Tiny single-celled organisms. 

Cell division 

Microscope 

400-1,000  Bacterial blights of beans, bacterial blights of small grains, fireblight, bacterial wilt of cucumber.

Viruses  Very tiny rod-shaped  or spherical particles, composed of RNA with a protein coat.

Cause host to manufacture virus 

Electron microscope

20,000-100,000

Wheat streak mosaic, barley stripe mosaic, potato viruses, tobacco  mosaic, cucumber mosaic.

Phytoplasmas 

Very tiny organisms without a cell wall, no definite shape. 

Division  Electron microscope 

20,000-50,000

Aster yellows (purple top in potato and tomato).

Nematodes  Tiny Roundworms 

Eggs  Microscope (naked eye for larger forms) 

1-60  No serious problem in North Dakota. Soybean cyst nematode and pine wilt nematode occur in neighboring states.

*The number of times a pathogen must be magnified to be visible. For example, a pathogen 1/1,000 inch in size when magnified 100 times would appear to be 1/10 inch in size; a pathogen 1/1,000,000 inch in size when magnified 100,000 times would appear to be 1/10 inch in size.

EB-31, Revised, February 2001

PhytoremediationFrom Wikipedia, the free encyclopediaJump to: navigation, search

Phytoremediation (from the Ancient Greek φυτο (phyto, plant), and Latin remedium (restoring balance or remediation) describes the treatment of environmental problems (bioremediation) through the use of plants that mitigate the environmental problem without the need to excavate the contaminant material and dispose of it elsewhere.

Phytoremediation consists of mitigating pollutant concentrations in contaminated soils, water, or air, with plants able to contain, degrade, or eliminate metals, pesticides, solvents, explosives, crude oil and its derivatives, and various other contaminants from the media that contain them.

Contents

[hide] 1 Application 2 Advantages and limitations 3 Various phytoremediation processes

o 3.1 Phytoextraction o 3.2 Phytostabilization o 3.3 Phytotransformation

4 The role of genetics 5 Hyperaccumulators and biotic interactions

o 5.1 Table of hyperaccumulators 6 See also 7 References 8 Bibliography

9 External links

[edit] Application

Phytoremediation may be applied wherever the soil or static water environment has become polluted or is suffering ongoing chronic pollution. Examples where phytoremediation has been used successfully include the restoration of abandoned metal-mine workings, reducing the impact of sites where polychlorinated biphenyls have been dumped during manufacture and mitigation of on-going coal mine discharges.

Phytoremediation refers to the natural ability of certain plants called hyperaccumulators to bioaccumulate, degrade,or render harmless contaminants in soils, water, or air. Contaminants such as metals, pesticides, solvents, explosives[1], and crude oil and its derivatives, have been mitigated in phytoremediation projects worldwide. Many plants

such as mustard plants, alpine pennycress and pigweed have proven to be successful at hyperaccumulating contaminants at toxic waste sites.

Phytoremediation is considered a clean, cost-effective and non-environmentally disruptive technology, as opposed to mechanical cleanup methods such as soil excavation or pumping polluted groundwater. Over the past 20 years, this technology has become increasingly popular and has been employed at sites with soils contaminated with lead, uranium, and arsenic. However, one major disadvantage of phytoremediation is that it requires a long-term commitment, as the process is dependent on plant growth, tolerance to toxicity, and bioaccumulation capacity.

[edit] Advantages and limitations

Advantages: o the cost of the phytoremediation is lower than that of traditional processes

both in situ and ex situo the plants can be easily monitoredo the possibility of the recovery and re-use of valuable metals (by

companies specializing in “phyto mining”)o it is potentially the least harmful method because it uses naturally

occurring organisms and preserves the environment in a more natural state.

Limitations: o phytoremediation is limited to the surface area and depth occupied by the

roots.o slow growth and low biomass require a long-term commitmento with plant-based systems of remediation, it is not possible to completely

prevent the leaching of contaminants into the groundwater (without the complete removal of the contaminated ground, which in itself does not resolve the problem of contamination)

o the survival of the plants is affected by the toxicity of the contaminated land and the general condition of the soil.

o bio-accumulation of contaminants, especially metals, into the plants which then pass into the food chain, from primary level consumers upwards or requires the safe disposal of the affected plant material.

[edit] Various phytoremediation processes

A range of processes mediated by plants or algae are useful in treating environmental problems:

Phytoextraction — uptake and concentration of substances from the environment into the plant biomass.

Phytostabilization — reducing the mobility of substances in the environment, for example, by limiting the leaching of substances from the soil.

Phytotransformation — chemical modification of environmental substances as a direct result of plant metabolism, often resulting in their inactivation, degradation (phytodegradation), or immobilization (phytostabilization).

Phytostimulation — enhancement of soil microbial activity for the degradation of contaminants, typically by organisms that associate with roots. This process is also known as rhizosphere degradation. Phytostimulation can also involve aquatic plants supporting active populations of microbial degraders, as in the stimulation of atrazine degradation by hornwort.[2]

Phytovolatilization — removal of substances from soil or water with release into the air, sometimes as a result of phytotransformation to more volatile and/or less polluting substances.

Rhizofiltration — filtering water through a mass of roots to remove toxic substances or excess nutrients. The pollutants remain absorbed in or adsorbed to the roots.

[edit] Phytoextraction

Phytoextraction (or phytoaccumulation) uses plants or algae to remove contaminants from soils, sediments or water into harvestable plant biomass (organisms that take larger-than-normal amounts of contaminants from the soil are called hyperaccumulators). Phytoextraction has been growing rapidly in popularity worldwide for the last twenty years or so. In general, this process has been tried more often for extracting heavy metals than for organics. At the time of disposal, contaminants are typically concentrated in the much smaller volume of the plant matter than in the initially contaminated soil or sediment. 'Mining with plants', or phytomining, is also being experimented with.

The plants absorb contaminants through the root system and store them in the root biomass and/or transport them up into the stems and/or leaves. A living plant may continue to absorb contaminants until it is harvested. After harvest, a lower level of the contaminant will remain in the soil, so the growth/harvest cycle must usually be repeated through several crops to achieve a significant cleanup. After the process, the cleaned soil can support other vegetation.

Advantages: The main advantage of phytoextraction is environmental friendliness. Traditional methods that are used for cleaning up heavy metal-contaminated soil disrupt soil structure and reduce soil productivity, whereas phytoextraction can clean up the soil without causing any kind of harm to soil quality. Another benefit of phytoextraction is that it is less expensive than any other clean-up process.

Disadvantages: As this process is controlled by plants, it takes more time than anthropogenic soil clean-up methods.

Two versions of phytoextraction:

natural hyper-accumulation, where plants naturally take up the contaminants in soil unassisted, and

induced or assisted hyper-accumulation, in which a conditioning fluid containing a chelator or another agent is added to soil to increase metal solubility or mobilization so that the plants can absorb them more easily. In many cases natural hyperaccumulators are metallophyte plants that can tolerate and incorporate high levels of toxic metals.

Examples of phytoextraction (see also 'Table of hyperaccumulators'):

Arsenic , using the Sunflower (Helianthus annuus), or the Chinese Brake fern (Pteris vittata), a hyperaccumulator. Chinese Brake fern stores arsenic in its leaves.

Cadmium , using Willow (Salix viminalis): In 1999, one research experiment performed by Maria Greger and Tommy Landberg suggested Willow (Salix viminlais) has a significant potential as a phytoextractor of Cadmium (Cd), Zinc (Zn), and Copper (Cu), as willow has some specific characteristics like high transport capacity of heavy metals from root to shoot and huge amount of biomass production; can be used also for production of bio energy in the biomass energy power plant.[3]

Cadmium and zinc, using Alpine pennycress (Thlaspi caerulescens), a hyperaccumulator of these metals at levels that would be toxic to many plants. On the other hand, the presence of copper seems to impair its growth (see table for reference).

Lead , using Indian Mustard (Brassica juncea), Ragweed (Ambrosia artemisiifolia), Hemp Dogbane (Apocynum cannabinum), or Poplar trees, which sequester lead in their biomass.

Salt-tolerant (moderately halophytic) barley and/or sugar beets are commonly used for the extraction of Sodium chloride (common salt) to reclaim fields that were previously flooded by sea water.

Caesium-137 and strontium-90 were removed from a pond using sunflowers after the Chernobyl accident.[4]

Mercury , selenium and organic pollutants such as polychlorinated biphenyls (PCBs) have been removed from soils by transgenic plants containing genes for bacterial enzymes.[5]

[edit] Phytostabilization

Phytostabilization focuses on long-term stabilization and containment of the pollutant. For example, the plant's presence can reduce wind erosion; or the plant's roots can prevent water erosion, immobilize the pollutants by adsorption or accumulation, and provide a zone around the roots where the pollutant can precipitate and stabilize. Unlike phytoextraction, phytostabilization focuses mainly on sequestering pollutants in soil near the roots but not in plant tissues. Pollutants become less bioavailable, and livestock,

wildlife, and human exposure is reduced. An example application of this sort is using a vegetative cap to stabilize and contain mine tailings.[6]

[edit] Phytotransformation

In the case of organic pollutants, such as pesticides, explosives, solvents, industrial chemicals, and other xenobiotic substances, certain plants, such as Cannas, render these substances non-toxic by their metabolism. In other cases, microorganisms living in association with plant roots may metabolize these substances in soil or water. These complex and recalcitrant compounds cannot be broken down to basic molecules (water, carbon-dioxide, etc.) by plant molecules, and, hence, the term phytotransformation represents a change in chemical structure without complete breakdown of the compound. The term "Green Liver Model" is used to describe phytotransformation, as plants behave analogously to the human liver when dealing with these xenobiotic compounds(foreign compound/pollutant).[7] After uptake of the xenobiotics, plant enzymes increase the polarity of the xenobiotics by adding functional groups such as hydroxyl groups (-OH).

This is known as Phase I metabolism, similar to the way that the human liver increases the polarity of drugs and foreign compounds (Drug Metabolism). Whereas in the human liver enzymes such as Cytochrome P450s are responsible for the initial reactions, in plants enzymes such as nitroreductases carry out the same role.

In the second stage of phytotransformation, known as Phase II metabolism, plant biomolecules such as glucose and amino acids are added to the polarized xenobiotic to further increase the polarity (known as conjugation). This is again similar to the processes occurring in the human liver where glucuronidation (addition of glucose molecules by the UGT (e.g. UGT1A1) class of enzymes) and glutathione addition reactions occur on reactive centres of the xenobiotic.

Phase I and II reactions serve to increase the polarity and reduce the toxicity of the compounds, although many exceptions to the rule are seen. The increased polarity also allows for easy transport of the xenobiotic along aqueous channels.

In the final stage of phytotransformation (Phase III metabolism), a sequestration of the xenobiotic occurs within the plant. The xenobiotics polymerize in a lignin-like manner and develop a complex structure that is sequestered in the plant. This ensures that the xenobiotic is safely stored, and does not affect the functioning of the plant. However, preliminary studies have shown that these plants can be toxic to small animals (such as snails), and, hence, plants involved in phytotransformation may need to be maintained in a closed enclosure.

Hence, the plants reduce toxicity (with exceptions) and sequester the xenobiotics in phytotransformation. Trinitrotoluene phytotransformation has been extensively researched and a transformation pathway has been proposed.[8]

[edit] The role of genetics

Breeding programs and genetic engineering are powerful methods for enhancing natural phytoremediation capabilities, or for introducing new capabilities into plants. Genes for phytoremediation may originate from a micro-organism or may be transferred from one plant to another variety better adapted to the environmental conditions at the cleanup site. For example, genes encoding a nitroreductase from a bacterium were inserted into tobacco and showed faster removal of TNT and enhanced resistance to the toxic effects of TNT.[9] Researchers have also discovered a mechanism in plants that allows them to grow even when the pollution concentration in the soil is lethal for non-treated plants. Some natural, biodegradable compounds, such as exogenous polyamines, allow the plants to tolerate concentrations of pollutants 500 times higher than untreated plants, and to absorb more pollutants.

[edit] Hyperaccumulators and biotic interactions

A plant is said to be a hyperaccumulator if it can concentrate the pollutants in a minimum percentage which varies according to the pollutant involved (for example: more than 1000 mg/kg of dry weight for nickel, copper, cobalt, chromium or lead; or more than 10,000 mg/kg for zinc or manganese).[10] This capacity for accumulation is due to hypertolerance, or phytotolerance: the result of adaptative evolution from the plants to hostile environments through many generations. A number of interactions may be affected by metal hyperaccumulation, including protection, interferences with neighbour plants of different species, mutualism (including mycorrhizae, pollen and seed dispersal), commensalism, and biofilm.

[edit] Table of hyperaccumulators

Plant Nutrient Deficiencies

Identifying Plant Problems

By Marie Iannotti, About.com Guide

See More About: plant problems fertilizer garden maintenance

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Not all plant problems are caused by insects or diseases. Sometimes an unhealthy plant is suffering from a nutrient deficiency or even too much of any one nutrient. Plant nutrient deficiencies often manifest as foliage discoloration or distortion. The following chart outlines some possible problems. Unfortunately many problems have similar symptoms and sometimes it is a combination of problems.

Be sure you eliminate the obvious before you kill your plants with kindness.

Check first for signs of insects or disease. Foliage discoloration and stunted plants can easily be caused by soil that is too

wet and drains poorly or soil that is too compacted for good root growth. Extreme cold or heat will slow plant growth and effect flowering and fruit set. Too much fertilizer can result in salt injury. Your plants may look scorched or

they may wilt, even when the soil is wet.

For a definitive diagnoses, contact your local cooperative extension service.

Plants require a mix of nutrients to remain healthy. Nutrients that are needed in relatively large amounts are called the macronutrients. Plant macronutrients include: nitrogen, potassium, phosphorus, calcium, sulfur and magnesium.

There are a handful of additional nutrients that are required for plant growth, but in much smaller quantities. These micronutrients include: boron, copper, iron, manganese, molybdenum and zinc.

All of these nutrients are taken in through the roots. Water transfers the nutrients from the soil to the plant roots. So one requirement of sufficient plant nutrition is water. A second requirement is the appropriate soil pH for the plant being grown. Each plant prefers a

specific pH range to be able to access the nutrients in the soil. Some plants are fussier than others, but if the soil pH is too acidic or alkaline, the plant will not be able to take in nutrients no matter how rich your soil may be.

Plant Nutrient Deficiency Symptoms

MacronutrientsCalcium (Ca)

o Symptoms: New leaves are distorted or hook shaped. The growing tip may die. Contributes to blossom end rot in tomatoes, tip burn of cabbage and brown/black heart of escarole & celery.

o Sources: Any compound containing the word 'calcium'. Also gypsum. o Notes: Not often a deficiency problem and too much will inhibit other

nutrients.

Nitrogen (N) o Symptoms: Older leaves, generally at the bottom of the plant, will yellow.

Remaining foliage is often light green. Stems may also yellow and may become spindly. Growth slows.

o Sources: Any compound containing the words: 'nitrate', 'ammonium' or 'urea'. Also manure.

o Notes: Many forms of nitrogen are water soluble and wash away.

Magnesium (Mg) o Symptoms: Slow growth and leaves turn pale yellow, sometimes just on

the outer edges. New growth may be yellow with dark spots. o Sources: Compounds containing the word 'magnesium', such as Epson

Salts.

Phosphorus (P) o Symptoms: Small leaves that may take on a reddish-purple tint. Leaf tips

can look burnt and older leaves become almost black. Reduced fruit or seed production.

o Sources: Compounds containing the words 'phosphate' or 'bone'. Also greensand.

o Notes: Very dependent on pH range.

Potassium (K) o Symptoms: Older leaves may look scorched around the edges and/or

wilted. Interveinal chlorosis (yellowing between the leaf veins) develops. o Sources: Compounds containing the words 'potassium' or 'potash'.

Sulfur (S) o Symptoms: New growth turns pale yellow, older growth stays green.

Stunts growth. o Sources: Compounds containing the word 'sulfate'. o Notes: More prevalent in dry weather.

MicronutrientsBoron (B)

o Symptoms: Poor stem and root growth. Terminal (end) buds may die. Witches brooms sometimes form.

o Sources: Compounds containing the words 'borax' or 'borate'.

Copper (Cu) o Symptoms: Stunted growth. Leaves can become limp, curl, or drop. Seed

stalks also become limp and bend over. o Sources: Compounds containing the words 'copper', 'cupric' or 'cuprous'.

Manganese (Mn) o Symptoms: Growth slows. Younger leaves turn pale yellow, often starting

between veins. May develop dark or dead spots. Leaves, shoots and fruit diminished in size. Failure to bloom.

o Sources: Compounds containing the words 'manganese' or 'manganous'

Molybdenum (Mo) o Symptoms: Older leaves yellow, remaining foliage turns light green.

Leaves can become narrow and distorted. o Sources: Compounds containing the words 'molybdate' or 'molybdic'. o Notes: Sometimes confused with nitrogen deficiency.

Zinc (Zn) o Symptoms: Yellowing between veins of new growth. Terminal (end)

leaves may form a rosette. o Sources: Compounds containing the word 'zinc'. o Notes: Can become limited in higher pH.

OsmolarityFrom Wikipedia, the free encyclopedia

Jump to: navigation, search This article is about osmolarity. For the osmole unit, see Osmole (unit).

Osmolarity is the measure of solute concentration, defined as the number of osmoles (Osm) of solute per litre (L) of solution (osmol/L or Osm/L). The osmolarity of a solution is usually expressed as Osm/L (pronounced "osmolar"), in the same way that the molarity of a solution is expressed as "M" (pronounced "molar"). Whereas molarity measures the number of moles of solute per unit volume of solution, osmolarity measures the number of osmoles of solute particles per unit volume of solution.[1]

Osmolality is a measure of the osmoles of solute per kilogram of solvent (osmol/kg or Osm/kg).

Molarity and osmolarity are not commonly used in osmometry because they are temperature dependent. This is because water changes its volume with temperature (See: Vapour pressure of water). However, if the concentration of solutes is very low, osmolarity and osmolality are considered equivalent.

Contents

[hide] 1 Types of solutes 2 Definition 3 Osmolarity vs. tonicity 4 Plasma osmolarity vs. osmolality 5 See also

6 References

[edit] Types of solutes

Osmolarity is distinct from molarity because it measures moles of solute particles rather than moles of solute. The distinction arises because some compounds can dissociate in solution, whereas others cannot.[1]

Ionic compounds, such as salts, can dissociate in solution into their constituent ions, so there is not a one-to-one relationship between the molarity and the osmolarity of a solution. For example, sodium chloride (NaCl) dissociates into Na+ and Cl- ions. Thus, for every 1 mole of NaCl in solution, there are 2 osmoles of solute particles (i.e., a 1 M NaCl solution is a 2 Osm NaCl solution). Both sodium and chloride ions affect the osmotic pressure of the solution.[1]

Nonionic compounds do not dissociate, and form only 1 osmole of solute per 1 mole of solute. For example, a 1 M solution of glucose is 1 Osm.[1]

Multiple compounds may contribute to the osmolarity of a solution. For example, a 3 Osm solution might consist of: 3 moles glucose, or 1.5 moles NaCl, or 1 mole glucose + 1 mole NaCl, or 2 moles glucose + 0.5 mole NaCl, or any other such combination.[1]

[edit] Definition

The osmolarity of a solution can be calculated from the following expression:

where

φ is the osmotic coefficient, which accounts for the degree of non-ideality of the solution. In the simplest case it is the degree of dissociation of the solute. Then, φ is between 0 and 1 where 1 indicates 100% dissociation. However, φ can also be larger than 1 (e.g. for sucrose). For salts, electrostatic effects cause φ to be smaller than 1 even if 100% dissociation occurs (see Debye-Hückel equation);

n is the number of particles (e.g. ions) into which a molecule dissociates. For example: glucose has n of 1, while NaCl has n of 2;

C is the molar concentration of the solute; the index i represents the identity of a particular solute.

Osmolality can be measured using an osmometer which measures colligative properties, such as Freezing-point depression, Vapor pressure, or Boiling-point elevation.

[edit] Osmolarity vs. tonicity

Osmolarity and tonicity are related, but different, concepts. Thus, the terms ending in -osmotic (isosmotic, hyperosmotic, hyposmotic) are not synonymous with the terms ending in -tonic (isotonic, hypertonic, hypotonic). The terms are related in that they both compare the solute concentrations of two solutions separated by a membrane. The terms are different because osmolarity takes into account the total concentration of penetrating solutes and non-penetrating solutes, whereas tonicity takes into account the total concentration of only non-penetrating solutes.[1]

Penetrating solutes can diffuse through the cell membrane, causing momentary changes in cell volume as the solutes "pull" water molecules with them. Non-penetrating solutes cannot cross the cell membrane, and therefore osmosis of water must occur for the solutions to reach equilibrium.

A solution can be both hyperosmotic and isotonic.[1] For example, the intracellular fluid and extracellular can be hyperosmotic, but isotonic - if the total concentration of solutes in one compartment is different from that of the other, but one of the ions cannot cross the membrane, drawing water with it and thus causing no net change in solution volume.

[edit] Plasma osmolarity vs. osmolality

Plasma osmolarity can be calculated from plasma osmolality by the following equation:[2]

Osmolarity = osmolality * (ρsol - ca)

Where:

ρsol is the density of the solution in g/ml, which is 1.025 g/ml for blood plasma.[3]

ca is the (anhydrous) solute concentration in g/ml - not to be confused with the density of dried plasma

Since ca is slightly larger than 0.03 g/ml, plasma osmolarity is 1-2%[2] less than osmolality.

According to IUPAC, osmolality is the quotient of the negative natural logarithm of the rational activity of water and the molar mass of water, whereas osmolarity is the product of the osmolality and the mass density of water (also known as osmotic concentration).

In more simple terms, osmolality is an expression of solute osmotic concentration per mass, whereas osmolarity is per volume of solvent (thus the conversion by multiplying with the mass density).

ROLE OF PLANT HORMONES IN LATERAL BUD GROWTH OF ROSE AND APPLE IN VITROAuthors: H. Telgen, V. Elagöz, A. Mil, A. Paffen, G. Klerk

Abstract: The effects of different growth regulators and inhibitors on sprouting and outgrowth of isolated axillary buds of rose and apple in vitro were evaluated. Budbreak in ‘Madelon’ rose was stimulated by benzyladenine (BA). The response was dependent on the original position of the axillary bud on the main stem. Using a gas-permeable closure, shoot length was increased in ‘Madelon’ but not in ‘Motrea’ rose, suggesting an inhibiting effect of accumulating ethylene. Addition of silverthiosulphate (STS), an inhibitor of ethylene action, increased shoot length significantly in both rose varieties and in apple. In apple, the stimulating effect of STS depended upon the auxin concentration in the medium. Inhibiting ethylene biosynthesis with aminoethoxyvinylglycine (AVG) or stimulating it with 1-aminocyclopropane-1-carboxylic acid (ACC) had no effect on average shoot length in rose. Only in ‘Madelon’, the presence of ACC decreased the multiplication rate. Abscisic acid (ABA) reduced or inhibited BA-induced prouting of isolated axillary buds of ‘Madelon’ rose.

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