UNIT 7

Chapter 36: Transport in PlantsChapter 37: Plant NutritionChapter 38: Plant Reproduction

Types of TransportTransport of water and minerals occurs on three levels:In/out of individual cellsShort distanceLong distanceDifferences in water potential drive transport of water in plant cells = s + p

Pure water, s = 0Addition of solutes decreases s Water moves from areas of high water potential to areas of low water potential

Water potential impacts uptake and loss of waterFlaccid cellCell loses waterCell will eventually plasmolyzeTurgid cellCell gains/maintains waterTurgid plant

Absorption of Water & MineralsRoot hairs and mycorrhizae increase surface area and enhance absorptionWater and minerals transported = xylem sapAscent of xylem sap depends mainly on transpirationXylem sap not pushed by pressure in roots, but pulled upForce created by transpiration and cohesion/adhesion of water molecules extends down shoot, into roots and even into soil

Water potential gradient drives water upAir in the xylem, cavitation, breaks the chain

Control of TranspirationA plant can transpire more than its weight in water every dayFlow in xylem can reach 75cm/minGuard cells control the size of the stomata and balance the plants water needs and loss

Transpiration-to-photosynthesis is amount of water loss per gram of CO2 fixedMany plants ~600:1 600g of water lost per 1g of CO2 fixedC4 plants (ex. corn) ~300:1Transpiration also results in evaporative cooling, which can cool leaves 10-15CPrevents denaturation of enzymes and disruption of metabolism

Each stoma is flanked by a pair of guard cells suspended by other epidermal cells over an air spaceWater into guard cells = turgid stoma openWater out of guard cells = flaccid stoma closed

K+ plays an important role in osmosis in guard cellsPresence of K+ ions lowers s and water flows in or out based on where s is lower

There are three cues that initiate the opening of stomata in the morning1. Blue-light receptors promote active uptake of K+ into guard cellsPhotosynthesis provides ATP2. Depletion of CO2 as photosynthesis begins3. Internal clock within guard cells that cycles on a 24-hour basis circadian rhythm

XerophytesXerophytes are plants that are well adapted to arid (very dry) environmentsCAM plants are xerophytes (family Crassulaceae)A number of anatomical adaptations exist to reduce water lossConcentration of stomata on lower surface of leafPresence of trichomesPlacement of stomata within crypts

Phloem TransportPhloem sap is typically moved from sugar sources to sugar sinksSources: sugar being produced by photosynthesis, esp. in mature leavesSinks: growing parts of the plant, fruitsSinks usually receive sugar from the sources closest to themCan be in any direction even against pull of gravityPressure flow is the primary force behind the translocation of phloem sap

Phloem sap flows from source to sink at about 1m/hrFlow is fastest near sourcesSugar concentration is highestWater is recycled because of differences in END

Nutrition RequirementsPlants obtain the molecules they need from the air and soilMineral nutrients absorbed from soil as inorganic ionsNitrogen acquired as nitrate ions NO3-Essential nutrients are required for plants to survive

Macronutrients are elements required in relatively high concentrationsC, O, H, N, S, P, K, Ca, MgMicronutrients are elements required in relatively low concentrationsFe, Cl, Cu, Zn, Mn, Mb, B, NiTable 37.1

Nitrogen Requirements80% of the atmosphere is nitrogen (N2), but plants can still suffer deficienciesPlants can only use nitrogen in certain formsAmmonium (NH4+) or nitrate (NO3-) ionsBacteria in the soil metabolize unusable forms of nitrogenNitrogen fixation makes nitrogen available

Nitrogen fixing bacteria convert N2 into NH4+ and ammonifying bacteria convert decomposing organic material into NH4+Nitrifying bacteria convert NH4+ into NO3-ALL (eukaryotic) life on earth depends on nitrogen fixation

Symbiotic RelationshipsMany plant families have special relationships with bacteria and fungiNitrogen-fixing bacteria live in plants roots to provide live-in source for nitrogenVery common in legumes (peas, beans, soybeans, peanuts)Roots of legumes have swellings, nodules, composed of plant cells containing bacteria

Rhizobium is a genus of nitrogen-fixing bacteriaBacteria provide usable nitrogen, plant provides carbohydrates and other organic compounds

Parasitic & Carnivorous PlantsSome plants supplement or replace their photosynthesis by taking advantage of other plantsex. Indian pipe

Epiphytes are autotrophic plants, but they simply live on other plantsNot truly parasiticex. some mosses and ferns

Carnivorous plants supplement their nutrition by digesting animalsTypically found in areas with poor soil conditionsUse photosynthesis for carbohydrates, but get some nitrogen and minerals from animalsModified leaves trap animals and secrete enzymesEND

Flower Structure: Review

Pollination is the attachment of pollen to a flowers stigmaPollen released is carried by wind or animalsPollen grain produces a pollen tubeGrows through style, into ovary and discharges spermZygote gives rise to an embryoOvule develops into a seed, ovary develops into a fruit containing seed(s)

Pollination

Plant biologists distinguish between complete flowers, those having all four organs, and incomplete flowers, those lacking one or more of the whorlsA bisexual flower (perfect flower) is equipped with both stamens and carpalsA unisexual flower is missing either stamens or carpelsPlant Reproduction Terminology

A monoecious plant has male and female flowers on the same individual plantex. cornA dioecious species has male flowers and female flowers on separate plantsex. date palms

Some flowers self-fertilize, but most angiosperms have mechanisms that make this difficultBarriers prevent self-fertilization to maintain genetic varietyPrevention of Self-Fertilization

In some species stamens and carpels mature at different timesMay be arranged so that it is unlikely that an animal pollinator could transfer pollen from the anthers to the stigma of the same flower

Most common anti-selfing mechanism is self-incompatibilityAbility to reject its own pollenBiochemical block prevents fertilizationSelf-incompatibility systems are analogous to the immune response of animalsDifference is that the animal immune system rejects non-self, self-incompatibility in plants is a rejection of self

Based on genes for self-incompatibility, called S-genes, with as many as 50 different alleles in a single populationIf a pollen grain and the carpels stigma have matching alleles at the S-locus, pollen grain fails to initiate or complete the pollen tube

Pollen grain lands on stigma, absorbs moisture and begins producing a pollen tubeDirected by a chemical attractant, the pollen tube enters the ovary through the micropyle and discharges two sperm within the embryo sacDouble Fertilization

Both sperm fuse with nuclei in the embryo sacOne sperm fertilizes the egg to form the zygoteOther sperm combines with the two polar nuclei to form a triploid nucleus in the central cellGives rise to the endosperm, a food-storing tissue of the seed

Fig. 38.9

After double fertilization, the ovule develops into a seed, and the ovary develops into a fruit enclosing the seed(s)As the embryo develops, the seed stockpiles proteins, oils, and starch

As the seeds are developing from ovules, the ovary of the flower is developing into a fruitPollination triggers hormonal changes that cause the ovary to begin its transformation into a fruitIf a flower has not been pollinated, fruit usually do not develop, and the entire flower withers and falls awayFruit Development

The wall of the ovary becomes the pericarp, the thickened wall of the fruitIn some angiosperms, other floral parts contribute to what we call a fruit

As a seed matures, it dehydrates and enters a dormancy phaseExtremely low metabolic rate, suspension of growth and developmentConditions required to break dormancySome seeds germinate as soon as they are in a suitable environmentRemains dormant until some specific environmental cue causes them to break dormancySeeds & Seed Germination

Seeds of many desert plant germinate only after a substantial rainfall, ensuring enough waterWhere natural fires are common, many seeds require intense heat to break dormancyOther seeds require a chemical attack or physical abrasionex. through an animals digestive tract before they can germinate

Germination of seeds depends on imbibition, the uptake of water due to the low water potential of the dry seedExpanding seed ruptures its seed coat and triggers metabolic changes in the embryo that enable it to resume growth

Enzymes begin digesting the storage materials of endo- sperm and the nutrients are transferred to the growing regions of the embryoEND

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