Plant nutrition

Plant Nutrition

1. What is meant by “plant nutrition”

2. The chemical elements required by plants

3. How plants take up mineral elements from soil

4. Problems in plant nutrition

6. Leaf senescence and withdrawal of nutrients to the plant

5. Nitrogen and the effects of soil organic matter on plant nutrition

1. What is meant by “plant nutrition”

Uptake from the soil of mineral elements

“Plant nutrition” specifically does not refer to photosynthesis.

In this lecture the uptake of nutrients from the soil directly by roots

In the next lecture mutualistic relationships between plants and fungi and microrganisms

Plants require 13 mineral nutrient elements for growth.

The elements that are required or necessary for plants to complete their life cycle are called essential plant nutrients.

Each has a critical function in plants and are required in varying amounts in plant tissue, see table on next slide for typical amounts relative to nitrogen and the function of essential nutrients .

The nutrient elements differ in the form they are absorbed by the plant, by their functions in the plant, by their mobility in the plant and by the plant deficiency or toxicity symptoms characteristic of the nutrient.

2. The chemical elements required by plants

Name Chemical Relative Function in plant symbol % in plant to NPrimary macronutrientsNitrogen N 100 Proteins, amino acidsPhosphorus P 6 Nucleic acids, ATPPotassium K 25 Catalyst, ion transport

Secondary macronutrientsCalcium Ca 12.5 Cell wall componentMagnesium Mg 8 Part of chlorophyllSulfur S 3 Amino acidsIron Fe 0.2 Chlorophyll synthesis

MicronutrientsCopper Cu 0.01 Component of enzymesManganese Mn 0.1 Activates enzymesZinc Zn 0.03 Activates enzymesBoron B 0.2 Cell wall componentMolybdenum Mo 0.0001 Involved in N fixationChlorine Cl 0.3 Photosynthesis reactions

3. How plants take up mineral elements from soil

Dominant in mineral soils:

Dominant in organic soils:

A. Bulk flow: Uptake in the transpiration stream

B. Mycorrhizae: symbiotic relationship with fungi

Nutrients diffuse to regions of low concentration and roots grow into and proliferate in soil zones with high nutrient concentrations (horse manure in sand).

Roots are slow growing but mycorrhizal fungi proliferate and ramify through the soil. Symbiotic relationship: carbon-nitrogen exchange.

The concentration of dissociated water in freshly-distilled water is 10-7 M. This is used to describe acidity-alkalinity, originally called the pouvoir Hydrogéne, which we know now as pH.

Mineral soilsSoil acidity determines how nutrients become available to plants

Nutrients are available through WATER in the soil

Small quantities of water molecules dissociate:

H2O OH- + H+

pH = - log [H+] = - log [10-7M] = 7 for fresh distilled water

Small values for acid, e.g., the water in Sphagnum bogs can be ~3

Large values for alkaline, e.g., soils on limestone ~8

Mineral soils

How clay particles provide nutrients

The root hair cells of plant roots secrete H+ into the water around nearby clay particles. These smaller H cations replace the larger macro- and micro-nutrient cations:

The released cations are now available for uptake into roots.

A clay particle (much enlarged here) is covered with negative charges, anions:

Opposites attract, so metal ions with positive charge(s), cations, stick all over the surface of the clay particle:

2H+

Ca2+

Summary of soil water chemistry

In this summary occurrence of H+ in soil water is shown as the result of respiration of CO2 and disassociation of carbonic acid H2CO3 that forms

Water flow

Single cell root hairs

Water and cations can be taken up by roots:

1. apoplastically, i.e. through the cell walls and intercellular spaces,

2. symplastically, i.e. from protoplast to protoplast via plasmodesmata

However, at the endodermis the apoplastic pathway is blocked by a waxy deposit of the wall called the Casparian strip.

In some plants is the Casparian strip located in the exodermis so that the apoplastic barrier works sooner.

Apoplastic and Symplastic TransportRecall transport of sucrose from photosynthesizing cells to phloem

Casparian strip

Cross section of endodermis with the Casparian strip stained pink. The Casparian strip contains suberin and lignin

Cross section of Smilax root showing heavily thickened endodermis walls

Cross section of Zea mays root using fluorescence microscopy showing thickened cell walls on the inside of endodermis

See Equivalent Fig. 32.2B

Uptake of water and nutrients by roots

The ions that have passed through the endodermis are contained within the vascular tissue.

Water can then be drawn into the root from the soil by osmosis, the endosmotic root pressure. This can be sufficient to force water up through the xylem and may be particularly important when there is not a strong water potential gradient due to transpiration

Some plants have hydathodes at their leaf margins that secrete water as droplets, a process called guttation.

Water uptake by the root

Film clip

4.Problems in plant nutritionPlant Nutrient Type Visual symptomsNitrogen Deficiency Light green to yellow appearance of leaves, especially older leaves; stunted growth; poor fruit development. Excess Dark green foliage which may be susceptible to lodging, drought, disease and insect invasion. Fruit and seed crops may fail to yield.

Phosphorus Deficiency Leaves may develop purple coloration; stunted plant growth and delay in plant development. Excess Excess phosphorus may cause micronutrient deficiencies, especially iron or zinc.

Potassium Deficiency Older leaves turn yellow initially around margins and die; irregular fruit development. Excess Excess potassium may cause deficiencies in magnesium and possibly calcium.

W.F. Bennett (editor), 1993. Nutrient Deficiencies & Toxicities in Crop Plants, APS Press, St. Paul, Minnesota.

Excess frequently operates through imbalance

5. Nitrogen and the effects of soil organic matter on plant nutrition

Nitrogen is the element most required by plants, in terms of weight.

It is not a product of weathering of soil particles.

There are two sources: fixation of atmospheric nitrogen by bacteria decomposition of organic matter, usually decaying plant material.

N-fixing bacteria

Fig. 32.13

Most uptake from the soil is in the form of nitrate

Spodic soil

Organic material is important in agricultural soils both as a source of nitrogen and because it can increase water holding capacity, e.g. biosolids application effects

A characteristic of non-agricultural soils is accumulation of organic material and acidification of the soil. Such soils typically develop a very distinct stratification, with organic mater at the top.

The organic layers in such soils can have a considerable total quantity of nitrogen but little may be available due to the high acidity, and sometimes lack of oxygen, in the organic layer.

6. Leaf senescence and withdrawal of nutrients to the plant

Senescence is a term for the collective process leading to the death of a plant or plant part, like a leaf. Leaf senescence is a part of the process by which a plant goes into dormancy and is induced by a change in day length.

As daylength decreases, the plants ability to synthesize chlorophyll becomes reduced.

Yellow and orange carotinoids and xanthophylls, always present within the leaf, begin to show.

Water and nutrients are drawn into the stems and from the leaves.

Senescing cells also produce other chemicals, particularly anthocyanins, responsible for red and purple colors.

Some species, particularly oaks, contain high quantities of tannins in the leaves

which are responsible for brown colors.

Changing leaf color

Nutrient retention during senescence

In deciduous tree species some 60 – 70% of N, 60 – 70% of P, 30% of K, 25% of Mg, and 15% of Ca are withdrawn from leaves prior to them being shed. Storage is in the bark and elements are re-mobilized in spring

A decline in photosynthesis with aging can be prevented by the decapitation of the plant above the leaf in question. This implies a regulatory action by the growing point. Results from primary leaves of bean (Das, 1968).

Leaf Abscission

The final stage in leaf senescence is abscission ("cutting off")

Abscission is controlled by a special layer of cells at the base of the petiole, the abscission layer.

This layer releases ethylene gas that stimulates production of cellulase. This in turn breaks down cells walls so that eventually the leaf is held on to the plant only by xylem fibers. Wind eventually weakens these and leaf falls

Another special layer of cells adjacent to the abscission layer produces cells impregnated with suberin. These form a protective layer, which is seen as the leaf scar

Tyloses, as well as gums are formed inside the vessels and plug them up before abscission occurs

Leaf

Axilliarybud

Abscission layer

Developing leaf scar

Stem

Vascular tissue

Sections you need to have read

32.6 32.7 32.8 32.9 32.13

Courses that deal with this topic

Botany 371/372 Plant physiology laboratory