PLANT MINERAL NUTRITION

I. Criteria of essentiality (DI Arnon & PR Stout, 1939)

A. The element must be essential for normal growth or reproduction, which can not proceed without it.

B. The element cannot be replaced by another element.

C. The requirement must be direct, that is not the result of some indirect effect such as relieving toxicity caused by some other substance.

II. Essential elements for normal plant growth:

C HOPKNS CaFe Mg Na Cl .

(Mighty good)

(Not always) (Clean)

CuMn CoZn MoB(y)!

C = Carbon = Major structural component of organic molecules

H = Hydrogen = Major structural component of organic molecules

O = Oxygen = Major structural component of organic molecules, final electron acceptor in Oxidative Phosphorylation

P = Phosphorus = Important structural component of nucleic acids, phospholipids, coenzymes

K = Potassium = Important cofactor of some enzymes, stomatal opening, membrane potentials, osmotic balance

N = Nitrogen = Important structural component of nucleic acids, proteins, chlorophyll, some phytohormones

S = Sulfer = Important structural component of some amino acids, forms disulfide bridges that are important to enzyme activity

Ca = Calcium = Important structural component of middle lamella, cofactor for some enzymes, involved in membrane transport phenomenon, forms crystals of toxic waste products

Fe = Iron = Site of catalytic reaction in may redox enzymes, essential for formation of chlorophyll

Mg = Magnesium = Involved in stabilization of ribosome particles, cofactor for many enxymes, structural component of chlorophyll

Na = Sodium = Benificial to Halophytes (Mangrove, Atriplex, etc)

Cl = Chlorine = Involved in photolysis of water in photosynthesis

Cu = Copper = site of catalytic reaction for some enzymes

Mn = Manganese = Resiratory enzyme cofactor, involved in photolysis of water, required for auxin synthesis

Co = Cobalt = Structural component of vitamin B12, necessary for nitrogen fixation

Zn = Zinc = Involved in auxin synthesis, enzyme cofactor

Mo = Molybdenum = Involved in reduction of nitrates

B = Boron = Involved in translocation and absorption of sugar, interacts with Ca flux

III. Commercial Fertilizers sold as

%N, %P, %K

eg. 20-10-5 = 20%N, 10%P, 5%K

Good Soils = 25% Air, 25% Water,

2% Organics, 48% Mineral

Mineral particles (-) bind elements with + charges (Ca++) but not elements with

- charges (PO - 4)

Recent estimates indicate 70% of all Nitrogen within Nitrogen cycle on Earth is currently contributed by human activity!

I. Soils

A. Important reservoirs for water, organic matter, and elements.

II. Pathway of soil solution (Water + Elements)

A. Flow of solution is from Soil -> Roots -> Stems -> Leaves -> Atmosphere

B. Initial absorption in roots

1. Most readily available in spaces between soil particles

2. Roots and associated Root Hairs grow into these spaces

3. Passive uptake into cell wall Active uptake into cytoplasm of root hairs & epidermis

4. Passive movement in cell walls of cortex Passive/active movement in cytoplasm of cortex

5. Casparean Strip of Endodermis prevents passive movement into central xylem, confers active selective uptake into cytoplasm by Endodermis

6. Casparean Strip of Endodermis prevents passive backflow of water from central Xylem to Cortex, Diffusion Gradient favors net flow of water into central Xylem =

Root Pressure (up to 30-45 psi)

C. Transpiration occurs in xylem

1. Primary xylem in roots -> Secondary xylem in roots -> Secondary xylem in stems -> Primary xylem in stems ->

Midvein in leaves ->

Lateral veins in leaves ->

Minor veins in leaves -> Intercellular space in Mesophyll of leaves ->

Stomatal Pores associated with Guard Cells

2. Cohesion-Tension Theory

(H. Dixon, early 1900's)

a. Concentration of water vapor inside leaves greater than in atmosphere so concentration gradient favors flow of water vapor to atmosphere = Transpiration = evaporation of water from leaves and stems

b. Transpiration puts water in xylem in a state of tension

c. Continuous tension on water column pulls it upward from roots to leaves

d. Cohesive nature of polar water molecules strong enough to resist rupturing of liquid water column in xylem but not strong enough to resist phase change and diffusion into mesophyll air spaces

3. Dynamics of water flow

a. Cuticle and wax of stem/leaf epidermis prevents excessive water loss in primary plant body

b. Suberin in cork cells prevents excessive water loss in secondary plant body

c. Flow of water in xylem follows Poiseiulles Law = Flow is proportional to fourth power of xylem element radius

d. Probability of rupture of water column is proportional to xylem element radius

e. C3 and C4 Plants can regulate transpiration by changes in size of stomatal pores which is related to turgor pressure of guard cells and photosynthetic activity

f. CAM Plants open stomatal pores at night and close them during the day to minimize loss of water

g. Current hypothesis is that stomata aperture Regulated by Potassium flux in and out of Guard Cells

h. We haven't a clue in regard to water dynamics in Secondary Plant Bodies - What, if anything, regulates water loss from TreeTrunks??

TRANSPORT OF SUGARS AND WATER = PHLOEM TRANSLOCATION

I. Primary sugar translocated is Sucrose = Glucose + Fructose

A. From leaves that are actively photosynthesizing

B. From storage tissue (primarily parenchyma and collenchyma)

C. From organs about to abscise

II. Translocation occurs in Phloem

A. Bidirectional - From Sink to Source Tissues

B. Source Storage/Mesophyll Tissue -> Sieve Tubes (Follows Pathways in Primary and Secondary Phloem -> Sink Storage/Growing Tissue

C. Sieve Tubes consisting of Sieve Tube Elements conduct sugar + water

D. Companion Cells (Specialized Parenchyma) provide energy for active loading & unloading

E. Pressure-Flow Hypothesis

(E. Munch, 1927)

1. Sucrose actively loaded into sieve tube elements at Source Tissue

2. Water moves into STE by osmosis in response to increase in sucrose concentration

3. Hydrostatic pressure increases in STE as water moves in

4. Bulk flow of water and sucrose driven by hydrostatic pressure within ST toward Sink Tissue

5. Sucrose actively unloaded from STE into Sink Tissue

6. Water moves out of STE by osmosis into Sink Tissue

7. Translocation continues as long as pressure is maintained by active loading and unloading of sucrose and osmosis of water

If you are interested in learning more about Plant Nutrition and Transport consider enrolling in Plant Physiology (BOT 351) offered Every Spring Semester