1. Mineral Nutrition and Absorption & Assimilation
HORT 301 – Plant Physiology
September 12, 2008
Taiz and Zeiger, Chapters 5, 6 (p ) & 12, Web Topics 5.1 and 5.2
courtesy of Burkhard Schulz
Plant mineral nutrition – essential elements required by plants
Mineral absorption and assimilation – uptake by roots and incorporation into bio-molecules

2. Plant mineral nutrition - elemental nutrients (inorganic, simplest chemical form) are required for metabolism, and growth and development of plants
Essential elements – products of soil organic matter recycling and weathering that are absorbed by roots from soil solution, uptake with water
These nutrients together with CO2 and H2O, and sunlight (light energy) allow plants to synthesize all other necessary molecules, essentially autotrophic (self-feeding)
Essential mineral nutrients often limit plant growth and development reducing maximal biomass and crop production
Agricultural practice is to optimize plant nutrient status by soil amendment with fertilizers

3. A nutrient is essential if:
It is a required component of structure (silicon in the cell wall) or plant
metabolism OR
It is necessary for plant growth, development or reproduction, i.e. species survival

4. Identification of essential elements was facilitated by the use of solution culture systems or hydroponics
Some micronutrients are required in trace amounts, essentially was difficult to establish because soils often contain sufficient amounts of these trace elements
Solution culture requires a synthetic “medium” containing essential nutrients, e.g. Hoagland’s solution

5. Macronutrient – up to 1.5%, 15,000 ppm in dry matter
Essential elements are categorized as macronutrients or micronutrients based on relative concentration in plant tissue (dry weight)
Macronutrient – up to 1.5%, 15,000 ppm in dry matter %

6. Micronutrient - 100 ppm (dry matter) or less

7. Classification of essential mineral nutrients by function
Group 1 (N and S) – components of organic molecules
N – amino acids/proteins, nucleotides/nucleic acids
S – amino acid (cysteine), lipids, intermediary molecules (acetyl-CoA)
Group 2 (P, Si, B) – P- energy storage (ATP) or Si & B - cell wall structure

8. Group 3 (K, Ca, Mg, Cl, Mn, Na) – present as ions in cells, enzyme co-factors, osmotic adjustment, signaling

9. Group 4 (Fe, Zn, Cu, Ni, Mo) – metals involved in redox reactions (electron transfer)

10. Nutrient deficiency symptoms – soils have a finite mineral nutrient load capacity
Plant nutrient deficiency symptoms may be used to determine when and what type of soil nutrient amendment (fertilization) is necessary
Symptoms are complex, occurring from deficiency of different individual nutrients and further complicated by stresses, see Web Topic 5.1 for an in-depth treatise of plant nutrient deficiency symptoms

11. Nutrient deficiency symptoms – N, P, K, Ca
Epstein & Bloom Mineral Nutrition of Plants 2004

12. Mobile nutrients (N, K, Mg, P, Cl, Na, Zn and Mo) - symptoms evident first in older leaves, re-translocated to younger leaves
Less mobile nutrients (Ca, S, Fe, B and Cu) - symptoms develop first in the younger leaves

13. Plant tissue analysis – a precise method to assesses nutrient status of tissues, used to optimize fertilizer application (increased crop production & reduced pollution)

14. Tissue mineral nutrient content zones for plant growth:
Deficiency zone - critical concentration, minimum tissue nutrient content for maximum growth or yield
Toxic concentration zone – content at which yield declines because the nutrient is in excess
Adequate zone - determination of adequate zones minimizes fertilization inputs, varies because of species differences in nutrient use efficiency

15. Mineral nutrients in the soil - plants access virtually all mineral nutrients from the soil solution
Mineral nutrients – derived from inorganic as well as organic components of the soil rhizosphere
Organic decomposition (microbes) “releases” mineral nutrients to the soil solution (mineralization)

16. Mineral nutrient forms
Macronutrients,
N – NH4+ or NO3-
P – H2PO4-
S – SO42-
Micronutrients

17. Cation exchange capacity (CEC) of soil particles facilitates nutrient availability to plants - soil particles, both inorganic (gravel, >2 mm to clay < 2 µm), and organic matter, have a negative charge, CEC
CEC – cations form electrostatic interactions with soil particles, exchange occurs during equilibrium, net exchange is concentration & charge strength dependent
CEC facilitates availability of cations (positively charged elements or molecules) for absorption by plant roots

18. Negatively charged ions (anions), e. g
Negatively charged ions (anions), e.g., NO3-, H2PO4-, Cl- - remain in the soil solution between particle spaces, adhesion of water
Limited anion exchange capacity of soils - anions form bridges with multivalent cations like Fe2+or Al3+, and H2PO2-
OR, anions are present in relatively insoluble compounds e.g., SO42- in gypsum (CaSO4), which are gradually released
However, anions are repelled by surface particle charge and tend to be leached through the soil to the ground water

19. Decomposition of organic material lowers the pH
pH and mineralization – affect mineral nutrient availability in soil solution, pH 5.5 to 6.5 is optimal
Decomposition of organic material lowers the pH
Soil amendments alter pH - lime (CaO, CaCO3, Ca(OH)2, attract protons) increases pH (alkaline)
Sulfur - reduces pH (mineralization results in release of sulfate and hydrogen ions) of the soil solution
Shaded area is the relative nutrient availability to plants

20. Nutrients move in the soil solution by pressure-driven bulk flow and diffusion, directly linked to water movement

21. Root structure and mineral nutrient absorption – roots acquire water and mineral nutrients
Plants vary in root development based on adaptation to local soil conditions,
water and nutrients

22. Plants respond to water and nutrient deprivation by remodeling their root architecture to maximize root surface area (secondary roots and root hairs) and “seek” water and nutrients
Hydrotropism – roots have the capacity to sense water, higher w

23. Effect of localized supply of PO4,2- , NO3- , NH4+ , or K+ on barley root growth
+ - portion of root system receiving complete nutrient solution
- - Part of the root system receiving the solution deficient
in specified nutrient
Drew (1975) New Phytol. 75 :

24. Mineral Nutrition and Absorption & Assimilation
HORT 301 – Plant Physiology
September 12, 2008
Taiz and Zeiger, Chapters 5, 6 (p ) & 12, Web Topics 5.1 and 5.2
courtesy of Burkhard Schulz
Plant mineral nutrition – essential elements required by plants
Mineral absorption and assimilation – uptake by roots and incorporation into bio-molecules

25. Mineral Absorption and Assimilation
Main regions of a primary root are the meristematic zone, elongation and maturation zones
Meristematic – root cap protects the root, gravitropic (gravity response), quiescent zone of meristem initials, progenitors of other cells
Elongation zone (0.7 to 1.5 mm from apex) – reduced cell division, rapid cellular elongation and development of cell types, including endodermis with Casparian strip, xylem and phloem

26. Maturation zone – root hair zone that increases the surface area for absorption of water and mineral nutrients

27. Mycorrhizal fungi facilitate water and mineral nutrient uptake into roots – extend the root absorption surface area
Mycorrhiza fungi – symbiotic (sugar for mineral nutrients) association between a fungus and plant roots, 83% of dicot species, 79% of monocots and all gymnosperms
Ectomorphic mycorrhizal fungi – hyphae extend into the cortex (apoplast) of plants and into the soil, up to 100% increase in surface area for nutrient absorption, reduces the nutrient depletion zone at the root surface

28. Vesicular arbuscular mycorrhizal fungi – hyphae are less dense and penetrate into cortical cell symplast where they branch (arbuscule) and transfer nutrients to the plant root, hypae extend from the root facilitating nutrient acquisition beyond the root surface
It is not known precisely how nutrients move from the hyphae to the plant cells, i.e. diffusion or release at hyphal death

29. Mineral nutrient (ion) uptake into roots, xylem loading and movement to shoots – absorption by roots, radial movement to the xylem, uptake to shoots in the transpiration stream (movement of water)
Movement of ions through the soil is due primarily to pressure- driven bulk flow, with water
Ion uptake from soil into roots occurs predominantly in the maturation/root hair zone (extension of the epidermis) of primary and secondary roots

30. Radial transport and xylem loading – through the apoplast or symplast of the root hair, epidermis and cortex with water
At the endodermis, ions must enter the symplast of endodermal cells because the suberized Casparian strip restricts apoplast movement
Uptake in the root cell symplast is by diffusion based on the electrochemical potential

31. Xylem loading – movement from the endodermis to the tracheary elements (tracheids or vessel elements
Xylem parenchyma cells - directly connected to the endodermis and tracheary elements (tracheids or vessel elements) and regulate ion movement into the xylem
Transport proteins regulate ion transport into and out of the xylem
Ion movement from root to shoot is primarily in the transpiration stream, pressure-driven bulk flow

32. Mineral nutrient assimilation – incorporation of mineral nutrients into organic molecules
Assimilation - requires substantial energy, e.g. 25% of the plant energy budget is consumed for N assimilation
Assimilated mineral nutrients - N either NH4+ or NO3-, SO42-, and H2PO42-

33. Nitrogen – biogeochemical cycling of nitrogen
N2 (N≡N) - 78% of the atmospheric volume
N2 – fixed biologically or by the Haber-Bosch process into NH4+, oxidized to NO3-

34. N2 (N≡N) fixation symbiosis - primarily legumes by bacterial symbionts (Rhizobia) into ammonium (NH3) (nitrogenase), which at physiological pH is converted to NH4+
Otherwise, nitrogen absorbed into roots as NO3- (NO3-H+ symporter) or NH4+ (uniporter)
NO3- is reduced to NH4+ (nitrate and nitrite reductases w/ferrodoxin as the electron donor)
NH4+ is assimilated into glutamine and then glutamate (glutamine synthase, glutamate synthase), sometimes ureides (legumes), 12 ATP/N assimilated is required

35. Sulfur – SO42- is a product of soil weathering
SO42- is absorbed by roots (SO H+ symporter) and translocated
SO42- is assimilated into 5’-adenylsulfate/adenosine-5’-phosphosulfate (SO42- + ATP → APS + PPi), reaction catalyzed by ATP sulfurylase
APS is then reduced to produce SO32-(APS reductase), SO32- is reduced to sulfide (S2-, sulfite reductase, ferrodixin), which condenses with O-acetylserine (OAS) (S2- + OAS → cysteine) to form cysteine (then methionine)
Assimilation occurs primarily in leaves, photosynthesis produces reduced ferrodoxin and photorespiration generates serine, 14 ATP consumed per S assimilated

36. Phosphorous – HPO42-, uptake (PO42- -H+ symporter) and translocated form
Assimilated into ATP (ATP synthase), photosynthesis, oxidative phosphorylation (respiration)
Cation mineral nutrients (K, Ca, Mg, Fe, Mn Cu, Co, Na, Zn) – function as ions or exist in complexes with organic molecules via noncovalent bonds, metals facilitate redox reactions
Coordination bonds (several oxygen or nitrogen atoms share electrons) to form a bond with a cation nutrient, chlorophyll a

37. Electrostatic interactions – charge group attraction, e. g
Electrostatic interactions – charge group attraction, e.g., Ca2+ for carboxylate groups in pectin (Ca2+-pectate)