Mineral Nutrition and Absorption & Assimilation

Mineral Nutrition“The study of how plants obtain and use mineral nutrients “

Plant mineral nutrition – essential elements required by plants

Mineral absorption and assimilation – Absorbed by roots primarily in the form of inorganic ions from the soil, translocated into various plant parts and incorporation into bio-molecules

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

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

ESSENTIAL NUTRIENTS

An essential element is Without which a plant cannot complete its life cycle Has a clear physiological role

If plants are provided with these essential elements and energy from sunlight, they can synthesize all the compounds they need for normal growth.

Classification of essential mineral nutrients by function

Group 1 (N and S) – components of organic moleculesN – amino acids/proteins, nucleotides/nucleic acidsS – amino acid (cysteine), lipids, intermediary molecules (acetyl-

CoA)

Group 2 (P, Si, B) – P- energy storage (ATP) or Si & B - cell wall structure

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

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

Special Techniques Are Used in Nutritional Studies

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

Mineral Deficiencies Disrupt Plant Metabolism and Function

Inadequate supply of an essential element results in a nutritional disorder manifested by characteristic deficiency symptoms.

In hydroponic culture, withholding of an essential element can be readily correlated with a given set of symptoms for acute deficiencies

Nutrient deficiency symptoms in a plant are the expression of metabolic disorders resulting from the insufficient supply of an essential element. These disorders are related to the roles played by essential elements in normal plant metabolism and function

Diagnosis of deficiency symptoms can be more complex, because:

Deficiencies of several elements may occur simultaneously.

Deficiencies or excessive amounts of one element may induce deficiencies or excessive accumulations of another.

Some plant diseases may produce symptoms similar to those of nutrient deficiencies.

Some essential element participates in many different metabolic reactions and have multiple roles in plant metabolism.

Diagnosis

When relating acute deficiency symptoms to a particular essential element, an important clue is the extent to which an element can be recycled from older to younger leaves.

Some elements, such as nitrogen, phosphorus, and potassium, can readily move from leaf to leaf; others, such as boron, iron, and calcium, are relatively immobile in most plant species

If an essential element is mobile, deficiency symptoms tend to appear first in older leaves.

Deficiency of an immobile essential element will become evident first in younger leaves

Important Consideration

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 Plant Physiolgy by Taize & Ziger for an in-depth study of plant nutrient deficiency symptoms

Nutrient deficiency symptoms – N, P, K, Ca

Epstein & Bloom Mineral Nutrition of Plants 2004

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 facilitates availability of cations (positively charged elements or molecules) for absorption by plant roots

5.5 The principle of cation exchange on the surface of a soil particle

CEC – cations form electrostatic interactions with soil particles, exchange occurs during equilibrium, net exchange is concentration & charge strength dependent

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

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

5.4 Influence of soil pH on the availability of nutrient elements in organic soils

Shaded area is the relative nutrient availability to plants

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

Root structure and mineral nutrient absorption – roots acquire water and mineral nutrients

5.7 Taproot system of two adequately watered dicots: sugar beet (A), alfalfa (B)

Plants vary in root development based on adaptation to local soil conditions, water and nutrients

Hydrotropism – roots have the capacity to sense water, higher w

5.6 Fibrous root systems of wheat (a monocot)

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

+ - portion of root system receiving complete nutrient solution

- - Part of the root system receiving the solution deficient in specified nutrient

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

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

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

5.10 Root infected with ectotrophic mycorrhizal fungi

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

5.11 Association of vesicular–arbuscular mycorrhizal fungi with a section of a plant root

It is not known precisely how nutrients move from the hyphae to the plant cells, i.e. diffusion or release at hyphal death

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

1.1 Schematic representation of the body of a typical dicot (Part 1)

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

4.4 Pathways for water uptake by the root

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 xylem6.20 Tissue organization in roots (Part 2)

Ion movement from root to shoot is primarily in the transpiration stream, pressure-driven bulk flow

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-

Nitrogen – biogeochemical cycling of nitrogen 12.1 Nitrogen cycles through the atmosphere

N2 (N≡N) - 78% of the atmospheric volume

N2 – fixed biologically or by the Haber-Bosch process into NH4+,

oxidized to NO3-

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

Sulfur – SO42- is a product of soil weathering

SO42- is absorbed by roots (SO4

2- - H+ symporter) and translocated

APS is then reduced to produce SO32-(APS reductase), SO3

2- is reduced to sulfide (S2-, sulfite reductase, ferrodixin), which condenses with O-acetylserine (OAS) (S2- + OAS → cysteine) to form cysteine (then methionine) 12.15 Structure and pathways of compounds involved in sulfur assimilation

Assimilation occurs primarily in leaves, photosynthesis produces reduced ferrodoxin and photorespiration generates serine, 14 ATP consumed per S assimilated

SO42- is assimilated into 5’-adenylsulfate/adenosine-5’-phosphosulfate

(SO42- + ATP → APS + PPi), reaction catalyzed by ATP sulfurylase

Phosphorous – HPO42-, uptake (PO4

2- -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

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