Mineral Nutrition of Plants

the assimilation by plants from the environment of the ions of mineral salts needed for normal development. Mineral nutrients include the elements nitrogen, phosphorus, sulfur, potassium, calcium, and magnesium, as well as such trace elements as iron, boron, copper, zinc, and magnesium. Mineral nutrition of plants involves the absorption of minerals in the form of ions, their transport within plants, and their inclusion in metabolism. Unicellular organisms and aquatic plants absorb ions over their entire surface, whereas higher terrestrial plants absorb the ions on the surface cells of the roots, predominantly on root hairs. After absorption by the cell membranes, the ions penetrate the cytoplasm through its surrounding lipoprotein membrane (plasmalemma). Cations (except K⁺) penetrate the membrane passively, by diffusion; anions and K⁺ (at low concentrations) do so actively, by means of molecular “ion pumps,” which transport ions with the expenditure of energy. The rate of active ion transport depends on the quantity of carbohydrates available to the cell and on the respiration rate, whereas the rate of passive absorption depends on the permeability of biological membranes and the difference in concentrations and electric potentials between the solution and the cell. Membrane permeability is not the same for the different ions: for the cation K⁺ it is 100 times higher than for Na⁺ and 500 times higher than for anions. Absorbed ions travel from cell to cell by way of the connecting cytoplasmic bridges (plasmodesmata). The roots and stems of higher plants have a special vascular system for transporting minerals and their organic com-pounds (which are also partly synthesized in the roots) to the leaves. As the lower leaves age, some mineral substances flow from them into the growing organs, where they can be used again.

Every element of mineral nutrition plays a specific role in metabolism and cannot be completely replaced by another element. Nitrogen forms part of the proteins, the main substances of the cytoplasm, and of the amides, nucleic acids, hormones, alkaloids, vitamins (B₁, B₂, B₆, and PP) and chlorophyll. Nitrgen is absorbed in the form of the anion NO₃^- (nitrate) and cation NH₄⁺ (ammonium), which are created by the decomposition of humus by soil microorganisms. Molecular nitrogen (N₂), the main constituent of air (79 percent), can be assimilated only by certain species of lower plants. Nitrates are reduced to ammonium by means of the enzyme nitroreductase. Ammonium combines with organic acids to form amino acids, which are then incorporated into proteins.

Phosphorus forms part of the nucleoproteids of the cell nucleus, phospholipids of cell membranes, phosphatides, and phosphoric esters of sugars. The participation of phosphorus in photophosphorylation is particularly important. In this process solar energy accumulated in the form of energy-rich adenosine triphosphate (ATP) bonds is used to take up CO₂ from the air and to form organic matter. Energy released during respiration by the oxidation of organic matter formed in the course of photosynthesis is also stored in the form of macroergic ATP bonds. Phosphorus is absorbed as an anion of orthophosphoric acid (PO₄^3-, or phosphate) and inalterably incorporated into organic compounds in hundredths of a second. In addition, plants always contain a large quantity of inorganic phosphate, whose physiological function is unknown.

Sulfur, like nitrogen, is a constituent of all proteins, as well as peptides (glutathione) and some amino acids (cystine, cysteine, and methionine) and essential oils. Sulfur is taken up by plants in the form of the anion (SO₄^2-, or sulfate), which is reduced in the cells to form disulfide (—S—S—) and sulfhydryl (—SH) groups (the latter form bonds that strengthen the configuration of the protein macromolecule).

Potassium is absorbed in the form of the K⁺ cation and remains in the same form in the cell without forming stable organic compounds. It participates only in weak adsorption interactions with proteins and in exchange reactions with inorganic acids. Unlike nitrogen, phosphorus, and sulfur, which participate directly in the creation of organic material in the plant cell, potassium is not a nutrient in the full sense of the word. It increases the water-retention capacity of the cytoplasm and the rate of photosynthesis and outflow of assimilated material and participates in the functioning of the stomata.

Calcium and magnesium are absorbed in the form of bivalent cations, Ca²⁺ and Mg²⁺. The main function of calcium is stabilization of the cell structures. The Ca²⁺ ions (”calcium bridges”) connect the lipid molecules to one another and ensure their orderly arrangement in cell membranes. Compounds of calcium with pectins cement the membranes of adjacent cells. Unlike other elements of mineral nutrition, calcium is almost immobile in plants. It is virtually not reused, and it accumulates in aging organs. It is essential to maintain the structure of the ribosomes, where protein is synthesized.

Manganese is a constituent of chlorophyll, and it activates the enzymes that transport phosphate from ATP to the sugar molecule.

Iron is part of several enzymes, including the respiratory enzymes (cytochromes). It is involved in the synthesis of chlorophyll, although it does not become part of it. Plants can also receive mineral nutrients through the leaves.

Together with aerial nutrition (photosynthesis), the mineral nutrition of plants constitutes a single process of exchange between the plants and the environment. It affects all physiological functions (respiration, growth, development, photosynthesis, water conditions, and so on) and, in turn, is dependent on them. Therefore, one of the most effective means of controlling crop productivity is regulation of mineral nutrition by means of fertilizers.