Plant Physiology - Musienko M. M. 2001

Root Nutrition of Plants
Physiological and Biochemical Role of Macro- and Microelements

Phosphorus

Phosphorus is the second most important mineral nutrient for plants. Alongside nitrogen and potassium, it frequently acts as a limiting factor in ecosystem functioning. Phosphorus enters the plant ROOT system in the form of oxidized compounds, primarily as orthophosphoric acid residues (Н₂РО₄^-, НРО₄^2-, РО₄^3-). The biochemistry of phosphorus is mainly restricted to orthophosphate ion derivatives. Phosphates play a pivotal role in METABOLISM; they serve as Structural components of phytin, phosphatides, Nucleic Acids, sugar phosphates, and phosphoric esters. Phosphorus is characterized by its ability to form A wide variety of bonds with both low and high energy potentials, as well as by an inherent bond instability that facilitates metabolic turnover. Hence its exceptional role in the biochemistry and Bioenergetics of physiological processes.

A unique property of phosphorus is its ability to phosphorylate cellular Proteins via protein Kinases.

Protein phosphorylation and dephosphorylation affect nucleic acid and Lipid Metabolism, Cell Differentiation and proliferation, and the synthesis of numerous Organic compounds. These processes play a crucial role in shaping Intracellular Signaling across All living organisms:

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In this diagram, the signal can be any stimulus (phytohormone, growth factor, light), while the receptor is a protein molecule that specifically perceives the signal and activates the effector system. The effector system may consist of Enzymes or Ion Channels, whereas The secondary messenger is typically cyclic AMP, Inositol trisphosphate, or others (Yavorska, 1989) (Fig. 135). Protein kinase is an intracellular enzyme activated by the binding of a secondary messenger. A number of protein kinases are activated by reducing Cyclic NUCLEOTIDES, hydrolyzing ATP to ADP, and covalently modifying Threonine or Serine within the protein substrates of protein kinases.

Proteins and Phosphoproteins serve as protein kinase substrates, which may include enzymes, regulatory proteins, and transporters of ions and various metabolites. As a result of phosphorylation and dephosphorylation processes, their catalytic activity, regulatory and transport properties, and the Intracellular Localization of various components are altered. Protein phosphatase is the enzyme responsible for protein dephosphorylation. The greatest contributions to The Development of this theory were made by E. Sutherland, who discovered cyclic AMP, adenylate cyclase, and cyclic AMP phosphodiesterase (Nobel Prize in 1972), and by E. Krebs and E. Fischer, who discovered protein kinases and protein Phosphatases (Nobel Prize in 1992). This framework paves the way for understanding the mechanisms of coordination in time, space, and reaction rate, as well as the Organism's ability to respond to all endogenous and exogenous influences.

Having entered The Root System as oxidized compounds, phosphorus retains its oxidation state throughout all subsequent transformations. All changes reduce to phosphorylation and transphosphorylation.

The biogeochemical cycle of phosphorus is relatively simple: phosphorus from organic residues and humus is mineralized by soil microorganisms, and a significant portion of it ultimately ends up in sedimentary rocks:

Mineral phosphates accumulate in The Cell sap, forming Buffer solutions that regulate its acidity. Up to 50% of phosphoric acid remains in the plant in ionic form. Phosphorus plays an exceptionally vital role in lipid, carbohydrate, and Protein metabolism, as well as in Respiration and Photosynthesis.

Two distinct periods stand out in the plant's utilization of phosphorus. The first is the onset of seed germination. The close relationship between phosphorus compounds and growth processes is evidenced by the consistent concentration of phosphorus in the growing Tissues of root and SHOOT tips. Maximum phosphate uptake is observed at pH 4–6.

Characteristically, just 5–7 minutes after entering the root system, the phosphoric acid ion is found incorporated into a wide range of organic compounds.

The second period encompasses seed ripening. The primary storage form of phosphorus in plants is phytin (C₁₂H₁₁O₄₄P₁₀Ca₇Mg).

Phytin is a complex ester of the hexahydric alcohol inositol, representing a mixture of calcium and magnesium salts of inositol-phosphoric acid. Flax, soybean, and sunflower seeds may contain 1–3% phytin. In vegetative storage Organs—such as leaves, stems, and roots—phytin is either entirely absent or present in negligible amounts. The rapid breakdown of phytin at the beginning of seed germination is carried out by the enzyme phytase. It must be emphasized that even during seed germination, phosphorus is of paramount importance for material and energy transformation processes, the interplay of which dictates the direction and intensity of GROWTH AND DEVELOPMENT, as well as overall productivity.

Phosphorus is capable of repeated reutilization by the plant. The radioisotope method makes it possible to determine not only The rate of phosphorus reuse but also the participation of various organs in the element's internal cycling. It has been proven that even the most distant organs containing meristematic tissues serve as sites of phosphorus accumulation. Outflow most commonly occurs from Aging leaf blades.

The possibility of utilizing soil organic phosphorus compounds through the action of plant extracellular phosphatases has also been established. One of the reserves for increasing phosphorus availability and mobility in the soil is mycorrhiza.

Different plant species depend on mycorrhiza to varying degrees. The hyphae of mycorrhizal Fungi extend several centimeters away from each colonized root, thereby increasing the volume of effectively utilized soil. In addition to expanding the root absorptive surface, The Significance of mycorrhizal fungi lies in their ability to extract phosphorus from diluted soil solutions as well as from sources otherwise unavailable to the plant.

Characteristically, phosphorus supply in citrus plants depends so heavily on mycorrhiza that without it they can only grow in soils with a fairly high phosphorus content. For many species, the percentage increase in growth in the presence of mycorrhiza has been documented (wheat — 220%, corn — 122%, onion — 3155%).

An excess of phosphorus in the soil inhibits mycorrhizal development.

There is a close interdependence between nitrogen and phosphorus Nutrition in plants. Phosphorus deficiency drastically reduces nitrogen uptake. The external symptoms of phosphorus deficiency include a bluish-green leaf coloration. Leaves become smaller, and growth processes are stunted. A shortage of phosphorus affects virtually all vital physiological processes in plants.

Sulfur

Among the elements involved in biochemical cycling, sulfur occupies an important place. Annually, 170 million tons of sulfur are extracted from the lithosphere, all of which is drawn into the global cycle:

In recent decades, human industrial activity has reached such a scale that its impact on the biogeochemical cycling of nutrients has become comparable to natural geological processes. For instance, the natural sulfur flux via river runoff is about 100 million tons, and an equivalent amount of anthropogenic sulfur is introduced through wastewater and acid rain. The latter have severe environmental consequences.

Sulfur enters the soil through several pathways, the primary ones being the decomposition of plant residues, precipitation, and the weathering of rocks. Most plants acquire sulfur both from sulfates dissolved in soil Water and directly from the atmosphere. Green plants absorb sulfur as the sulfate ion (SO₄²) and reduce it to the level of organic SH-groups. Mineralization is carried out by saprophytic microorganisms that convert it into H₂S. Hydrogen sulfide oxidation is driven by colorless aerobic sulfur Bacteria and anaerobic Purple and green sulfur bacteria:

Sulfate activation and its reduction in plants occur in three stages.

In The First stage, sulfate is activated by ATP to form adenosine-5′-phosphosulfate (APS) and 3-phosphoadenosine-5′-phosphosulfate (PAPS):

The enzyme ATP sulfurylase catalyzes the reactions:

This enzyme is found in various plant organs (roots, leaves), with its activity being lower in the root system than in the leaves.

More than 40% of ATP sulfurylase is localized in the chloroplast fraction, with an optimum pH of 7.5–9.0. It is activated by magnesium and cobalt ions and inhibited by nitrates, molybdate, and selenate. APS can be phosphorylated by APS kinase to form 3-phosphoadenosine-5′-phosphosulfate (PAPS). Both phosphosulfate derivatives are then enzymatically reduced to sulfite.

In the second stage, the sulfate group of APS and PAPS is transferred to Other Compounds with the participation of sulfotransferase:

APS sulfotransferase is widespread in photosynthetic organs and is inhibited by phosphate.

In the Third Stage, sulfite is either directly reduced to sulfide (in a reduction reaction requiring 6e^-; NADPH, and the presence of the enzyme sulfite reductase), or it reacts with serine or o-acetylserine to form Cysteine, or finally, it is reduced to thiosulfate with subsequent conversion into cysteine via the intermediate stage of S-sulfocysteine:

Sulfite reductase is localized in the leaves. The Biosynthesis of cysteine from sulfite and serine is also a two-step process. The first reaction is catalyzed by serine transacetylase, leading to The formation of o-acetyl-L-serine, while in the second reaction, o-acetylserine reacts with sulfide to form cysteine through the action of the enzyme o-acetylserine sulfhydrylase.

The overall process of the complete reductive sulfur cycle is as follows:

Light stimulates sulfate reduction reactions; therefore, sulfur reduction in leaves is closely linked to photosynthesis (which supplies ATP, ferredoxins, NADPH, and ensures electron flow) and is localized in the Chloroplasts (Fig. 136).

Fig. 136. Sulfate metabolism in the leaf

A number of secondary sulfur compounds are formed from cysteine through the substitution and oxidation of sulfhydryl groups. In the formation of this type of compound, cysteine is first alkylated to yield alliins. The latter are cleaved into alkanesulfenic and α-aminoacrylic acids by the action of the enzyme alliinase. Among secondary metabolites, sulfur is present in various plant oils, such as mustard and garlic oils.

It should be noted that one of the essential FEATURES OF PLANT metabolism is the close interrelation between nitrogen and sulfur nutrition, as evidenced by the similarity of sulfate and nitrate assimilation pathways in plants:

To transport sulfur throughout plant organs, its organic compounds are converted into sulfate, which is then transformed back into organic compounds in the roots and seeds.

Part of the absorbed sulfur is utilized by the root, while the rest is transported via the Transpiration stream to young, growing organs, where it is actively integrated into metabolism and loses its mobility. Sulfates and reduced forms of sulfur can move through the phloem from the leaves. Although plant Cells require only small amounts of sulfur, almost all of it is consumed in fulfilling structural Functions.

The synthesis of many proteins cannot take place without Sulfur-Containing Amino Acids (Methionine, cystine, cysteine). Methionine is an essential amino acid with unique properties: it is a component of the active sites of numerous enzymes, and methionyl-tRNA acts as the initiator of polypeptide chain growth.

Methionine residues impart hydrophobic properties to the protein molecule, which plays a crucial role in stabilizing the active conformation of enzymes in a saline environment.

An important function of sulfur in proteins and Polypeptides is the involvement of SH-groups in forming covalent, hydrogen, and mercaptide bonds that maintain the Tertiary Protein Structure. Disulfide S-S- bridges provide linkages between individual polypeptide chains, thereby stabilizing their structure.

Sulfur is also a constituent of Glutathione, which plays a vital role in redox processes due to its ability to interconvert between sulfhydryl (SH-) and disulfide (-S-S-) forms. Furthermore, sulfur is part of essential biological compounds such as coenzyme A and Vitamins (Lipoic Acid, biotin, thiamine). It is precisely because of all these functions that sulfur may rank third in plant nutrition, right after nitrogen and phosphorus. Many plant species contain volatile sulfur compounds in small amounts, such as R-S-R sulfoxides, which are components of phytoncides found in onions or garlic.

Sulfur deficiency manifests as pale, yellowish leaves, particularly the younger ones, with symptoms quite similar to nitrogen deficiency. At high sulfate concentrations in the nutrient solution, so-called sulfur stress can be observed, accompanied by the release of hydrogen sulfide into the air. A large portion of sulfur is removed from the biological cycle with the harvest and is not returned to the soil. Cabbage is characterized by a high rate of sulfur removal, whereas potatoes, cereals, and grain crops remove lesser amounts.

Recently, gypsum and elemental sulfur have been used as sulfur fertilizers, the latter becoming available to plants after being converted into sulfates by microorganisms.

Evidence indicates that sulfur deficiency inhibits the reduction and assimilation of nitrogen by plants. This is of significant importance for cultivated crops, which synthesize plant protein by utilizing nitrates, sulfates, and phosphates—highlighting this unique feature in the metabolism of such crucial inorganic anions as nitrogen, phosphorus, and sulfur.