- Entry.
Zinc is a heavy metal; its atomic mass is 65.38. It exhibits relatively good thermal and electrical conductivity. It forms compounds only in the +2 oxidation state. Zinc dissolves even in dilute acids, displacing hydrogen. It never occurs naturally in its native state. Zinc, like copper, forms complex compounds, although they are less stable.
In minimal amounts, zinc is an essential nutrient for all plants (both higher and lower). Zinc deficiency symptoms in higher plants were first described by Mazć (1914) in corn grown in aquaculture. However, researchers became more broadly interested in zinc problems only when it was recognized that certain diseases of fruit trees and other crops are caused by a lack of this nutrient. These included "small leaf" and "rosette" disease, occurring on various fruit trees (apple, pear, plum, cherry, peach, apricot, etc.), and "spotted leaf" disease in citrus trees. In the 1930s, these diseases occurred in large orchards in California, Florida, Hawaii, Australia, and elsewhere, and for many years, no significant cause could be identified. It was only by chance that they were recognized as diseases caused by zinc deficiency, and it was then found that a radical remedy was a small dose of zinc applied to the soil or foliarly by spraying diseased trees with a ZnSO4(Hoagland, 1948).
2. Zinc uptake and content in plants.
Zinc uptake by plants is influenced by a number of factors. This uptake is closely related to the available zinc content in soils, which in turn is largely dependent on pH. Zinc availability in the soil is greatest at pH 6.5, and the presence of organic matter in the soil also improves zinc availability. Zinc uptake can be limited by high phosphate concentrations. Phosphorus has been shown to precipitate insoluble zinc compounds in the soil. Zn availability to plants also depends on the microflora present in the soil.
There are significant species (and even cultivars) differences among plants in their ability to absorb zinc from the soil and in their sensitivity to deficiency or excess of this nutrient. This is due to specific differences in the requirements of individual species (or cultivars) for this nutrient, varying rates of its uptake, and the influence of other ions (phosphates, manganese, etc.) absorbed along with zinc by plant roots in very different proportions. The extent of zinc reutilization within the plant also plays a role. A large portion of the zinc contained in cells is stored in soluble form in the cell sap. However, zinc mobility throughout the plant is not high.
Sensitivity to excess zinc also varies among plants. Researchers are attempting to determine so-called zinc thresholds in plants, but knowledge on this topic is still very incomplete. For most plants, 30-50 ppm Zn is considered adequate. However, plants generally demonstrate a high tolerance to higher zinc concentrations in tissues. It is also known from agricultural practice that widely used zinc fungicides at concentrations of 0.1-0.2% do not damage sprayed plants. Establishing zinc thresholds for plant zinc supply can be very helpful in diagnosing deficiencies (or excesses) of this nutrient in the soil. Researchers emphasize, however, that not only zinc content should be determined in plants, but also the P:Zn ratio and the Mn:Zn ratio. Low manganese levels reduce plant zinc demand, while high manganese levels raise the tolerance threshold for high zinc concentrations.
Plant zinc requirements also depend on the type of crop. Zinc deficiencies can occur, for example, under conditions of intense light.
3. Distribution and forms of zinc in the plant cell.
A characteristic feature of zinc's distribution within the cell is the presence of a significant portion of it in the cell sap, where this trace element is found in a soluble form. Interestingly, this form is also present under conditions of zinc deficiency. Studies of individual cellular fractions have revealed the presence of zinc in all subcellular structures (including the cell wall), but none of them is particularly rich in this trace element. Mitochondria contain slightly more zinc than chloroplasts; in both organelles, it is predominantly bound to a high-molecular-weight substance, likely proteins.
Research findings indicating that a large portion of zinc contained in cells is soluble are inconsistent with information on the mobility of this trace element in plants. Specifically, it has been observed that this mobility is not high, especially in old leaves and roots, where zinc is presumably precipitated into a form that is difficult to move. Researchers, however, agree that zinc is more mobile in young leaves.
4. Symptoms of zinc deficiency and excess in plants.
The first symptom of zinc deficiency in fruit trees is mottled leaf chlorosis on the upper branches. Leaves become stunted (sometimes reaching only 1/20th their normal size), stiff and brittle, and the chlorotic tissues die, turning reddish-brown. Leaves often drop prematurely. Branches are stunted and die from the tip. Lower leaves develop yellow streaks between the veins, followed by white, necrotic spots that eventually turn brown, and the entire leaf dies. Young leaves are often pale yellow or white. Plants become stunted due to stunted internode growth.
The literature also describes symptoms of zinc excess, caused, for example, by soil contamination with large amounts of zinc. Plants under these conditions experience stunted growth, become chlorotic, and leaf tips become necrotic or dry. Ultimately, the plant dies.
5. Physiological and biochemical functions of zinc in plants.
Although zinc has been recognized as an essential trace element for all higher plants, its biochemical functions in plants are not yet well understood. Carbonic anhydrase has been discovered in plants and identified as a zinc protein. This enzyme catalyzes the reversible reaction of hydrating carbon dioxide to carbonic acid. It has been hypothesized that this reaction could be important in respiration and photosynthesis, but experimental confirmation is lacking. It has also been found that, among plant dehydrogenases, the common triosephosphate dehydrogenase is a zinc protein. Other zinc-dependent dehydrogenases have also been detected in plants. Little is known about other "zinc" plant enzymes, but it is suspected that zinc functions in plants in similar enzymatic systems as in animal tissues and microorganisms
One consequence of zinc deficiency in plants is a reduction in protein content, while a significant increase in free amino acids and amides is observed. Therefore, it is speculated that zinc functions in plants in the process of protein synthesis at the level of peptide formation. Furthermore, a direct effect of zinc on ribonucleic acids has been demonstrated; specifically, zinc deficiency in plants significantly reduced RNA content due to increased ribonuclease activity. Recently, it was found that cytoplasmic ribosomes in Euglena gracilis contain significant amounts of zinc; in its absence, these organelles disintegrate. Zinc's influence on RNA metabolism may be sufficient justification for the role of this trace element in protein synthesis. Furthermore, a specific role of zinc in the synthesis of tryptophan has been demonstrated—an amino acid that is important not only as a protein-forming component but also plays a role in the synthesis of growth factors.
Researchers wonder whether the observed effect of zinc deficiency on the content of certain enzymes is the result of zinc's indirect effect, through reduced synthesis of protein substances, and therefore enzymatic proteins. Zinc deficiency in plants induces changes in phosphorus metabolism. Specifically, when zinc was excluded from the nutrient solution, the accumulation of inorganic phosphorus in tissues significantly increased, while the content of nucleotide phosphorus decreased, as did the phosphorus content in lipid and nucleic acid fractions.
The effect of zinc on the reaction catalyzed by triose dehydrogenase indicated its involvement in glycolysis. It was found that zinc deficiency reduced tomato leaf respiration. This finding was associated with a weakening of reactions in the glycolytic system and the Krebs cycle, while an increased reaction in the pentose phosphate pathway was observed. The effect of zinc on plant respiration requires further study.
Zinc deficiency symptoms in plants, such as inhibited growth cone development and the development of abnormally small leaves, have long suggested a specific relationship between zinc and growth factors. The effect of light on the severity of zinc deficiency symptoms in plants also allows for certain analogies to the effect of this climatic factor on auxins. Direct evidence for zinc's role in auxin synthesis was obtained by Skoog (1940). In his experiments, tomatoes growing on zinc-free medium contained minimal amounts of auxins; the addition of zinc salts to the medium first increased the levels of these substances, and then the plants themselves. Skoog's findings were confirmed by Tsui, who observed a decrease in auxin content in plants in water cultures without zinc supplementation, which occurred even before the appearance of external symptoms of zinc deficiency. Tsui also found that the content of tryptophan, which is likely a precursor of auxin (namely, indoleacetic acid), was significantly lower in deficient plants than in control plants. When zinc was added to the zinc-deficient culture, the content of this amino acid increased within 3 days. Tsui concluded that zinc was required for tryptophan synthesis and thus, indirectly, for auxin synthesis
-Anna Nowotna-Mieczyńska "Physiology of mineral nutrition of plants. PWRiL 1965,
-Lityński T., Jurkowska H "Soil fertility and plant nutrition" PWN 1982,
– Franck B. Salibury, Cleon Ross "Plant Physiology" PWRiL 1975,
-Zurzycki J. Michniewicz M. "Plant Physiology" PWRiL 1979,
-Otis F. Daniel G. Curtis Clark "Introduction to Plant Physiology" PWRiL 1958.