Zinc and its role in plant life.

Published on 11 February 2024 at 20:40

Zinc and its role in plant life.




Zinc is a heavy metal; its atomic mass is 65.38. It shows quite good thermal and electrical conductivity. Forms compounds only in the +2 oxidation state. Zinc dissolves even in dilute acids, displacing hydrogen from them. It never occurs in nature in its native state. Zinc, like copper, forms complex compounds, but less durable.


In minimal amounts, zinc is an essential nutrient for all plants (higher and lower). Symptoms of zinc deficiency in higher plants were first described by Mazć (1914) on maize growing in water cultures. However, researchers became more interested in the problem of zinc only when it was discovered that some diseases of fruit trees, as well as other crop plants, are caused by a lack of this component. These were: "small leaf" disease and "rozette", occurring on various fruit trees (apples, pears, plums, cherries, peaches, apricots, etc.), and "mottled leaves" disease of citrus trees. In the 1930s, these diseases occurred in large areas of California orchards, in Florida, Hawaii, Australia and others. and for many years they were unable to find a significant reason for them. It was only by chance that4 , (Hoagland, 1948).


  1. Zinc uptake and content in plants.


A number of factors affect the uptake of zinc by plants. This uptake is closely related to the content of available zinc in soils, which in turn is highly dependent on pH. The highest availability of zinc in the substrate occurs up to pH 6.5, the availability of zinc is also improved by the presence of organic matter in the substrate. Zinc uptake may be limited by high phosphate concentrations. Under the influence of phosphorus, insoluble zinc compounds were precipitated in the soil. The availability of Zn for plants is also dependent on the microflora present in the substrate.


Among plants, there are large species (and even cultivar) differences in the ability to absorb zinc from the soil and in terms of their sensitivity to deficiency or excess of this component. This is due to both specific differences in the demand of individual species (or varieties) for this component, different rates of its uptake, as well as the influence of other ions (phosphates, manganese, etc.), absorbed together with zinc by plant roots in very different proportions; the scope of zinc reuse in the plant also comes into play here. A large part of the zinc contained in the cell is contained in the cell sap in a soluble form. However, the mobility of zinc throughout the plant is not high.


The sensitivity to excess zinc is also different in different plants. Researchers are trying to define the so-called limits of zinc content in plants, but the information on this subject is still very incomplete. Zn levels of 30-50 ppm Zn are considered appropriate for most plants. Plants, however, generally show high tolerance to higher zinc concentrations in tissues. It is also known from agricultural practice that widely used fungicidal zinc preparations in concentrations of 0.1-0.2% do not damage the plants sprayed with them. Determining the limit ranges of zinc supply to plants can be of great help in diagnosing deficiencies (or excesses) of this component in the soil. Researchers point out, however, that not only the zinc content in the plant should be determined, but also the P:Zn ratio, as well as the Mn:Zn ratio.


Zinc requirements of plants also depend on the type of crop. Zinc deficiencies can, for example, occur in conditions of intense light.


  1. Distribution and forms of zinc in the plant cell.


A characteristic feature of the distribution of zinc within the cell is the presence of a significant part of it in the cell sap, where this micronutrient is in a soluble form. Interestingly, this form is also present in conditions of zinc deficiency. Studies on individual cell fractions have shown the presence of zinc in all subcellular structures (also in the cell wall), none of which, however, is distinguished by any special abundance in this micronutrient. Mitochondria contain slightly more zinc than chloroplasts; in these two organelles it is mostly associated with some macromolecular substance, probably with proteins.


The research results stating that a large part of the zinc contained in the cell is soluble do not coincide with the information on the mobility of this microelement in the plant. Namely, it was observed. that this mobility is not high, especially in old leaves and roots, where zinc is presumably precipitated in hard | a moving form. Researchers, however, agree that zinc is more mobile in young leaves.


  1. Symptoms of zinc deficiency and excess in plants.


The first symptom of zinc deficiency in fruit trees is mottled leaf chlorosis on the top branches. The leaves are stunted (sometimes their dimensions reach only 1/20 of their normal size), they are stiff and brittle, and the chlorotic tissues die, taking on a red-brown color. Leaves often drop prematurely. Twigs are stunted in growth and die from the top. On the lower leaves, yellow streaks appear between the veins, and then white, necrotic spots that turn brown with time, and the entire leaf dies. Young leaves are often pale yellow or white. Plants are stunted due to inhibition of internode growth.


Symptoms of zinc excess, caused, for example, by soil contamination with large amounts of zinc, have also been described in the literature. Under these conditions, plants are stunted in growth, become chlorotic, the tips of the leaves are covered with necrosis or dry up. Eventually the plant dies.


  1. Physiological and biochemical functions of zinc in plants.


Although zinc has been recognized as a micronutrient necessary for all higher plants, its biochemical functions in plants are not yet well understood. Carbonic anhydrase has been discovered in plants and has been identified as a zinc protein. This enzyme catalyzes the reversible hydration of carbon dioxide to carbonic acid. It was assumed that this reaction could be of importance in the processes of respiration and photosynthesis, but there is no experimental confirmation of this. Moreover, it was found. Among plant dehydrogenases, the common phosphotriose dehydrogenase is a zinc proteid. Other zinc-dependent dehydrogenases have also been detected in plants. Little is known about other "zinc" plant enzymes, but there are assumptions that zinc functions in similar enzyme systems in plants,


One of the consequences of zinc deficiency in plants is a decrease in the content of protein substances, while a significant increase in the level of free amino acids and amides. Thus, it is assumed that in plants zinc acts in the process of protein synthesis at the level of peptide formation. On the other hand, the direct effect of zinc on ribonucleic acids has been proven; namely, in the absence of zinc in plants, the RNA content decreased significantly due to the increase in ribonuclease activity. Recently, the cytoplasmic ribosomes of Euglena gracilis contain significant amounts of zinc; in its absence, these organelles disintegrate. The influence of zinc on RNA metabolism may be a sufficient justification for the role of this microelement in protein synthesis. On the other hand, the specific role of zinc in the synthesis of tryptophan - an amino acid, has been proven

Researchers wonder whether the observed effect of zinc deficiency on the content of some enzymes is not the result of an indirect effect of zinc, by reducing the synthesis of protein substances, and thus enzyme proteins. Zinc deficiency in plants causes changes in phosphorus metabolism. Namely, with the exclusion of zinc from the medium, the accumulation of inorganic phosphorus in tissues strongly increased, while the content of nucleotide phosphorus, as well as phosphorus in the fraction of lipids and nucleic acids, decreased.


The effect of zinc on the reaction catalyzed by trioz dehydrogenase indicated the participation of this component in glycolysis. It was found that in the absence of zinc there was a reduction in tomato leaf respiration. This symptom was associated with a weakening of the reaction in the glycolysis system and the Krebs cycle, while an increase in the reaction of the pentose phosphate pathway was observed. The effect of zinc on plant respiration requires further research.


Symptoms of zinc deficiency in plants, such as inhibition of the development of growth tips and the formation of abnormally small leaves, have long suggested the existence of a specific relationship between zinc and growth substances. Also, the effect of light on the severity of zinc deficiency symptoms in plants allows us to draw some analogies to the effect of this climatic factor on auxins. Direct proof of the participation of zinc in the synthesis of auxins was obtained by Skoog (1940). In his experiments, tomatoes grown on a zinc-free medium contained minimal amounts of auxins; the addition of zinc salts to the medium caused an increase in the level of these substances first, and then in the plants themselves. The results of Skoog's research were confirmed by Tsui in water cultures without the addition of zinc, he found a decrease in the content of auxins in plants, which occurred even before the onset of external symptoms of zinc deficiency. Tsui further found that the content of tryptophan, which is probably a precursor to auxin (namely indole acetic acid), was significantly lower in deficient plants than in control plants. After adding zinc to the deficient culture, the content of this amino acid increased within 3 days. Tsui concluded that zinc was necessary for the synthesis of tryptophan and thus, indirectly, for the synthesis of auxin