- Entry.
Copper is a heavy metal; its atomic weight is 63.54. After silver, it is the best conductor of heat and electricity. In compounds, it occurs in oxidation states I or II. Copper has a very pronounced tendency to form complexes. The stability constants of copper complexes are generally higher than those of other divalent metals and are second only to the stability of certain complexes with ivalent iron.
The presence of minimal amounts of copper in plant and animal tissues was discovered in the early 19th century, but was thought to be entirely coincidental. However, as early as 1847, it was discovered that copper was a component of hemocyanin—the respiratory pigment in mollusc blood. A milestone in copper research was the 1924 discovery of its role in hemoglobin synthesis. This discovery initiated the search for copper in all living tissues, as well as in food products; it also stimulated research into the relationship between copper and certain enzymatic systems.
On the other hand, the fungicidal effects of copper compounds were already recognized in the second half of the 19th century. Initially, it was believed that copper also had only a toxic effect on higher plants. Only at the beginning of this century did it begin to be assumed that copper might be an essential nutrient for plants. Chemical analyses showed that all plants contained from 3 to 40 ppm or more of Cu on a dry weight basis. It was also observed that spraying crops with Bordeaux liquid (basic copper-lime sulfate) resulted in increased yields, sometimes even when the plants were not infected by fungi. However, it was not until Sommer, Lipman, and Mackinney, using specially purified mineral salts and distilled water in Pyrex glass apparatus, that they demonstrated that plants did not grow in a medium devoid of copper and that this trace element was essential for them.
2. Uptake and content of copper in plants.
It is assumed that most copper in soil is absorbed by organic and inorganic complexes or is incorporated into the crystal lattice of minerals. In mineral soils, copper is generally readily exchangeable. However, in soils rich in humus, copper absorbed by soil colloids, as Cu2+ or CuOH+, is bound much more strongly than other cations and is displaced with great difficulty, relatively best by hydrogen ions (H+). It is assumed that certain forms of humus bind copper particularly strongly, but this issue is not yet fully understood. In any case, due to this strong absorption, copper in organic soils is poorly mobile. Presumably, copper in soil does not undergo valence changes and occurs almost exclusively in its divalent form. Some copper in the soil is found in complex combinations with organic substances in the form of so-called chelates. Chelate connections are of particular importance because it has been found that copper in this form is more easily absorbed by plants than when absorbed in the soil sorption complex.
Copper is absorbed by plants in the form of Cu2+ or copper-chelate complexes. Opinions regarding the influence of environmental pH on copper uptake have not yet been sufficiently agreed upon. Although the general belief is that copper absorption decreases with increasing alkalinity, results have been presented demonstrating the lack of any correlation between soil pH and copper uptake by plants. In aquatic cultures, an increase in copper uptake was even observed with increasing pH from 3.7 to 7.0. Mengel argued that the effect of other cations on copper uptake was practically negligible and that copper tended to displace other cations, as evidenced by, among other things, the strong copper binding by plant roots. However, a clear interaction between copper and iron was found in eliminating the toxic effects of both trace elements. The range of copper content in plants depends on a number of factors, including the plant species, its organ, growth stage, environmental conditions, and fertilization. In principle, however, the copper content in plants varies within relatively narrow limits: from about 1 ppm Cu in the dry weight of the above-ground parts to about 30 ppm Cu. The copper concentration in the roots is usually higher.
3. Symptoms of copper deficiency in plants.
The initial symptom of copper deficiency in fruit trees is unusually dark green leaves, indicating high nitrogen concentrations. However, with severe copper deficiency, leaves take on a yellow-green tint and then drop prematurely. Disease symptoms are very characteristic and become apparent early. Plants initially grow normally, then, usually after 2-3 weeks, chlorosis appears at the leaf margins; the leaf tips wilt and die, taking on a yellowish-gray color. These symptoms also occur on young leaves, which die without developing their leaf blades. Lower leaves remain green for a longer period, and numerous secondary shoots may develop from the base, but these, too, eventually show disease symptoms.
4. Physiological and biochemical functions of copper in plants.
The role of copper in plant metabolism is closely linked to this element's participation in enzymatic systems involved in redox processes. In most cases, it has been demonstrated that copper functions as an electron carrier in these processes by changing its oxidation state. Copper's functioning is highly similar to iron. The best-studied enzymes in which copper is a significant component are three oxygen oxidases: catechol oxidase, p-diphenol oxidase, and ascorbate oxidase. In these oxidases, the copper content ranges from 0.20 to 0.26% Cu, with the metal atoms bound to the enzyme protein in a highly stable complex. A common feature of these enzymes is the ability to transfer electrons exclusively to oxygen; the reduction product is always water. It has also been shown that 75% of the copper contained in leaves is located in chloroplasts as an organic bond, suggesting the element's involvement in CO2assimilation. Copper's beneficial effect on photosynthesis intensity and the Hill reaction has also been reported. This effect became clear when a copper-containing protein, named plastocyanin, was discovered in green plant parts. The copper content of plastocyanin is 0.58%, indicating the presence of two Cu atoms per enzyme molecule. Plastocyanin occurs only in photosynthetic organs. It has been calculated that the copper in plastocyanin constitutes half of the total copper content in chloroplasts. Studies by Bishop and others have shown that plastocyanin functions as an electron carrier in the chain of reactions between photosystems II and I, or only in photosystem II. Copper is involved, often in as yet unspecified ways, in other metabolic processes in the plant, namely lipid metabolism, iron metabolism, and has a beneficial effect on plant protein content and chlorophyll content. However, its role in nitrate reduction has been questioned.
-Anna Nowotna-Mieczyńska "Physiology of mineral nutrition of plants" PWRiL 1965,
-Konrad Mengel, Ernest A. Kirkby "Basics of plant nutrition" PWRiL 1983,
– Mark Szkolnik "Microelements in plant life" PWRiL 1980,
-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.