©1997 Bernard Knezek, All Rights Reserved

COPPER

9.3.1 GENERAL

-A transition element, copper shares similarities with iron, such as the transformation of highly stable complexes and easy electron transfer. The divalent copper ion (Cu2+) is strongly bound in soils to humic and fulvic acids.



-Divalent copper is readily reduced to monovalent copper, which is unstable.

-Copper has a high affinity for peptide and sulfhydryl groups, and thus, to cysteine-rich proteins as well as for carboxylic and phenolic groups. In soil solutions up to 98% of the copper is complexed to low-molecular-weight organic compounds.

9.3.2 COPPER PROTEINS

-According to Sandmann and Boger copper is present in three different forms in proteins: (a) blue proteins without oxidase activity (e.g., plastocyanin), which function in one-electron transfer; (b) non-blue proteins, which produce peroxidases and oxidize monophenols to diphenols; and (c) multicopper proteins containing at least four copper atoms per molecule, which act as oxidases (e.g., ascorbate oxidase and laccase) and catalyze the reaction

2AH2 + O2 --> 2A + 2H2O

-Cytochrome oxidase is a mixed copper-iron protein catalyzing the terminal oxidation in mitochondria.

-Under conditions of copper deficiency the activity of these copper enzymes decreases fairly rapidly.

9.3.2.1 PLASTOCYANIN

-In general, more than 50% of the copper localized in chloroplasts is bound to plastocyanin.

-With copper deficiency, there is a greater decrease in the plastocyanin content and the activity of photosystem I than there is in the content of other chloroplast pigments and the activity of photosystem II. (Table 9.9, 9.10)





-In copper-deficient plants, the rate of photosynthesis can also be reduced for other reasons directly related to the role of copper in chloroplasts.

9.3.2.2 SUPEROXIDE DISMUTASE

-The various types of SOD isoenzymes and their requirement for the detoxification of superoxide radicals (O2-) have been discussed.

-The CuZnSOD is located in the cytoplasm in mitochondria and glyoxysomes, but occurs also in the chloroplasts together with the FeSOD. Under copper deficiency, CuZnSOD activity declines drastically in leaves (chloroplastic and cytosolic) with simultaneous corresponding increase in activity of the MnSOD.

9.3.2.3 CYTOCHROME OXIDASE

-The terminal oxidase of the mitochondrial electron transport chain (Fig. 5.5) contains two copper atoms and two iron atoms in the heme configuration. As respiration rates either remain unaffected or are only moderately decreased by copper deficiency, cytochrome oxidase seems to be present in large excess in the mitochondria.

9.3.2.4 ASCORBATE OXIDASE

-Ascorbate oxidase catalyzes the oxidation of ascorbic acid to L-dehydroascorbic acid.

-The enzyme occurs in cell walls and in the cytoplasm. There is a close positive correlation in the suboptimal concentration range between the copper content of leaf tissue and its ascorbate oxidase activity (Fig. 9.10).



-Resupplying copper to deficient plants can restore the activity of ascorbate oxidase only in very young, but not mature leaves (Table 9.11).



9.3.2.5 DIAMINE OXIDASE

-Polyamine oxidases are flavoproteins which catalyze the degradation of polyamines. Its activity decreases in copper-deficient plants (Table 9.9) and is confined to very young leaves (Table 9.11).

-Diamine oxidase is located in the apoplasm of the epidermis and the xylem of mature tissues where it presumably functions as an H2O2 -delivery system for peroxidase activity in the process of lignification and suberization.

9.3.2.6 PHENOL OXIDASES

-These enzymes catalyze oxygenation reactions of plant phenols. Laccase can be found in the thylakoid membranes of chloroplasts, where it is presumably required for the synthesis of plastoquinone, a constituent of the photosynthetic e- transport chain. Phenolase has two distinct enzyme functions: (a) hydroxylate monophenols to diphenols, resembling tyrosinase activity, and (b) oxidize diphenols to o-quinones such as dihydroxyphenylalamine (DOPA), resembling polyphenol oxidase activity. Both reactions need molecular oxygen.

-Under conditions of deficiency, the decrease in phenolase is quite severe (Table 9.12) and is correlated with an accumulation of phenolics and a decrease in the formation of melanotic substances.



-The decline in phenolase activity may be at least indirectly responsible for the delay in flowering and maturation often observed in copper-deficient plants and shown for the flowering of Chrysanthemum in Table 9.12. It is known that certain phenols are active inhibitors of IAA oxidase and that ascorbic acid also strongly inhibits peroxidase-catalyzed oxidation of IAA.

9.3.3 CARBOHYDRATE, LIPID AND NITROGEN METABOLISM

-In plants suffering from copper deficiency the content of soluble carbohydrates is considerably lower than normal during the vegetative stage. In wheat, after anthesis, when the grain has developed as a dominant sink, copper-deficient plants have only a few grains, remain green (i.e. actively photosynthesizing) and build up excessive levels of soluble carbohydrates both in the leaves and in the roots.(Fig. 9.11)



-Given the role of copper in photosynthesis (PS I), a lower content of soluble carbohydrates would be expected during vegetative growth. Under severe copper deficiency alterations in polypeptides of PS II occur.

-Low carbohydrate contents in copper-deficient plants are involved in impaired pollen formation and fertilization and depressed nodulation and N2 fixation in legumes.

-It has been shown repeatedly that nitrogen application accentuates copper deficiency, and when the nitrogen supply is large, the application of copper fertilizers is required for maximum yield.

9.3.4 LIGNIFICATION

-Impaired lignification of cell walls is the most typical anatomical change induced by copper deficiency in higher plants. This gives rise to the characteristic distortion of young leaves, bending and twisting of stems and twigs ("pendula" forms in trees).

-Copper has a marked effect on the formation and chemical composition of cell walls (Table 9.13.)



-The effect on lignification is even more distinct in the sclerenchyma cells of stem tissue (Fig. 9.12)



-Lignification responds rapidly to copper supply; transition periods of copper deficiency during the growth period can be readily identified by variations in the degree of lignification in stem sections.

-The inhibition of lignification in copper-deficient tissue is related to two copper-enzymes in lignin biosynthesis (polyphenol oxidase, diamine oxidase)

9.3.5 POLLEN FORMATION AND FERTILIZATION

-Copper deficiency affects grain, seed, and fruit formation much more than vegetative growth (Table 9.14). It is important to have adequate copper supply during fertilization for final seed and fruit yield.



-The main reason for decrease in formation of generative organs is the nonviability of pollen from copper-deficient plants (microsporogenesis)

9.3.6 COPPER DEFICIENCY AND TOXICITY

9.3.6.1 COPPER DEFICIENCY

-Copper deficiency is often observed on soils inherently low in total copper or high in organic matter. High nitrogen availability can also accentuate copper deficiency.

-Stunted growth, distortion of young leaves, necrosis of the apical meristem and bleaching of young leaves, and/or 'summer dieback' in trees are typical visible symptoms of copper deficiency.

-The critical deficiency level of copper in vegetative parts is generally in the range of 3 to 5 g/g dry wt.

-Foliar applications of copper in the form of inorganic salts, oxides, or chelates are required to correct copper deficiency rapidly in soil-grown plants.

9.3.6.2 COPPER TOXICITY

-For most crop species, the critical toxicity level of copper in the leaves is considered to be above 20 to 30 g/g dry wt. There are marked differences in copper tolerance among plant species; these differences are directly related to the copper content of the shoots.

-A large copper supply usually inhibits root growth before shoot growth. This does not mean, however, that roots are more sensitive to high copper concentrations; rather, they are the sites of preferential copper accumulation when the external copper supply is large, as shown in Table 9.15 for tomato plants.



-In nontolerant plants, inhibition of root elongation and damage to the plasma membrane of root cells are an immediate response to a large copper supply.

-For various reasons there is increasing concern about copper toxicity in agriculture.

9.3.6.3 MECHANISMS OF COPPER TOLERANCE

-Genotypical differences in tolerance to copper and other heavy metals are well-known in certain species and ecotypes of natural vegetation.

-The mechanisms of copper tolerance in higher plants can be grouped as follows: (a) exclusion or restriction of copper uptake, (b) immobilization of copper in cell walls, (c) compartmentation of copper in insoluble complexes, (d) compartmentation of copper in soluble complexes and (e) enzyme adaptation (Fig. 9.13).


©1997 Bernard Knezek, All Rights Reserved