-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
-Under conditions of copper deficiency the activity of these copper enzymes decreases
-In general, more than 50% of the copper localized in chloroplasts is bound to
-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.
126.96.36.199 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.
188.8.131.52 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.
184.108.40.206 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).
220.127.116.11 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.
18.104.22.168 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
-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
-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
-The effect on lignification is even more distinct in the sclerenchyma cells of stem tissue
-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
22.214.171.124 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
-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.
126.96.36.199 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.
188.8.131.52 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).