Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

Nutrient Stress
201
11. COPPER
Copper is a transition element and forms stable complexes. It occurs in two oxidation
states (Cu + and Cu 2+ ). Divalent form of copper is readily reduced to monovalent copper
which is unstable. Copper occurs in soils mainly as a divalent cation. Copper is not
easily mobile in plants but it is known to move from older to younger leaves and this
movement depends on copper status of the plant.
Copper is an important redox component directly related to electron transfer
reactions in photosynthesis, respiration, lignification of cell walls and detoxification of
superoxide radicals (Fox and Guerinot, 1998). Copper is associated with three different
forms of proteins (Sandmann and Boger, 1983) which include Blue proteins without
oxidase activity (eg. Plastocyanin), Non-blue proteins which oxidize monophenols
and Multi-copper proteins, containing around four copper atoms per molecule. (eg.
Ascorbic acid oxidase and diphenol oxidase). Polyphenol oxidases are involved in the
synthesis of lignin and alkaloids. Copper-deficient leaves of subterranean clover showed
lower levels of polyphenol oxidase activity (Delhaize et al., 1985).
Nearly 50% of the copper localized in chloroplasts is associated with plastocyanin.
Chlorophyll and plastocyanin levels and certain enzyme activities were more
with the increased copper content in dry matter of Pisum sativum (Table 11)
Table 11. Relationship between copper content, some constituents of chloroplast
and activities of certain of certain enzymes in Pisum sativum leaves*
Copper Chlorophyll Plastocyanin Diamine Ascorbate Superoxide
(mg/g dry (m moles/g (n moles/m mole oxidase oxidase dismutase
wt) dry wt) chlorophyll activity (m activity activity
mole/g (enzyme
protein/hr)
units/mg
protein)
2.2 4.4 0.3 0.24 220 3.6
3.8 3.9 1.1 0.43 470 13.5
6.9 4.9 2.4 0.86 730 22.9
* Adapted from Ayala and Sandmann (1988).
Davies et al. (1978) observed that copper deficiency decreased the activities
of polyphenol oxidase, IAA oxidase and peroxidase. Flowering was also affected by
copper deficiency (Table 12).