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Reaction of Plants to Excessive Supply of Heavy Metals

At the same EC level sensitive as well as tolerant plants to stress by heavy metal generally show similar reactions when exposed to this kind of stress.

The first reaction complex may be considered an avoidance strategy, aiming at immobilisation of the heavy metals outside the protoplasts of root cells in order to prevent physiological effec-tivity. Complexing Cu2+ ions by root exudates in the soil, but also in the cell wall, on the one hand reduces the supply to the protoplast but, on the other, requires continuous root growth. Depending on the composition of the cell wall and the types and amounts of the exudates, the

Heavy metal Activity

A Time after application of cadmium (min)

B Transpiration (as % of maximum)

ifig. 1.7.7. Inhibition by cadmium of photosynthesis and transpiration of maize and sunflower. a Time kinetics of the influence of various Cd2+ concentrations on the rates of photosynthesis. n = control (27 mM KCI); A = 4.5 mM CdCI2; 0 = 9 mM CdCI2; •= 18 mM CdCI2. b Relationship between the inhibition of photosynthesis and transpiration by cadmium. The regression line shows a quasi-linear relationship between the two processes. As the regression line does not cross at zero, transpiration, i.e. stomatal conductance, must be inhibited to a greater extent than photosynthesis. This suggests that inhibition of photosynthesis by cadmium principally occurs through closure of stomates. The data points have been produced with both plant species. (After Bazzaz et al. 1974)

Table 1.7.7, Respiration and enzyme activity of leaves from soybean seedlings after 10 days growth at various concentrations of cadmium chloride. The measured activity is normalised to 1 g fresh weight. Respiration as well as activity of catabolic enzymes show a clear "injury effect" (Lee et al. 1976)

Table 1.7.7, Respiration and enzyme activity of leaves from soybean seedlings after 10 days growth at various concentrations of cadmium chloride. The measured activity is normalised to 1 g fresh weight. Respiration as well as activity of catabolic enzymes show a clear "injury effect" (Lee et al. 1976)

Activity

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