Effect of cadmium and nickel on photosynthesis and the enzymes of the photosynthetic carbon reduction cycle in pigeonpea (Cajanus cajan L.)

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The effect of Cd(2+) and Ni(2+) on the rate of photosynthesis and activities of key enzymes of the photosynthetic carbon reduction cycle was examined in leaves from pigeonpea (Cajanus cajan L., cv. UPAS-120) grown in nitrogen free sand culture. Two different concentrations of Cd(2+) and Ni(2+) were applied through the rooting medium at two growth stages. The application of Cd(2+) and Ni(2+) (0.5 and 1.0 mM) at an early vegetative stage (30 days after sowing) resulted in about 50% and 32% reduction in net photosynthesis, respectively. However, enzyme activities were decreased to different levels (2-61%) depending upon the enzyme and the concentration of the metal ion.These concentrations (0.5 and 1.0 mM of Cd(2+) and Ni(2+)) had no effect when applied at a later vegetative stage i.e. 70 days after sowing. However, when the concentration of Cd(2+) was increased to 10 mM, there was about an 86% reduction in the rate of photosynthesis but the enzyme activities were reduced by only about 40%. Although Ni(2+) reduced the photosynthetic rate by 65%, it had little effect on enzyme activities. The reduction in photosynthesis seems to occur indirectly through a decrease in chlorophyll content and stomatal conductance but not due to decreased enzyme activities. Oxygen evolution by leaf discs was inhibited by Cd(2+) and Ni(2+) in parallel with a reduction in photosynthesis. These data confirm the earlier reported effects of Cd(2+) and Ni(2+) on O2 evolution in isolated chloroplasts.