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 2รท and Ni 2 รท were applied through the rooting medium at two growth stages. The application of Cd 2 รท and Ni 2 รท (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 2รท and Ni 2รท ) 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 2รท and Ni 2รท in parallel with a reduction in photosynthesis. These data confirm the earlier reported effects of Cd 2รท and Ni 2รท on 02 evolution in isolated chloroplasts.