EFFECTS OF CADMIUM AND NICKEL ON INVIVO CARBON-DIOXIDE EXCHANGE-RATE OF PIGEON PEA (CAJANUS-CAJAN L)

Pigeon pea plants, grown in sand culture and exposed to varying concentrations of Cd2+ and Ni2+ (5, 10, 15 and 20 mM) at different stages of their growth were examined for carbon dioxide exchange rate (CER), stomatal conductance, transpiration, internal CO2 concentration and chlorophyll content. In general, Cd2+ was more inhibitory than Ni2+ at all concentrations. The extent of inhibition increased with duration of exposure. At the vegetative stage, CER was inhibited by 87% and 30% with 20mM Cd2+ and Ni2+, respectively one day after treatment. Stomatal conductance decreased in parallel with transpiration rate. After 10 days of treatment, wilting occurred in plants receiving 20 mM Cd2+. Reduction in CER was more pronounced at the flowering stage as compared to vegetative and pod-filling stages. However, decrease in chlorophyll content was marked at the pod-filling stage. Reduction of CER in pigeon pea by these metals could not be attributed to any single factor and appeared to be due to the combined effects on stomatal conductance, chlorophyll content and on the functioning of photosynthetic apparatus.