Effects of cadmium on carbon and nitrogen assimilation in shoots of mungbean [Vigna radiata (L.) wilczek] seedlings

Increased cadmium (Cd) uptake from contaminated soils damages plant metabolism. The purpose of this study was to determine Cd-induced time-related changes in some shoot growth and physiological attributes, and their interrelationships in Cd-tolerant (NM-98) and sensitive (NM-28) mungbean varieties. Shoot Cd and leaf chlorosis increased with a concomitant reduction in shoot dry weight, leaf area, relative growth rate (RGR), net assimilation rate (NAR) and relative leaf expansion rate. Reduction in transpiration rate (E) and stomatal conductance (g(s)) and increase in substomatal CO2 level (C-i), indicated that Cd reduced net photosynthesis (P-n) by reducing CO2 fixation by Rubisco, albeit these changes were less pronounced in NM-98. A positive correlation of chlorosis with shoot Cd, and negative relationships of chlorosis and shoot Cd with P-n revealed that Cd damages the photosynthetic apparatus in mungbean. Time course decrease in in vivo nitrate reductase activity (NRA) and an increase in soluble nitrate in NM-28 revealed that Cd markedly hampers nitrogen assimilation. Positive correlations of RGR and NAR with P-n and NRA and negative ones with chlorosis, shoot dry weight, shoot Cd and C-i in NM-98 suggested that mungbean sensitivity to Cd is due to perturbed C and N assimilation.