Effect of cadmium on photosynthesis, nutrition and growth of mungbean

Contamination of soils with cadmium is a major threat to ecosystems. Root uptake of cadmium from contaminated soils induces physiological changes such as a decrease in plant growth. Plant species and varieties show differential physiological mechanisms of cadmium tolerance. Here, we studied the effect of cadmium chloride on leaf chlorosis, gas exchange attributes and some essential nutrients in the shoots of selected tolerant and sensitive mungbean (Vigna radiata) varieties at the seedling, vegetative and reproductive growth stages. Our results show that elevated concentrations of cadmium led to accumulation of cadmium in the shoot and roots, intervein chlorosis of leaves and loss of pigments. Tolerant mungbean showed steady-state contents of potassium, magnesium, manganese and iron, and photosynthetic pigments at all growth stages. A decrease in the net photosynthesis, increase in substomatal CO2 level and a decline in the ratio of net photosynthesis and substomatal CO2 level revealed that cadmium prevented CO2 fixation by Rubisco. Correlations of shoot cadmium concentration and chlorosis with its nutrient and pigment content, although negative in both varieties, were closer in the tolerant but absent, or weaker if present, in the sensitive mungbean. Most nutrients had close association with the content of photosynthetic pigments of tolerant mungbean, which indicated their involvement in maintaining steady levels of pigments. This finding indicates the involvement of nutrients and pigments in cadmium tolerance. In conclusion, cadmium tolerance in mungbean was attributed to low cadmium uptake and its accumulation in the shoot, leaf chlorosis, improved pigment, nutrient levels and carboxylation efficiency of Rubisco throughout the mungbean phenology. Based on these findings the tolerant mungbean can be grown in moderately cadmium-contaminated soils.