Photosynthetic activities of Pisum sativum seedlings grown in presence of cadmium

One-month-old pea seedlings (Pisum sativum L. cv. Bonneville) raised in sand culture, were provided with a nutrient solution containing 0, 2.5, 5, 7.5 and 10 mM cadmium and the effect on various aspects of photosynthesis were investigated after 6 and 12 d of the treatment. The rate of photosynthesis, chlorophyll content, activities of photosystem I (PS I) and II (PS II) and a few selected photosynthetic enzymes viz. ribulose-1,5-bisphosphate carboxylase (EC 4.1.3.9), NADP-glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.13), fructose-1,6-bisphosphatase (EC 3.1.3.11) and NADP-malate dehydrogenase (EC 1.1.1.82) declined progressively with increasing concentration of applied cadmium. As compared to the other parameters such as per dm2 leaf area, per mg fresh weight (FW) and per mg chlorophyll (chl), the rate of photosynthesis showed maximum decline on per plant basis. On extending the period of exposure to 12 d, the rate of photosynthesis and activities of enzymes showed a further decline. Cadmium had a more pronounced effect on the activity of PS II during the initial stages; however, on prolongation of the exposure for 12 d to the heavy metal, the functioning of PS I was also equally affected. Addition of cadmium to the chloroplasts isolated from untreated plants impaired the functioning of PS II without any discernible effect on that of PS I. In presence of 0.1 mM cadmium in the reaction mixture, activity of PS II was inhibited by 46 % whereas that of PS I remained unaffected. Compared to the effect on the rate of photosynthesis (per mg chl basis) and on photosystems, activities of photosynthetic enzymes showed far greater inhibition 12 d after the treatment.