Effects of cadmium on the uptake,distribution and assimilation of nitrate in Pisum sativum

The net uptake, distribution and assimilation of NO ₃ ^– were studied in pea plants subjected to either long-term continuous Cd treatment for 10 d (10 or 50 M Cd) or short-term treatment (72 h) with 50 M Cd. In the latter treatment, the effects of transferring the plants to a Cd-free nutrient solution for a 'recovery period' of 96 h were also studied. All these treatments were compared with 'controls', plants which received no Cd. In both experiments, the reduction in fresh weight was associated with a decrease in the content (%) of shoot and root water and in transpiration rate as Cd concentration increased. The concentration of ₃ ^– in the shoots and sap decreased dramatically and net ₃ ^– uptake was severely inhibited, effects associated with a loss of shoot nitrate reductase (NR) activity. In the short-term Cd treatment, net ₃ ^– uptake was almost completely inhibited after 24 h, but recovered after the transfer of plants to a Cd-free nutrient solution. Similarly, a dramatic decrease in the shoot NR activity was observed. The uptake, distribution and tissue partitioning of K was also studied, which is considered to be the major counterion of ₃ ^– . Potassium uptake was similarly affected by Cd, as inferred from the ratio ₃ ^– /K uptake, which was ca. 10. The ratio K/ ₃ ^– tissue content increased in the shoot concomitantly to Cd in both long-term and short-term metal supply. These parameters showed a tendency of K similar to that observed for ₃ ^– , although its relative tissue distribution was not affected by Cd.