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

The net uptake, distribution and assimilation of NO3- were studied in pea plants subjected to either long-term continuous Cd treatment for 10 d (10 or 50 mu M Cd) or short-term treatment (72 h) with 50 mu 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 NO3- in the shoots and sap decreased dramatically and net NO3- uptake was severely inhibited, effects associated with a loss of shoot nitrate reductase (NR) activity. In the short-term Cd treatment, net NO3- 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 NO3-. Potassium uptake was similarly affected by Cd, as inferred from the ratio NO3-/K uptake, which was ca. 10. The ratio K/NO3- 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 NO3-, although its relative tissue distribution was not affected by Cd.