Cadmium exposure in Brassica juncea causes a decline in transpiration rate and leaf expansion without effect on photosynthesis

Brassica juncea L. is able to accumulate more than 400 mu g g(-1) DW Cd in the shoot, a physiological trait which may be exploited for the bioremediation of contaminated soils and waters, Cd accumulation is accompanied by metabolic adaptation, in particular, the rapid induction of phytochelatin (PC) biosynthesis. Sequestration of Cd by PCs provides an essential cellular mechanism for Cd detoxification, To address the effects of Cd exposure on leaf physiology as compared to induction of PC synthesis, the accumulation of Cd in relation to growth rate, transpiration rate, CO2 assimilation, and PC synthesis, has been analysed in a time-course study using the same leaf material. Furthermore, expression of the rate-limiting enzyme for glutathione (GSH) synthesis, gamma-glutamylcysteine synthetase (gamma-ECS), has been assessed by RNA blot, and was compared to expression of metallothionein class 2 (MTP). These results indicate that while photosynthesis was not affected by exposure to 25 mu M CdNO3, transpiration showed a significant decline, in particular, under lower light conditions (less than or equal to 300 mu mol photons m(-2) s(-1)), starting 48 h after the onset of Cd exposure. Reduced transpiration correlated with reduced leaf expansion growth, and a decrease in Cd accumulation rate. A quantitative comparison revealed that during the entire time-course, PC content was theoretically sufficient to chelate all Cd taken up. Expression of gamma-ECS appeared to correlate closely with Cd accumulation and PC synthesis, whereas transcript amounts for MT2 increased only later. These results suggest that stringent control of Cd detoxification by PCs protects photosynthesis, but does not prevent a decline in transpiration rate.