Cadmium exposure in Brassica juncea causes a decline in transpiration rateand 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 bioremediat ion of contaminated soils and waters, Cd accumulation is accompanied by met abolic adaptation, in particular, the rapid induction of phytochelatin (PC) biosynthesis. Sequestration of Cd by PCs provides an essential cellular me chanism for Cd detoxification, To address the effects of Cd exposure on lea f 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 ma terial. Furthermore, expression of the rate-limiting enzyme for glutathione (GSH) synthesis, gamma-glutamylcysteine synthetase (gamma-ECS), has been a ssessed by RNA blot, and was compared to expression of metallothionein clas s 2 (MTP). These results indicate that while photosynthesis was not affecte d 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. Red uced transpiration correlated with reduced leaf expansion growth, and a dec rease in Cd accumulation rate. A quantitative comparison revealed that duri ng the entire time-course, PC content was theoretically sufficient to chela te all Cd taken up. Expression of gamma-ECS appeared to correlate closely w ith Cd accumulation and PC synthesis, whereas transcript amounts for MT2 in creased only later. These results suggest that stringent control of Cd deto xification by PCs protects photosynthesis, but does not prevent a decline i n transpiration rate.