Journal of Experimental Botany, Vol 50, 1827-1835, Copyright © 1999 by Oxford University Press
A Haag-Kerwer, H Schafer, S Heiss, C Walter and T Rausch
Brassica juncea L. is able to accumulate more than 400
ARTICLES
Cadmium exposure in Brassica juncea causes a decline in transpiration rate and leaf expansion without effect on photosynthesis
Botanisches Institut, Im Neuenheimer Feld 360, D-69120 Heidelberg, Germany; Corresponding author; Fax: +49 6221 545859; E-mail: ahaag@Botanik1.bot.uni-heidelberg.de
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, 
-glutamylcysteine synthetase (-ECS), has been assessed by RNA blot, and was
compared to expression of metallothionein class 2 (MT2). These results
indicate that while photosynthesis was not affected by exposure to
25M CdNO3, transpiration showed a significant
decline, in particular, under lower light conditions (
300 
mol photons m-2
s-1), starting 48 h after the onset of 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 
-ECS appeared to correlate closely with
Cd accumulation and PC synthesis, whereas transcript amounts for MT2
increased only later. These results suggest stringent control of Cd
detoxification by PCs protects photosynthesis, but does not prevent a
decline in transpiration rate.Keywords:
Brassica juncea, gas exchange, chlorophyll
fluorescence, growth, phytochelatin, -ECS, MT2, cadmium
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