Effect of Cadmium Repartition on Nitrogen Metabolism in Tobacco Seedlings

Thirty-day-old tobacco seedlings (Nicotiana tabaccum, Bureley v) were subjected during one week to increasing cadmium (Cd) concentrations (0, 10, 20, 50 and 100 µM CdCl 2 ).Increasing Cd stress led to a gradual decrease of dry weight (DW) production, water and nitrate contents.More than the half of Cd accumulated per plant was sequestered in the oldest leaf stage (S 1 leaves).Leaves from S 1 were the least affected by Cd stress.The activities of nitrate reductase (NR, EC 1.6.1.6),nitrite reductase (NiR, EC 1.7.7.1) were the least reduced in S 1 leaves despite of the high presence of Cd ions.At 100 µM Cd, glutamine synthetase activity (GS, EC 6.3.1.2) from S 1 leaves rose to become 2 times more important than control.Western Blot analysis showed that S 1 GS activity induction was correlated to the GS 1 and GS 2 protein accumulation.Young leaves (S 3 leaves) were more affected by Cd stress than old leaves.The GS activity reduction in S 3 leaves was correlated to GS 2 protein decrease detected by western-blot analysis.So, tobacco plant accumulated Cd ions in old leaves (S 1 leaves) to protect younger leaves which are more sensitive to Cd effects.Leaves from S 1 are a target organ to verify an eventual soil contamination per cadmium.This leaves may evolve adaptive process to partially inactivate Cd ions and maintain stable rate of nitrogen metabolism.