Analysis of cadmium translocation, partitioning and tolerance in six barley (Hordeum vulgare L.) cultivars as a function of thiol metabolism

Six barley cultivars widely differing for cadmium (Cd) tolerance, partitioning, and translocation were analyzed in relation to their thiol metabolism. Results indicated that Cd tolerance was not clearly related to the total amount of Cd absorbed by plants, resulting instead closely dependent on the capacity of the cultivars to trap the metal into the roots. Such behaviors suggested the existence of root mechanisms preserving shoots from Cd-induced oxidative damages, as indicated by the analysis of thiobarbituric acid-reactive substances —diagnostic indicators of oxidative stress—whose increases in the shoots were negatively related to Cd root retention and tolerance. Cd exposure differentially affected glutathione (GSH) and phytochelatin (PC) levels in the tissues of each barley cultivar. The capacity to produce PCs appeared as a specific characteristic of each barley cultivar, since it did not depend on Cd concentration in the roots and resulted negatively related to the concentration of the metal in the shoots, indicating the existence of a cultivar-specific interference of Cd on GSH biosynthesis, as confirmed by the existence of close positive linear relationships between the effect of Cd on GSH levels and PC accumulation in both roots and shoots. The six barley cultivars also differed for their capacity to load Cd ions into the xylem, which was negatively related to PC content in the roots. Taken as a whole, these data indicated that the different capacity of each cultivar to maintain GSH homeostasis under Cd stress may strongly affect PC accumulation and, thus, Cd tolerance and translocation.