Changes in glutathione and phytochelatins in roots of maize seedlings exposed to cadmium

The effects of excess Cd on the contents of free cysteine, total glutathione and phytochelatin (PC) were measured in roots of intact maize seedlings. Exposure to 3 /tmM Cd for 15 min caused PCs to appe substrates for formation of longer PCs. Total glutathione levels declined with PC synthesis, free cysteine contents changed little. The reactions to excess Cd differed along the length of roots. In the 1 cm apical region a high production of PCs occurred with a moderate loss of total glutathione. In the mature region, PC content was 2.5-fold less than in apices, several unidentified thiols accumulated, and total glutathione levels declined drastically. Exposure to 0.05 μM Cd for 24 h induced PCs, the contents rose as Cd concentrations were increased. The roots produced PCs in excess of that required to chelate the Cd present, as if some PCs were compartmentalized or had not yet formed Cd-PC complexes. Phytochelatin formation was stimulated most effectively by Cd, less by Zn and Cu and negligibly by Ni. Total glutathione declined with Cd and Zn exposure, however, with excess Cu the roots contained 45% more total glutathione than did the controls.