Ability of Agrogyron elongatum to accumulate the single metal of cadmium, copper, nickel and lead and root exudation of organic acids.

Agrogyron elongatum were grown in nutrient solution containing moderate to high amounts of separate heavy metal of Cd, Cu, Ni and Pb in a greenhouse for a 9-day. Cd, Cu, Ni and Pb generally led to decrease in the elongation of roots although the length of seedlings exposed to Cd and Pb at 0.05 and 0.5 mg/L showed to be slightly greater than that of controls. Of the four metals in the experiment, Pb was absorbed and accumulated to the highest level, with the concentrations of 92,754 mg/kg dry weight (DW) in roots and 11,683 mg/kg DW in shoots. Cd was moderately accumulated in Agrogyron elongatum, but the maximum bioaccumulation coefficients (BCs) for roots and shoots were observed. The patterns for Cu and Ni uptake and distribution in plants differed from those of Pb and Cd, as it was showed that the shoot accumulation of Cu and Ni was significantly higher than in roots. A. elongatum had the highest Ni concentration in shoots (30,261 mg/kg DW) at the external concentration of 250 mg/L. Cu ranked second, with a shoot concentration of 12,230 mg/kg DW when 50 mg/L Cu in solution was applied. For the four trace elements tested, the highest concentrations in shoots decreased by the order of Ni > Cu > Pb > Cd (mg/kg DW), and those in roots were Pb > Cd > Ni > Cu (mg/kg DW). Malic, oxalic and citric acids exuded by roots exposed to 1 and 50 mg/L of the metals were detected. Release of organic acids from plants significantly differed among the metal treatments. Cu was most effectively in inducing root exudation of the three types of organic acids. Cd, and Ni were also the inducers of secretion of malic and oxalic acids. With reference of Pb, a small amounts of malic and oxalic acids were detected in the root exudates, but few quantities of citric acid were found. However, no correlation between alternations in root exudation of organic acids and metal accumulation could be established.