Tolerance and accumulation of lead by species of Iris L

Seedling development, accumulation and distribution of lead (Pb) in Iris lactea var. chinensis (Fisch.) Koidz. and I. tectorum Maxim. were studied using plants grown in sand culture and exposed to 0-10 mmol l(-1) concentrations of Pb supplied as Pb(NO3)(2) for 28 days. A significant reduction in dry weight (dw) of shoots and roots of I. lactea var. chinensis was observed at 6 and 10 mmol l(-1), respectively, and a significant reduction in dw of shoots and roots of I. tectorum was observed at 6 mmol l(-1). Concentration of Pb in the shoots and roots of I. lacteal var. chinensis exposed to 4 mmol l(-1) Pb reached 1,109 microg g(-1) and 2,408 microg g(-1) dw, respectively. The index of tolerance (IT) of I. lactea var. chinensis among 0-8 mmol l(-1) Pb treatments were not significantly different, while those of I. tectorum at 6 mmol l(-1) Pb were significantly decreased. The results indicated that I. lactea var. chinensis was more tolerant to Pb than I. tectorum. Sub-cellular localization of Pb in root cells was evaluated using transmission electron microscopy (TEM) and Pb deposits were found along the plasma membrane of some root tip cells of I. lactea var. chinensis treated at 10 mmol l(-1) Pb. Deposits of Pd were also observed along the surface, in the root tip cell wall and in the cytoplasm of a few malformed cells of I. tectorum exposed at 10 mmol l(-1) Pb treatment. One possible mechanism to explain these observations may be that most cells can maintain normal activities in the plant by sacrificing a small number of cells that accumulate a large amount Pb and show toxicity. Future studies should be designed to test this hypothesis.