Effect of combined application of lead, cadmium, chromium and copper on grain, leaf and stem heavy metal contents at different growth stages in rice.

Most studies on plants' response to heavy metal toxicity have been focusing on single metals. However, soils are always contaminated by several kinds of heavy metals. In this study, pot experiments were carried out to investigate the effects of combined toxicity on two rice genotypes differing in Cd accumulation (Xiushui817, a low-grain-Cd-accumulation and Zheda821, a high-grain-Cd-accumulation genotype). Yield, heavy metal concentrations of grain and leaf/stem at different growth stages were measured under combined application of Cd, Cr, Pb and Cu. Yield was significantly decreased under higher Pb and Cd treatment in both genotypes with Xiushui817 showing greater reduction. Increasing soil Cu level showed no significant effect on grain yield. Zheda821 consistently showed a higher grain Cd content than Xiushui817. The application of Pb, Cd, Cr and Cu significantly affected grain Cd, Cr and Cu accumulations. Similar trends were also observed in leaves and stems at harvest stage. The critical levels of leaf/stem Cd and Cr for safe rice production were also estimated. Alleviation measures should be taken to decrease Cd or Cr accumulations in grain of rice if leaf or stem Cd or Cr concentrations at different growth stages exceed the critical levels.