Abscisic acid accumulation and cadmium tolerance in rice seedlings

Rice (Oryza sativa L.) seeds were soaked for 18 h in distilled water in the absence (-PBZ) or presence (+PBZ, a triazole) of 100 mg l(-1) paclobutrazol and then air dried. These air-dried seeds were germinated in the dark and then cultivated in a Phytotron. Twelve-day-old -PBZ and +PBZ seedlings were treated or not with CdCl2. Cd toxicity was judged by the decrease in biomass production, decrease in chlorophyll and protein content, increase in NH4+ content and induction of oxidative stress. The results indicated that PBZ applied to seeds was able to protect rice seedlings from Cd toxicity. On treatment with CdCl2, the abscisic acid (ABA) content increased in +PBZ leaves, but not in -PBZ leaves. The decrease in the transpiration rate of -PBZ seedlings by CdCl2 was less than that of +PBZ seedlings. Exogenous application of the ABA biosynthesis inhibitor, fluridone (Flu), reduced ABA accumulation, increased the transpiration rate and Cd content, and decreased the Cd tolerance of +PBZ seedlings. The effects of Flu on the Cd toxicity, transpiration rate and Cd content were reversed by the application of ABA. It seems that the PBZ-induced Cd tolerance of rice seedlings is mediated through an accumulation of ABA.