Abstract: A pot experiment was conducted using four Japonica rice genotypes differing in cadmium (Cd)-tolerance and grain Cd concentration, to study the effect of Cd on growth and nutrition metabolism. Cd was added into soil to form 2 levels, i.e. 0 (CK) and 10 mg·kg-1 (Cd stress), respectively. The results showed that Cd stress significantly reduced grain yield and panicle number per plant, spikelets per panicle, filled spikelet rate and grain weight, and shoot dry weight at various growth stages for all four genotypes. Furthermore, these reductions varied in genotypes. The inhibition extent of genotypes with higher Cd-tolerance was smaller than that of genotypes with lower Cd-tolerance when exposed to Cd stress, and the genotype with higher grain Cd concentration showed more inhibition than one with lower grain Cd concentration. It is indicated that Cd-tolerance is consistent at different growth stages, so Cd-tolerance can be evaluated at early seedling growth. Cd stress significantly increased Cd concentration and accumulation in shoots and grains for all four genotypes, and the Cd-induced increase varied in growth stage and genotype. Cd concentration in shoots and grains became less with the progress of growth, but more for Cd accumulation, being similar to the change of dry weight. The results also showed that the effect of Cd stress on eight mineral nutrient concentrations in grains differed among genotypes and mineral nutrients. When exposed to Cd stress, reduced effect could be noted for grain K, P, Mg, Mn and Zn, thus resulting in reduced concentration. On the other hand, increased effect was found for Fe, Cu and Ca. Moreover, the effect of Cd stress on free amino acid concentration in rice grains varied in amino acids and genotypes. It was interestingly noted that Cd stress inhibited synthesis of S-containing amino acid, such as glutamic acid and cysteine, and the Cd-induced inhibition was severer in Cd-sensitive genotypes than Cd-tolerant ones. Meanwhile, the higher Cd-tolerant genotypes had higher glutamic acid and cysteine concentration in grains than the sensitive ones in all treatments. It is suggested that the metabolism of these amino acids may be associated with the response of Cd tolerance of the rice genotype.

Key words: Rice (Oryza sativa L.), Cadmium, Rice grain, Genotype, Mineral nutrient, Free amino acid