Variations between rice cultivars in iron and manganese plaque on roots and the relation with plant cadmium uptake.

To understand certain mechanisms causing variations between rice cultivars with regard to cadmium uptake and tolerance, pot soil experiments were conducted with two rice cultivars of different genotypes under different soil Cd levels. The relationships between plant Cd uptake and iron/manganese (Fe/Mn) plaque formation on roots were investigated. The results showed that rice cultivars differed markedly in Cd uptake and tolerance. Under soil Cd treatments, Cd concentrations and accumulations in the cultivar Shanyou 63 (the genotype indica) were significantly higher than those in the cultivar Wuyunjing 7 (the genotype japonica) (P < 0.01, or P < 0.05), and Shanyou 63 was more sensitive to Cd toxicity than Wuyunjing 7. The differences between the rice cultivars were the largest at relatively low soil Cd level (i.e., 10 mg/kg). Fe concentrations in dithionite-citrate-bicarbonate root extracts of Shanyou 63 were generally lower than that of Wuyunjing 7, and the difference was the most significant under the treatment of 10 mg Cd/kg soil. The results indicated that the formation of iron plaque on rice roots could act as a barrier to soil Cd toxicity, and may be a "buffer" or a "reservoir" which could reduce Cd uptake into rice roots. And the plaque may contribute, to some extent, to the genotypic differences of rice cultivars in Cd uptake and tolerance.