Uptake, Translocation, and Remobilization of Zinc Absorbed at Different Growth Stages by Rice Genotypes of Different Zn Densities

Zinc (Zn) is an essential micronutrient for humans, and increasing Zn density in rice (Oryza sativa L.) grains is important for improving human nutrition. The characteristics of Zn translocation and remobilization were investigated in high Zn density genotype IR68144, in comparison with the low Zn density genotype IR64. Stable isotope tracer Zn-68 was supplied at various growth stages, either to the roots in nutrient solution or to the flag leaves to investigate the contribution of Zn-68 absorbed at different growth stages to grain accumulation and the remobilization ability of Zn-68 within plants. Significant differences in Zn-68 allocation were observed between the two rice genotypes: Much higher Zn-68 concentrations were found in grains, stems, and leaves of IR68144 than in IR64, but higher Zn-68 was found in roots of IR64. More than half of the Zn accumulated in the grains was remobilized before anthesis, accounting for 63 and 52% of the total Zn uptake for IR68144 and IR64, respectively. Without supply of external Zn, at vegetative or reproductive stages, more Zn-68 was retranslocated from old tissues to new tissues in IR68144 than in IR64. Retranslocation of Zn-68 from flag leaves to grains was twice as high in the former when Zn-68 was applied to the flag leaves during booting or anthesis. These results indicate that Zn density in rice grains is closely associated with the ability to translocate Zn from old tissues to new tissues at both early and late growth stages and with phloem remobilization of Zn from leaves and stems to grains.