Characteristics of translocation and remobilization of zinc absorbed from different stages into grain in dense rice genotype using a stable isotope tracing technique
Zinc (Zn) is an essential micronutrient for plant and human, and over 40% of children in the world are suffering from Zn malnutrition. Zinc deficiency in human is mainly caused from low Zn concentration and bioavailability in foods or edible parts of crop plants. Biofortification appears to be a most sustainable and cost-effective approach to solve human micronutrient malnutrition. Zinc deficiency is a widespread problem in rice, causing decreased crop yields and nutritional quality. Increasing Zn density in rice grain is important for improving crop yield and human nutrition. Large genotypic differences in Zn concentration in rice grains have been observed, however, the mechanisms of Zn dense acumulation in rice grain is not fully understood. In this paper, 68Zn stable isotope tracing technique was used to compare the translocation of 68Zn absorbed at different growth stages to grain and Zn remobilization between Zn-dense and Zn-indense rice genotype. The results showed that the Zn-dense rice genotype (IR68144) had greater translocation of 68Zn absorbed during early (from seedling to tillering stage) and late (from grain filling to harvest stage)growth stages, as compared with the indense genotype (IR64). More than half of Zn distributed in grains was translocated from those absorbed by rice plant before anthesis, accounting for 63% and 52% of the total for IR68144 and IR64, respectively. Even after grain filling, plenty of Zn could be transported into grains, with a two fold higher distribution rate to brown rice for IR68144 than for IR64. The Zn-indense rice genotype IR64 contained more 68Zn in roots whereas the Zn-dense rice genotype IR68144 contained more 68Zn in stems, leaves, and grains when 68Zn was supplied at all stages. The remobilization of 68Zn within the plant varied greatly between the two genotypes. When stopping 68Zn supplying at tillering stage, we found that more 68Zn was retranslocated into the new growing leaves and tillers, and more 68Zn was exported from the full-expanded leaf in the dense genotype than in the indense genotype. When stopping 68Zn supply at Anthesis stage, it was observed that more 68Zn was retranslocated into the growing grain, and more 68Zn remobilized from stem and flag leaf in IR68144 than in IR64. The retranslocation of 68Zn from the flag leaf to grain was found to be twice greater in IR68144 than in IR64 when applying 68Zn on the flag leaf either at booting stage or anthesis stage. These results indicated that the Zn-density in rice grain is closely associated with the ability of Zn translocation from “the source” to “the sink” at both early and late growth stages and phloem mobility of Zn, especially from stem to grain.