Does increased zinc uptake enhance grain zinc mass concentration in rice?

Rice (Oryza sativa) is the worlds' most important cereal and potentially an important source of zinc (Zn) for people who eat mainly rice. To improve Zn delivery by rice, plant Zn uptake and internal allocation need to be better understood. This study reports on within-plant allocation and potential Zn accumulation in the rice grain in four so-called aerobic rice cultivars (Handao297, K150, Handao502 and Baxiludao). Two controlled-condition experiments were carried out, one with a wide range of constant Zn concentrations in the medium and one with a range of plant growth rate-related supply rates. In both experiments, increased Zn supply induced increased plant Zn uptake rate throughout crop development, when expressed as daily Zn uptake (¿g day¿1) or as daily Zn uptake per gram of plant dry matter (¿g g¿1). Zinc mass concentration (ZnMC) in all plant organs increased with an increase in Zn supply but to various degrees. At higher uptake levels, the ZnMC in stems increased most, while the ZnMC in hulled grains (brown rice) increased least. The increase in leaf ZnMC was generally small, but at toxic levels in the medium, leaf ZnMC increased significantly. It appears that regulation of grain Zn loading differs from regulation of Zn loading to other organs. A milling test on seeds of Baxiludao and Handao502 showed that when ZnMC in brown rice increased from 13 to 45 mg kg¿1, ZnMC in polished rice grains (endosperm) also increased from 9 to 37 mg kg¿1 but remained three to five times lower than that in the bran. Irrespective of the ZnMC in the brown rice, around 75% of total grain Zn was present in the endosperm. In both cultivars, there was a major difference in ZnMC between bran and endosperm (120 and 37 mg kg¿1, respectively), suggesting a barrier for Zn transport between the two tissues. There seems to be a second barrier between stem and rachis, as their ZnMCs also differed greatly (300 and 100 mg kg¿1, respectively) in both cultivars at higher plant ZnMC. It is concluded that there is too little scope from a human nutrition perspective to enhance ZnMC in rice endosperm by simply increasing the Zn supply to rice plants because Zn allocation to the endosperm is limited, while observed genotypic differences indicate scope for improvement through breeding