Soil Zn availability and supply determine the success of rice grain Zn biofortification program. In flooded rice production system however, soil Zn availability is limited due to immobilization of Zn into unavailable forms. The present study investigated the effects of genotypes and planting density on soil Zn forms and distribution and plant Zn uptake under Zn-deficient and Zn-sufficient field soils during maturity. Rice genotypes with differing tolerance to Zn deficiency [Zn –biofortified (ZnB), Zn-efficient (ZnT) and Zn-inefficient (ZnS)] were grown at two planting densities [D1 (one plant per hill) and D5 (five plants per hill)] in BAY (Zn-deficient) and IRRI (Zn-sufficient) soils in the Philippines. Results revealed that Zn forms were significantly altered by genotypes rather than planting density where water-extractable Zn (W-Zn) and exchangeable Zn (Exc-Zn) in soils grown with ZnB had significantly higher concentrations than ZnS. This was translated into better (P<0.01) Zn concentrations and Zn uptake in the plant parts of ZnB compared with both ZnS and ZnT genotypes. Moreover, W-Zn showed significantly positive correlations with plant parts Zn concentration and Zn uptake, suggesting that W-Zn can effectively predict plant Zn uptake across soils. Interestingly, carbonate bound Zn (Car-Zn) and organic bound Zn (Org-Zn) consistently showed negative correlations with W-Zn, plant parts Zn concentration and Zn uptake suggesting that ZnB and ZnT genotypes can solubilize strongly bound Zn such as carbonates and organic matter to meet the demand for Zn during flowering or maturity. These results highlight the significance of understanding soil Zn mobility, composition and supply for an effective and sustainable agronomic management of high grain Zn rice genotypes towards a successful grain Zn biofortification program. Location: Bay and IRRI Years: 2015 Season Wet Season

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