Core Ideas Rice grain‐Zn concentration increased with more aerobic soil conditions. Fertilizer‐Zn applied to soil had limited effect on rice grain‐Zn concentration. Rice paddy water management affected soil redox potential and DTPA‐extractable Zn. Combined water and Zn management are useful for agronomic Zn biofortification. Agronomic biofortification was not sufficient to meet rice Zn nutrition targets. Increasing rice (Oryza sativa) grain‐Zn concentration is important for achieving improved human nutrition. Our objective was to understand how agronomic management practices, including water management and fertilizer‐Zn application rate and timing, affect plant growth, grain‐Zn concentration, and yield of rice genotypes. In a series of four‐field experiments over three seasons, we tested multiple combinations of water management techniques and fertilizer‐Zn application techniques. The use of alternate wetting and drying (AWD) water management increased (p < 0.001) soil redox potential and diethylene triamine pentaacetic acid (DTPA)‐extractable soil Zn compared with continuous flooding (CF) in the 0‐ to 2‐cm soil depth, but not always in the 2‐ to 10‐cm depth. On average, AWD grain‐Zn concentration increased 9% over CF without any yield penalty. Fertilizer‐Zn application increased DTPA‐extractable Zn only in the top soil layer and only temporarily, with a corresponding increase in grain‐Zn concentration only at rates > 10 kg Zn ha−1. Different timings of fertilizer‐Zn application (from basal to flowering) had no effect on grain‐Zn concentration or yield. Overall, our results indicated that AWD had a consistent and larger positive effect than fertilizer‐Zn application on grain‐Zn concentration. However, the increase in grain‐Zn concentration due to fertilizer‐Zn or water management was small, up to 2 mg Zn kg−1 brown rice, implying that improved agronomic management alone is not sufficient to meet the target increase of at least 10 mg Zn kg−1, but is a useful complementary strategy for enhancing the performance of Zn‐enriched rice by improving soil‐Zn availability.

Understanding Zn uptake dynamics is critical to rice grain Zn biofortification. Here we examined soil Zn availability and Zn uptake pathways as affected by genotype (high-grain Zn varieties IR69428 and IR68144), Zn fertilization and water management in two pot experiments. Results showed significant interactions (P < 0.05) between genotypes and Zn fertilization on DTPA (diethylenetriaminepentaacetic acid)-extractable soil Zn from early tillering to flowering. DTPA-extractable Zn in soils grown with IR69428 was positively correlated with stem (r = 0.78, P < 0.01), flagleaf (r = 0.60, P < 0.01) and grain (r = 0.67, P < 0.01) Zn concentrations, suggesting improved soil Zn availability and continued soil Zn uptake by IR69428 even at maturity. Conversely for IR68144, DTPA-extractable Zn was positively correlated only with leaf Zn uptake (r = 0.60, P < 0.01) at active tillering, indicating dependence on remobilization for grain Zn loading. Furthermore, the highest grain Zn concentration (P < 0.05) was produced by a combination of IR69428 and Zn fertilization applied at panicle initiation (38.5 μg g−1) compared with other treatments (P < 0.05). The results highlight that Zn uptake behavior of a rice genotype determines the fate of Zn from the soil to the grain. This has implications on overcoming Zn translocation barriers between vegetative parts and grains, and achieving grain Zn biofortification targets (30.0 μg g−1).