Zinc (Zn) is an essential element involved in human metabolism, which can be supplied by an appropriate diet. Enhancing Zn enrichment in rice grains through agronomic biofortification is advocated as an immediate and effective approach to combat micronutrient malnutrition in hu-man. It has been well-documented that high grain Zn accumulation in rice can be achieved by Zn fertilizers management. This study evaluated the effects of foliar nitrogen (N) and Zn applied at the flowering and milky stages of brown rice plants with and without soil Zn application. A glasshouse pot experiment was conducted using a completely randomized design with four replicates. Soil Zn in the form of ZnSO₄ was applied at 0 and 50 kg ha⁻¹. Foliar fertilizer of 1% urea along with 0.5% ZnSO₄ was applied and assigned as (1) nil foliar N and Zn (N0Zn0), (2) foliar N with nil Zn (N+Zn0), (3) nil foliar N with foliar Zn (N0Zn+), and (4) foliar N and Zn (N+Zn+) at flowering and milky stages. Foliar application of N and Zn increased grain yield and yield components in both soil Zn conditions. Grain Zn concentration in brown rice was the highest when foliar N and Zn were applied under nil soil Zn conditions; however, grain N concentration decreased by 13.1–28.5% with foliar application at flowering and 18.8–28.5% with application at the milky stage. The grain Zn content was increased by foliar application of N0Zn+ and N+Zn+ at flowering and milky stages. Applying foliar N and Zn at flowering or milky stages tended to increase the grain N content when Zn was applied to the soil, while nil soil Zn decreased the N content by 26.8% at flowering and milky stages under N0Zn+. The results suggest that the milky stage is the most suitable for foliar application of Zn for increasing (i) grain yield and (ii) N and Zn concentrations in brown rice without having a dilution effect.

Improving grain yield and zinc (Zn) concentration yields a double benefit for farmers and consumers, especially when accomplished through the common practice of nitrogen (N) and Zn application. The objective of this study was to evaluate responses of a modern improved rice variety (SPT1) to Zn and N fertilizer management of seed germination, seedling growth, yield, and grain Zn accumulation. A preliminary laboratory study was conducted by priming seeds with variation of N and Zn solutions, consisting of (1) 0% urea + 0% ZnSO₄ (N0Zn0), (2) 0% urea + 0.07% ZnSO₄ (N0Zn+), (3) 0.05% urea + 0.07% ZnSO₄ (N0.05Zn+), (4) 0.10% urea + 0.07% ZnSO₄ (N0.10Zn+), (5) 0.15% urea + 0.07% ZnSO₄ (N0.15Zn+), (6) 0.20% urea + 0.07% ZnSO₄ (N0.20Zn+), and (7) 0.25% urea + 0.07% ZnSO₄ (N0.25Zn+). Priming seeds with N0.15Zn+ led to a higher germination rate and growth performance. Seedling Zn concentration increased linearly along with the dry weights of root and coleoptile during germination. A second experiment in the field included priming the seed with (1) 0% urea + 0% ZnSO₄ (N0Zn0), (2) 0.15% urea + 0% ZnSO₄ (N+Zn0), (3) 0% urea + 0.07% ZnSO₄ (N0Zn+), and (4) 0.15% urea + 0.07% ZnSO₄ (N+Zn+); this experiment showed that simultaneous priming of seeds with 0.15% urea and 0.07% ZnSO₄ (N+Zn+) resulted in the highest coleoptile length and seedling dry weight. The highest seedling Zn concentration was observed when priming seeds with N0Zn+ followed by N+Zn+, but the effect disappeared at the later growth stages. A third experiment in the field was conducted by foliar application with four different treatments of (1) 0% urea + 0% ZnSO₄ (N0Zn0), (2) 1% urea + 0.5% ZnSO₄ (N+Zn0), (3) 0% urea + 0.5% ZnSO₄ (N0Zn+), and (4) 1% urea + 0.5% ZnSO₄ (N+Zn+). The highest grain yield increases were achieved by foliar application of N+Zn0 (28.5%) and foliar application of N+Zn+ (32.5%), as compared with the control (N0Zn0). Grain Zn concentration was the highest under foliar application of N+Zn+, with a 37.9% increase compared with N0Zn0. This study confirmed that seedling growth performance can be enhanced by initially priming seeds with N and Zn solution, while grain yield and Zn concentration can be improved by foliar application of N and Zn fertilizer. The information would be useful for the appropriate combined application of Zn and N fertilizers in the practical field to improve grain yield and Zn accumulation as well as Zn nutrition among humans with rice-based diets. The result should be extended to a wider range of rice varieties under suitable management of N and Zn fertilizer.

The rising interest in foliar nutrient spraying as a strategy to boost crop yields has led to investigations of how such application influences nutrient uptake and accumulation, especially in edible plant parts. Despite its importance, the effects of single versus simultaneous nutrient application on plant absorption, transport, and accumulation have been underexplored. This study addresses this knowledge gap by examining the physiological and molecular responses of rice to foliar application of nitrogen (N) and zinc (Zn) individually and in combination at different growth stages. We assessed how the treatments affect rice grain yield and nutrient accumulation in relation to the expression of Zn transport-related genes. Foliar application of N+Zn+ at the tillering stage resulted in a 62.01% increase in grain yield compared to the control. Additionally, Zn concentrations in brown rice were increased by 26.04% and 34.20% when N0Zn+ and N+Zn+ treatments, respectively, were applied at panicle initiation. Gene expression analysis revealed that the timing and nutrient combination significantly influenced rice productivity and grain Zn concentration. At the tillering stage, the N+Zn+ treatment elevated the expression of Zn transporters such as OsZIP3, OsZIP4, and OsZIP9 in leaves, thereby enhancing grain yield. At panicle initiation, the nutrient treatments influenced a broad range of genes, including OsZIP4, OsZIP9, OsHAM2, OsDUR3, OsAAP1, OsGS1;1, and OsFd-GOGAT, affecting grain Zn and N accumulation. These insights are crucial for developing targeted nutrient management strategies to optimize rice yield and grain nutritional quality for the benefit of consumers.

The objective of this study was to evaluate the responses in grain yield and zinc concentration of wetland and upland rice varieties to Zn fertilizer application and different growing conditions. The wetland (Chainat 1; CNT1) and upland (Kum Hom CMU; KH CMU) rice varieties were grown under waterlogged and well-drained soil conditions with or without Zn fertilizer application. Zinc fertilizer (ZnSO₄) was applied at 0 and 60 kg ha⁻¹ in three stages at tillering, booting, and flowering. In the wetland variety, CNT1, grain yield decreased by 18.0% in the well-drained soil compared to the waterlogged conditions, but there was an 8.9% decrease in grain yield in the waterlogged soil compared to the well-drained soil in the upland variety, KH CMU. Applying Zn fertilizer affected yields differently between the varieties, decreasing grain yield by 11.9% in CNT1 while having no effect in KH CMU. For grain Zn concentrations in brown rice, applying Zn fertilizer increased Zn concentration by 16.5–23.1% in CNT1 and KH CMU under both growing conditions. In the well-drained soil, applying Zn fertilizer increased straw Zn concentration by 51.6% in CNT1 and by 43.4% in KH CMU compared with the waterlogged conditions. These results indicated that the wetland and upland rice varieties responded differently to Zn fertilizer application when grown in different conditions. Applying Zn fertilizer in the appropriate rice variety and growing conditions would help farmers to improve both the desirable grain yield and Zn concentration in rice.