Abstract
INTRODUCTION: Magnesium (Mg) and nitrogen (N) are both essential elements for plant growth, and their synergistic interactions in plants have been widely reported. This study aims to clarify how foliar Mg application enhances photosynthesis performance, the transport of photosynthetic products, and yield formation under variable soil nitrogen applications. METHODS: Rice was cultivated in soil supplied with 0.4, 0.3, and 0.2 g kg(-1) N as urea in a pot experiment and subjected to foliar Mg application of 0%, 2%, and 4% as MgSO(4)·7H(2)O at the jointing and booting stages. RESULTS: The results demonstrated that foliar Mg application improved the net photosynthetic rate and soluble sugar content in leaves, and increased the starch, protein, and dry weight of grains. The increases were more pronounced under Mg(4) (4% MgSO(4)·7H(2)O), particularly when combined with the N(0.3) or N(0.2) treatments. For example, the highest grain weight (151 g pot(-1)) was recorded under N(0.4)Mg(4), 11.7% higher than N(0.4)Mg(0), and the grain weights of N(0.3)Mg(4) and N(0.2)Mg(4) were 17.0% and 7.9% higher than N(0.3)Mg(0) and N(0.2)Mg(0), respectively. The highest starch content (759.7 g kg(-1)) was observed under the N(0.4)Mg(2) and N(0.4)Mg(4) treatments. Compared with Mg(0), Mg(2) and Mg(4) increased starch contents by 0.44% and 0.45% under the N(0.4) level, by 0.35% and 0.57% under the N(0.3) level, and by 2.21% and 1.71% under the N(0.2) level, respectively. Mg application effectively regulated the photosynthetic framework by strengthening maximal fluorescence (Fm), photosystem II (PSII), reaction center pool size (Area), quantum yield of electron transport (φEo), and the total performance index (PI(total)) for energy conservation from exciton trapping to the reduction of photosystem I. DISCUSSION: This enhancement in photosynthetic efficiency promoted carbohydrate accumulation, thereby providing a physiological basis for improvements in grain yield and quality. Overall, our research demonstrated that N(0.4)Mg(4) is more favorable for improving rice yield and quality, while N(0.2)Mg(4) is more beneficial to raising nitrogen use efficiency. These findings prove the agronomic significance of foliar Mg application under variable soil nitrogen regimes for sustainable rice production.