Photosynthesis and yield enhancements in rice by foliar magnesium supply under variable soil nitrogen applications

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.

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 MgSO₄·7H₂O at the jointing and booting stages.

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 Mg₄ (4% MgSO₄·7H₂O), 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_0.4Mg₄, 11.7% higher than N_0.4Mg₀, and the grain weights of N_0.3Mg₄ and N_0.2Mg₄ were 17.0% and 7.9% higher than N_0.3Mg₀ and N_0.2Mg₀, respectively. The highest starch content (759.7 g kg^-1) was observed under the N_0.4Mg₂ and N_0.4Mg₄ treatments. Compared with Mg₀, Mg₂ and Mg₄ 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.

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_0.4Mg₄ is more favorable for improving rice yield and quality, while N_0.2Mg₄ 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.