Moderate Soil Water Tension Maintains Flag-Leaf Physiological Activity and Grain Development in Rice

Water-saving irrigation can substantially reduce water inputs in rice production, but excessive soil drying may impair physiological activity during grain filling and reduce yield. This study evaluated continuous flooding (CF), moderate water-saving irrigation (SW; −20 kPa), and intensified water-saving irrigation (IW; −40 kPa) in rice. Flag-leaf relative chlorophyll content and net photosynthetic rate, superior-grain development, yield components, grain yield, irrigation water input, and water productivity (WP) were evaluated. SW reduced irrigation water input by 57.4% relative to CF while maintaining a comparable grain yield (4545.0 vs. 4550.7 kg ha−1), resulting in the highest WP (0.54 kg m−3). In contrast, IW reduced grain yield to 3694.4 kg ha−1. During late grain filling, SW maintained flag-leaf chlorophyll status and photosynthetic activity comparable to CF, whereas IW showed greater reductions at 28 and 35 days after flowering. IW also exhibited lower relative grain volume during the middle-to-late grain-filling period. Plot-level regression analyses showed positive associations among flag-leaf chlorophyll status, photosynthetic rate, relative grain volume, and final grain yield. These results indicate that moderate soil water tension can substantially reduce irrigation requirements while maintaining late-stage physiological activity, grain development, and yield, whereas intensified soil drying provides limited additional water savings at the expense of crop productivity.