Water management during the grain-filling period affects rice grain quality due to changes in both the temperature and nitrogen content of paddy water

Water management techniques potentially mitigate rice quality deterioration caused by high temperatures during the grain-filling period. To elucidate the effects of water management practices on rice quality, the effects of paddy water temperature (T w ) and nitrogen concentration (N w ) on the occurrence of chalky grains (CG) were assessed in this study. Experiments were conducted in paddy fields under different irrigation water temperatures and nitrogen concentrations. The T w , N w , grain protein content (GPC), and CG percentage were measured at multiple locations within each paddy field. The data were subjected to path analysis, which consisted of two multiple regression equations: one with CG as the objective variable and the other with GPC as the objective variable. The path analysis indicated that water management influences the CG percentage through three mechanisms: positive, direct effects of T w ; negative, indirect effects of T w ; and negative, indirect effects of nitrogen supply from paddy water to the soil, which is proportional to N w . Moreover, the effects of water management on rice quality varied substantially based on the temperature of and nitrogen concentration in irrigation water. Rice quality improved with the inflow of irrigation water when irrigation water temperature was low but nitrogen content was high; however, rice quality deteriorated when the temperature and nitrogen content of irrigation water were low. The temperature and nitrogen status of irrigation and paddy water, and rice plants should be considered to develop optimal water and fertilizer management techniques for improving rice quality amidst the threat of global warming. • Chalky grains commonly occur because of high air temperatures. • Effects of water management on rice quality were analyzed using path analysis. • Both temperature and nitrogen content of paddy water affect rice quality. • Irrigation water conditions alter the effects of water management on rice quality. • Thermal and nitrogen environments of rice should be considered in water management.