Well-controlled soil drying mitigates starch synthesis impairment in inferior spikelets of rice under high-temperature stress

This study investigates the roles and mechanisms through which controlled moderate soil drying (MD) mitigates high-temperature (HT)-induced impairments in rice starch synthesis. Two rice cultivars were grown under normal-temperature (NT) and HT conditions with traditional well-watered (WW) and well-controlled MD regimes during grain development. Results showed that under HT conditions, MD effectively promoted root water uptake, improving water homeostasis and photoassimilate accumulation. Moreover, MD enhanced free polyamine biosynthesis (e.g., spermidine, spermine), increasing the activities of enzymes that convert sucrose to starch. This alleviated starch synthesis impairment and improved the physico-chemical properties of starch in late-flowering inferior spikelets, and this effect of MD was more pronounced under HT stress. Thus, well-controlled MD provided superior protection against HT-induced starch synthesis disruption in inferior spikelets, thereby reducing yield loss and quality deterioration compared with traditional WW.