The Higher Average Daily Grain Yield and Its Formation Mechanisms Underlying the Efficient Photo-Assimilate Transport in Rice Ratooning

Studying the physioecological mechanisms behind the high daily productivity of ratoon rice is essential for improving farming strategies and maintaining stable yields in intensive cropping systems. This study compared the physiological traits of main crop rice (MR), ratooning season rice (RSR), and single-cropping mid-late rice (LR) with the same genotype and flowering time as RSR, from 2021 to 2022. The results demonstrated that RSR maintained higher leaf SPAD values than both MR and LR at the heading stage. Moreover, the translocation efficiency of stem-stored non-structural carbohydrates (NSC) to grains was significantly greater in RSR compared to MR (24.18–58.1%) and LR (24.59–63.5%). 13C isotope labeling confirmed that RSR exhibited superior assimilate allocation to grain pools, with values 13.7% and 14.74% higher than those of MR and LR, respectively. Anatomical analysis of vascular bundles at panicle-stem nodes showed an increased phloem area proportion in RSR. Furthermore, RSR exhibited elevated expression levels of genes associated with nitrogen uptake and metabolism regulation (AMTs, NRTs, and NRFs), sucrose transport (SUTs, SWEET11, and INVs), and stress tolerance (TPSs and TPPs), particularly within its grain parts, relative to MR and LR. Enzymatic activities of sucrose synthase (SS), sucrose phosphate synthase (SPS), and invertase in RSR grains were also significantly higher than in MR and LR. Further analysis indicated that these advantageous physiological traits in RSR arise from the elevated gene expression and enzyme activities, which are positively modulated by endogenous hormones including zeatin riboside (ZR), abscisic acid (ABA), and indole-3-acetic acid (IAA). These hormones serve as key factors for the efficient coordination of matter production and translocation in rice ratooning. This study provides important theoretical and practical insights into the formation and regulation mechanisms of high and stable yield in ratoon rice.