During the later reproductive period of rice growth, the chlorophyll in the leaves degraded, accompanied by the nitrogen (N) transportation from leaves to panicle, resulting in a change in leaf color from green to yellow. This study aimed to investigate the effects of exogenous N supply on leaf color-changing, N accumulation, N transportation, and N loss of indica-japonica hybrid rice during the grain-filling stage. Two indica-japonica hybrid rice cultivars, Chunyou 167 (CY167) and Chunyou 927 (CY927), which exhibited significant differences in leaf color-changing during the grain-filling stage, were selected as materials for field experiment and hydroponic experiment with low, medium, and high N treatments (LN, MN, and HN). The dynamic changes in SPAD value from heading to maturity were measured and fitted with quadratic function to extract leaf color-changing parameters; labeled ¹⁵N was used as N source after heading to trace the source of N in the panicle and the remobilization of vegetative organ N. The results showed that 67.37–72.38% of the panicle N was transported from vegetative organs, the N transport efficiency was the upper three leaves > lower leaves > stem, and about 3.1–35.0% of the transported N was lost via volatilization. The effects of exogenous N concentration on N harvest index, N dry matter/grain production efficiency, N reuse efficiency, and N loss were closely related to leaf color-changing parameters. In MN and HN treatment, the N loss was negatively correlated with the onset time of leaf color-changing (T₀) and the final leaf color index (CI_f), but positively correlated with the leaf color-changing rate (R_mean). Increasing the supply of exogenous N increased T₀ and CI_f, but decreased R_mean, N transport/reuse efficiency, N harvest index, and N dry matter/grain production efficiency. Compared to the cultivar CY167 with normal leaf color-changing, the “stay-green” cultivar CY927 had higher T₀, CI_f, and lower R_mean, resulting in less N volatilization loss, lower N harvest index and N transport efficiency, while higher N reuse efficiency. In conclusion, the exogenous N supply affects leaf color by influencing the transportation and reuse of leaf N during the grain-filling stage.

Previous studies have shown that a one-time application of polymer-coated urea (PCU) can increase rice yield and nitrogen (N) uptake. However, the connection between rice root morphology and physiological traits and grain yield and N absorption has still not been well understood. The objective of this study was to explore whether one-time application of PCU could enhance shoot growth, improve plant physiological activity, and ultimately boost rice yield and NUE by optimizing root morphological and physiological traits. In this study, a super-large-panicle indica-japonica hybrid rice variety, Yongyou1540, was cultivated under three N treatments during 2022 and 2023: (1) 0N, throughout the entire growth period, no N fertilizer was applied; (2) LFP, local farmers’ N management practices were followed, using urea as the N source, and N fertilizer management was carried out according to the local farmers’ customary fertilization practices; and (3) PCU, a one-time application of PCU was performed at one day before transplanting. PCU is a controlled-release fertilizer in which urea granules are coated with a synthetic polymer layer; it has been widely used in rice cultivation. In both LFP and PCU treatments, N was applied at a rate of 200 kg N ha⁻¹. PCU is a type of controlled-release fertilizer in which urea granules are coated with a layer of synthetic polymer. Compared to LFP, PCU significantly improved several root morphological traits, including increased deep-root proportion and specific root length (SRL), throughout the entire growth period; increased root length and root length density at heading and maturity; and increased root biomass growth rate from jointing to heading and reduced reduction rate after heading. Additionally, PCU enhanced root oxidative activity (ROA) and increased zeatin and zeatin riboside (Z+ZR) content in both roots and root bleeding sap at the middle and late grain-filling stages. Furthermore, PCU markedly increased the flag-leaf net photosynthetic rate, Z+ZR content in leaves, and activities of key enzymes involved in sucrose-to-starch conversion in grains during the middle and late grain-filling stages. Correlation analysis indicated that root and shoot biomass growth rate showed a significant positive correlation before heading, and that root biomass reduction rate was significantly negatively correlated with shoot biomass growth rate after heading. ROA and Z+ZR content in both roots and root bleeding sap were significantly associated with flag-leaf photosynthetic rate, Z+ZR content in leaves, and the activities of key enzymes involved in the sucrose-to-starch conversion in grains. On average, PCU increased rice yield by 10.0% and agronomic NUE by 46.2%, compared to LFP. These findings suggest that PCU could optimize root morphological and physiological traits, and thereby promote shoot growth, enhance physiological activity, and ultimately increase both rice yield and NUE. Further research could also investigate the potential for combining PCU with other agronomic practices to enhance both rice yield and NUE.