The change in leaf color during the later reproductive period of rice is directly related to photoassimilate accumulation and nutrient reuse, and it ultimately affects grain filling and yield.  This study aimed to explore an assessment model that depicts the leaf color change process, and extract parameters that can precisely distinguish differences in leaf color changes among different treatments and varieties.  A total of 31 rice varieties were selected as the field experiment materials in 2019 and 2023.  The SPAD values of the flag, 2nd and 3rd leaves were measured after heading, and they were normalized to the leaf color index (CI).  A functional model for the variation of leaf CI with time (t) in the late reproductive stage of rice was established based on CI=at²+bt+c, and seven color change parameters were extracted for the quantitative comparison and assessment of leaf color changes, including three time related parameters for color change (onset time, T₀; midpoint time, T₅₀; and color change duration, T₁₀₀); one leaf color index (final value of CI, CI_f); and three parameters related to the color change rate (the rate during T₀−T₅₀, R₁; the rate during T₅₀−T₁₀₀, R₂; and the mean color change rate, R_m).  In 2023, Chunyou 927 (CY927) with a dark leaf color and Yongyou 1540 (YY1540) with a normal leaf color were used as materials, and three N fertilizer amounts were applied to explore the effects of N fertilizer on the leaf color change process through the established assessment system.  The T₀ of the flag leaf was delayed by 2.6−3.0 d compared to the 2nd and 3rd leaves.  The CI_f of the flag leaf was 12.12 and 21.15% higher than those of 2nd and 3rd leaves, respectively.  In addition, the R₁, R₂ and R_m of the 3rd leaf were 10.75–19.82%, 17.99–20.09% and 18.23–11.61% higher than the flag and 2nd leaves, respectively.  Rice yield was significantly positively correlated with T₀, positively correlated with T₅₀ and T₁₀₀, and negatively correlated with R₁, R₂ and R_m.  The average T₀, T₅₀, and T₁₀₀ of rice varieties with yields higher than 8,000 kg ha⁻¹ were 6.8, 22.2, and 31.8 d, respectively, with a CI_f of 0.563 and an R_m of 0.015 d^–1.  N applications delayed T₀ by 4.5–6.2 d, reduced R_m by 30.06–32.33%, and increased CI_f by 35.78–39.69%.  The established leaf color change model and extracted parameters quantitatively depicted the leaf color change process during the later reproductive period.  They also effectively distinguished the differences in leaf color change among leaf positions, rice varieties and N treatments.  This approach is valuable for selecting and cultivating high-yield and nutrient-efficient rice varieties, as well as for analyzing the underlying mechanisms.

To elucidate the relationship between leaf color-changing and stem NSC translocation during grain filling and their impact on yield formation, two indica-japonica hybrid varieties with distinct leaf color change patterns were planted under three N fertilizer dosages (LN 0 kg ha⁻¹; MN 150 kg ha⁻¹; HN 300 kg ha⁻¹). Leaf color change characteristics, photosynthetic productivity, stem NSC translocation, yield and harvest index were analyzed. The results showed that CY927 (slow leaf color change) achieved 10.45%−21.81% higher yields than YY1540 (fast leaf color change) under high-temperature conditions. Compared to YY1540, CY927 delayed the onset of leaf color-changing (T₀) by 2.1−4.1 d, enhanced the final leaf color indicator at maturation (CI_f) by 16.79−52.25%, contributing to 10.56−42.77% greater aboveground biomass accumulation through higher photosynthetic capacity, but significantly limited stem NSC remobilization, reduced total NSC translocation by 23.78−33.19% and NSC translocation ratio by 14.65−22.19%, resulting in a 2.66−8.43% lower harvest index. N application increased rice yield via a delay in leaf color-changing onset (T₀), a reduced color-changing rate (R_m), a shortened color-changing duration (T₁₀₀), and an improved final color index (CI_f). This retardation of senescence enhanced photosynthetic capacity, which was associated with elevated sucrose content and sucrose synthase activity. However, N reduced stem α-amylase activity (14.83−62.07%) and NSC translocation ratio (5.44−16.30%) in both varieties. Correlation analysis revealed significant positive relationships between T₀ and aboveground biomass (P<0.001), and between T₁₀₀ and stem NSC translocation (P<0.001). In conclusion, rice variety and N application indirectly regulate the dynamic balances between leaf photosynthetic carbon metabolism and stem NSC translocation by influencing the leaf color-changing dynamic, ultimately affecting yield and resource use efficiency. This integrative framework, connecting leaf color-changing, carbon allocation, and yield performance, provides scientific guidance for optimizing rice cultivars and N fertilization strategies.