Cultivar difference in rice leaf color change during the later reproductive stage and its relationship with grain filling and nitrogen level

To study the relationship between rice leaf color change and grain filling, two indica-japonica hybrids with distinct leaf colors were grown under three N fertilizer dosages (LN, 0 kg ha⁻¹; MN, 150 kg ha⁻¹; HN, 300 kg ha⁻¹). The leaf color change features of flag leaf, 2nd leaf and 3rd leaf, as well as grain filling traits of superior and inferior grains were compared. Compared to cultivar CY167 (normal green leaves) under the same N level, cultivar CY927 (dark green leaves) exhibited delayed leaf color change onset time (T₀) by 0–3.6 d, reduced mean leaf color change rate (R_m) by 1.98%–9.45%, and increased leaf color index at maturity (CI_f) by 3.77%–53.48%. Additionally, CY927 prolonged the grain filling period (D) of inferior grains by 0.6–2.0 d, resulting in a yield increase of 8.13%–25.46%. N supply significantly increased rice yield, primarily by delaying T₀ and reducing R_m of flag leaf, improving initial grain filling potential (R₀) and maximum grain weight (A), delaying the time to reach the maximum grain filling rate (T_max), and prolonging the grain filling activity period (D) of inferior grains. The time interval (T_L-G) between the T₀ of the flag leaf and T_max of inferior grains was negatively correlated with yield (−0.780, P < 0.01). Suggesting that rice yield can be improved by optimizing N fertilizer management to shorten the T_L-G. These findings provide valuable knowledge about the relationship between leaf senescence and grain filling, and benefit the understanding of the physiological mechanisms underlying high-yield rice production.