Model establishment and feature parameter extraction to capture rice leaf color-changing variations during the later reproductive period

▸ The established leaf color-changing model and extracted parameters quantitatively depict the leaf color-changing process during the later reproductive period. ▸ Significant differences in the leaf color-changing parameters are observed across leaf positions, varieties and nitrogen treatments. ▸ High-yielding varieties are characterized by a later onset time and a slower rate of leaf color-changing. 1 Leaf color-changing during the later reproductive period of rice is directly related to the photoassimilate accumulation and nutrient reuse, finally affects grain filling and yield. This study aimed to explore an assessment model to depict leaf color-changing process, and extract parameters to precisely distinguish leaf color-changing differences among different treatments and varieties. A total of 31 rice varieties were selected as field experiment materials in 2019 and 2023, the SPAD values of flag leaf, 2nd leaf and 3rd leaf were measured after heading, which were normalized to the leaf color index (CI). A functional model for the variation of leaf color index (CI) with time (t) in the late reproductive stage of rice was established based on CI=at 2 +bt+c, and seven color-changing parameters were extracted for quantitative comparison and assessment of leaf color-changing, including three color-changing time related parameters (onset time, T 0 ; midpoint time, T 50 ; and color-changing duration, T 100 ); one leaf color index (final value of CI, CI f ); and three parameters related to color changing rate (color-changing rate during T 0 −T 50 , R 1 ; color-changing rate during T 50 −T 100 , R 2 ; mean color-changing rate, R m ). In 2023, CY927 with dark leaf color and YY1540 with normal leaf color were used as materials and applied with three N fertilizer amounts, to explore the N fertilizer effects on leaf color-changing process through established assessment system. The T 0 of flag leaf was delayed by 2.6−3.0 d compared to the 2nd and 3rd leaves. The CI f of flag leaf was 12.12 and 21.15% higher than that of 2nd and 3rd leaf, respectively. Additionally, the R 1 , R 2 and R m of the 3rd leaves were 10.75−19.82%, 17.99−20.09% and 18.23−11.61% higher than the flag and 2nd leaves, respectively. Rice yield is significantly positively correlated with T 0 , positively correlated with T 50 and T 100 , and negatively correlated with R 1 , R 2 and R m . The average T 0 , T 50 , and T 100 of rice varieties with yields higher than 8000 kg ha −1 were 6.8, 22.2, and 31.8 d, respectively, with the CI f of 0.563 and R m of 0.015 d -1 . N applications delayed T 0 by 4.5−6.2 d, decreased R m by 30.06−32.33%, and increased CI f by 35.78−39.69%. The established leaf color-changing model and the extracted parameter quantitatively depicted the leaf color-changing process during the later reproductive period. They also effectively distinguished the differences in leaf color-changing among leaf positions, varieties and N treatments. This approach is valuable for selecting and cultivating high-yield and nutrient-efficiency rice varieties, as well as for analyzing underlying mechanisms.