Model Establishment and Feature Parameter Extraction to Capture Rice Leaf Color-Changing Variations During the Later Reproductive Period

Abstract

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. 31 varieties were selected as field experiment materials in 2019 and 2023, the changes of SPAD values of rice flag leaf, 2nd leaf and 3rd leaf were measured at 3-day intervals after heading, and the SPAD values were normalized to the leaf color index (CI). Three different mathematical functions were used to fit the change of CI, and the function model with the best fitting was screened out to extract the parameters of leaf color-changing. All tested varieties were clustered according to the leaf color-changing parameters, and the color-changing parameters and yields of varieties with different characteristics of leaf color-changing were compared. 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. 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 = at2 + 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, T0; midpoint time, T50; and color-changing duration, T100); one leaf color index (final value of CI, CIf); and three parameters related to color changing rate (color-changing rate during T0−T50, R1; color-changing rate during T50−T100, R2; mean color-changing rate, Rm). Significant differences were observed in the parameters of leaf color-changing among leaf positions, varieties and N treatments. The T0 of flag leaf was delayed by 2.6−3.0 d compared with that of 2nd and 3rd leaf, CIf of flag leaf was 12.12% and 21.15% higher than that of 2nd and 3rd leaf, respectively, and the R1, R2 and Rm of the 3rd leaf was 10.75%−19.82%, 17.99%−20.09% and 18.23%−11.61% higher than the flag and 2nd leaf, respectively. Rice yield is significantly positively correlated with T0, positively correlated with T50 and T100, and negatively correlated with R1, R2 and Rm. The average T0, T50, and T100 of rice varieties with yields higher than 8000 kg ha−1 were 6.8 d, 22.2 d, and 31.8 d, respectively, with the CIf of 0.563 and Rm of 0.015 d-1. N applications delayed T0 by 4.5−6.2 d, decreased Rm by 30.06%−32.33%, and increased CIf 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, as well as accurately distinguished the differences in leaf color-changing among leaf positions, varieties and N treatments, which is useful in selections and cultivations of high-yield and nutrient-efficiency rice varieties, as well as mechanism analysis.

Keywords: Rice, Leaf color-changing, Grain filling, Model, Feature parameters