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, CIf); 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, Rm). 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 CIf 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 Rm 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 Rm. 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 CIf of 0.563 and an Rm of 0.015 d–1. N applications delayed T0 by 4.5–6.2 d, reduced Rm by 30.06–32.33%, and increased CIf 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.
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The aim of the study was to evaluate the differences between rice yield and its components, as well as quality of double cropping early japonica rice which was selected from various accumulated temperature zones in cold region to plant in the lower reaches of the Yangtze River, so as to provide a theoretical basis to select the proper japonica varieties as the double cropping early season rice in this area.
Field experiments were carried out at Taizhou (121°13′ E, 28°78′ N) and Hangzhou (119°94′ E, 30°08′ N) of Zhejiang province in 2018, 2021 and 2022. Six (2018), thirteen (2021) and forty-one (2022) japonica varieties were selected from different accumulated temperature zones in Heilongjiang province, respectively, and the early indica rice Zhongzao 39 was picked out as a control in 2018 and 2022. The differences of productive panicle number, spikelets number per panicle, percentage of filled grain, thousand grains weight, number of spikelets unit area, yield, and harvest index were analyzed among double-cropping early japonica and indica rice; the quality indexes were compared among this two types rice too, such as head rice rate, diaphaneity, gel consistency, amylose content, chalkiness degree, and chalky grain rate.
The harvest dates of early japonica rice with beforehand sowing were about 0-15 days earlier than that of local double-season early indica rice. The total mean yield of early japonica rice was 6 637.77 kg·hm-2 in 2018; and the yield of Kongyu 131 were the highest with 7 724.70 kg·hm-2. Moreover, the field production verification in 2022 suggested that the yield of Kongyu 131 could reach 7 194.77 kg·hm-2. The averages of chalkiness degree, diaphaneity, gel consistency and amylose content in all varieties met the quality of second-class edible rice variety criterion, but the head rice rate was low. The total mean yield early japonica rice was 6 630.45 kg·hm-2 in 2022, in order as follows: Longken 257 (8 324.99 kg·hm-2), Longken 263 (8 170.94 kg·hm-2), and Liandao 1 (8 108.34 kg·hm-2). Compared with Zhongzao 39, the productive panicle number in early japonica rice were higher, however, the spikelets number per panicle, percentage of filled grain, thousand grains weight were lower than the control, which induced the yield decrease of 54.75%-93.28%. The average yield of the third accumulated zone was higher than that of other accumulated zones due to the relative superior percentage of filled grain. The correlation analysis within yield and its components of different japonica cultivars showed that, the yield had significantly positive correlation with productive panicle number (P<0.05) and number of spikelets unit area (P<0.01).
It was practicable to plant double cropping early japonica rice varieties in the Yangtze River area selected from different accumulated temperature zones in Heilongjiang province, confirmed by appropriate performances on growth duration, yield and quality. It was suggested that Kongyu 131, Longken 257, Longken 263, and Liandao 1 could be selected as double cropping early japonica rice for the next large-scale demonstration planting.
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