Spikelet Filling Characteristics in Early-Season Rice Experiencing High Temperatures during Ripening

Spikelet filling characteristics in early-season rice in southern China may be distinctive due to its exposure to high temperatures during the ripening period. However, limited information is currently available on these characteristics. This study aimed to characterize spikelet filling in early-season rice and identify the key factors contributing to its improvement. Field experiments were conducted over two years (2021 and 2022) to mainly investigate the proportions of fully-filled, partially-filled, and empty spikelets, along with the biomass-fertilized spikelet ratio and harvest index, in 11 early-season rice varieties. The results revealed significant varietal variation in spikelet filling, with the proportion of fully-filled spikelets ranging from 60.6% to 81.1% in 2021 and from 66.3% to 79.2% in 2022. Among the 11 varieties, Liangyou 42, Lingliangyou 942, and Liangyou 287 exhibited relatively superior performance in spikelet filling. Linear regression revealed that, although a significant negative relationship existed between the proportion of fully-filled spikelets and both partially-filled and empty spikelets, the relationship with partially-filled spikelets was stronger. Additionally, the proportion of fully-filled spikelets showed a significant positive relationship with the harvest index but not with the biomass-fertilized spikelet ratio. These findings indicate that increasing the harvest index and reducing the occurrence of partially-filled grains are essential strategies for improving spikelet filling in early-season rice.