Hidden Stigmas Enhance Heat Resilience: A Novel Breeding Trait for Sustaining Rice Spikelet Fertility Under Nocturnal Heat Stress

Heat stress during the flowering stage induces a remarkable decrease in rice spikelet fertility, mainly due to poor pollination manifesting as insufficient pollen deposited on the stigma. It is hypothesized that stigma exsertion, which confers a pollination advantage, may enhance pollen reception and improve female reproductive success under heat stress. The present study aimed to investigate the role of stigma exsertion in spikelet fertility under nocturnal heat. Four rice cultivars exhibiting distinct heat tolerance and twenty rice cultivars with varying degrees of stigma exsertion were grown and subjected to high nighttime temperature treatment at anthesis, in 2023 and 2019, respectively. Heat-tolerant rice cultivars had a relatively low percentage of spikelets with exserted stigmas, and vice versa. Under nocturnal heat stress, rice cultivars exhibiting higher stigma exsertion showed significantly greater reductions in spikelet fertility compared to lower stigma exsertion cultivars. The spikelet fertility of rice cultivars with a higher degree of stigma exsertion was reduced more seriously than that of cultivars with a lower degree of stigma exsertion. Rice spikelet fertility positively correlated with the percentage of hidden stigmas, and exogenous substance-induced increased stigma exsertion led to reduced spikelet fertility under nocturnal heat. These results indicate that a hidden stigma contributes to higher spikelet fertility, while increased stigma exsertion aggravates spikelet sterility in rice cultivars under nocturnal heat conditions. It is proposed that hidden stigmas could serve as a novel breeding trait for sustaining rice spikelet fertility against nocturnal heat stress.