Decreased anther dehiscence contributes to a lower fertilization rate of rice subjected to shading stress

Shading stress has become a worldwide phenomenon and can lead to rice (Oryza sativa L.) yield loss by decreasing the seed setting percentage (the percentage of filled spikelets to total spikelets per panicle), which depends primarily on spikelet fertility. However, the effect of shading stress on rice spikelet pollination and fertilization processes is not fully understood. Here, a field experiment, spanning two years (2019 and 2020), was conducted using two rice varieties (Huanghuazhan and Fyou 498) grown under full sunlight and 53 % shade. No significant difference in pollen I2-KI staining rate was observed between the full sunlight control and the shading treatment in 2019 and 2020. However, shading stress significantly decreased the in vitro pollen germination rate of spikelets that opened 5 d and 8 d after shading in 2020. Shading stress also significantly reduced the anther dehiscence rate of both varieties in both years (except for Huanghuazhan at 2 d after shading in 2019), and a mass of pollen grains remained in the anthers. Together with a reduction in the stigma exsertion rate, shading stress reduced the number of pollen grains per stigma by 29.4–71.2 % and the number of germinated pollen grains per stigma by 9.8–63.2 %. Consequently, shading stress significantly decreased the fertilization rate in these varieties, which contributed to a marked yield loss. We assert that shading stress decreased the fertilization rate by inhibiting key pollination and fertilization processes, particularly anther dehiscence and pollen grain release.