Shading decreases rice yield by impeding grain-filling progress after heading

Shading stress decreases both the yield and quality of rice (Oryza sativa L.), which has become a serious problem in rice production worldwide. However, the effect of shading stress on the grain-filling progress of rice is not well understood. In this study, field experiments were conducted to investigate the effect of shading on the grain-filling progress and grain yield of rice at Hanyuan and Wenjiang in Sichuan, China, between 2018 and 2019. The results showed that shading after heading significantly decreased the spikelet-filling rate and grain weight, as well as the dry weight of grains panicle(-1), 30 d after shading; these traits were closely related to the grain yield of rice. Hence, the grain yield decreased by 13.72-22.54% and 6.86-14.35% at Hanyuan and Wenjiang, respectively. However, the influence of shading stress on the spikelet-filling rate, grain weight, and dry weight of grains panicle(-1) 30 d after shading differed according to spikelet positions. In general, these influences were greater on spikelets at the bottom and middle positions than on those at the top position. Furthermore, the effects of shading stress on rice differed across study sites; shading stress had a greater effect on grain weight at Hanyuan, but a greater effect on spikelet-filling rate at Wenjiang. Therefore, it is critical to improve both spikelet-filling rate and grain weight of spikelets at the bottom and middle positions to maintain the grain yield of rice under shading stress.