Increasing the number of seedlings per hill with reduced number of hills improves rice grain quality by optimizing canopy structure and light utilization under shading stress

Under shading stress, increasing canopy light utilization is the primary means of increasing photosynthesis, and in turn, rice (Oryza sativa L.) productivity. Here, an artificial-shading field experiment was conducted to study the influence of planting and light conditions on canopy structure, light utilization by rice plants, and rice grain yield and quality at Wenjiang and Hanyuan, China. The results showed that both grain yield and quality were significantly reduced by shading stress, primarily because of a reduction in the photosynthetic capacity of rice plants. There was no significant difference in grain yield between the methods of increasing the number of seedlings per hill with a reduced number of hills (optimized planting pattern) and traditional high-density planting owing to the decrease in panicle/m², but there was an increase in spikelet filling rate or grain weight under the optimized planting pattern. Using the optimized planting pattern increased the canopy width and leaf area per hill, and decreased the leaf inclination angle, resulting in a more open rice canopy than that under the traditional high-density planting. Furthermore, the optimized planting pattern enhanced light incidence into the mid and bottom layers of the canopy under shading stress. As a result, the net photosynthetic rates of flag and second leaves were significantly increased by the optimized planting pattern under shading stress. This, in turn, increased the head rice rate at Wenjiang and taste value at both sites, and decreased chalky grain rate and chalkiness degree under shading stress at both sites. Therefore, the optimized planting pattern is a viable planting pattern for the maintenance of rice grain yield while improving grain quality through the optimization of canopy structure and light distribution and utilization, under shading stress.