Planting density affects the yield and quality of indica-japonica hybrid rice by regulating grain filling characteristics

Planting density and grain filling are important cultivation practices and physiological processes that affect the yield and quality of indica-japonica hybrid rice. Elucidating how planting density influences the yield and quality of indica-japonica hybrid rice by regulating physiological processes such as grain filling, and revealing the relationships between grain filling characteristics and yield as well as quality, is of great significance. This study selected two indica-japonica hybrid rice varieties with different yield levels as experimental materials and conducted a two-year field experiment under four planting densities. The results showed that increasing the planting density of two indica-japonica hybrids affected photosynthesis, enzyme activities, and grain filling rate, reducing the brown rice rate, milled rice rate, head rice rate, protein content, and amylose content, while increasing chalkiness. However, the increase in tiller number enlarged the population and extended the grain filling duration. Consequently, the yield of the two hybrids increased significantly by 5.47%–11.38% and 4.76%–10.93%, respectively, and the taste values of superior grains (SG) and inferior grains (IG) increased by 0.83%–5.35%, 0.54%–2.45% and 0.96%–2.24%, 0.73%–2.65%, respectively. Analysis of the relationships between grain filling characteristics and yield as well as quality in indica-japonica hybrid rice revealed that a higher grain filling rate contributed to increases in brown rice rate, milled rice rate, head rice rate, protein content, and amylose content, while reducing chalkiness degree and chalky grain percentage. In contrast, a longer grain filling duration not only helped improve rice taste value but was also one of the main reasons for high yield.