Planting density affects yield of indica-japonica hybrid rice by regulating population and photosynthetic characteristics

To increase the yield of indica-japonica hybrid rice, it is crucial to explore their photosynthetic and population characteristics at different yield levels and quantify their response to varying planting densities. In this study, a two-year field experiment was conducted to test two indica-japonica hybrid rice varieties with different yield potentials (12 t ha⁻¹ and 15 t ha⁻¹, respectively) at four planting densities (D1: 21 cm × 30 cm; D2: 18 cm × 30 cm; D3: 16 cm × 30 cm; D4: 14 cm × 30 cm). High yield indica-japonica hybrid rice was primarily charcterized by a high number of spikelets per panicle and a greater spikelet weight, which increased the single panicle’s weight. In addition, rational material translocation and a coordinated source-sink relationship contributed to dry matter accumulation. Open plant morphology optimized leaf enzyme activity at all stages, improved photosynthesis, and increased yield by 20.54%–21.60%. Increasing the planting density of indica-japonica hybrid rice somewhat restricted the growth of the rice population, leading to decreases in spikelets per panicle, 1000-grain weight, seed-setting rate, plant height, length of the top three leaves, leaf width, leaf angle, and the number of primary and secondary branches. However, the higher number of basic seedling resulted in more effective panicles and an increase in total spikelets, increasing the yield of each variety by 4.17%–8.48% and 2.39%–12.71%, respectively. Optimum dense planting of indica-japonica hybrid rice will benefit the sink capacity and a synergistic increase in both yield and economic benefit. This study offers crucial theoretical insights and practical significance for increasing indica-japonica hybrid rice yield and ensuring food security.