Selection of Rice Cultivars with Superior Photosynthetic Carbon Metabolism and Decreasing Transplanting Hill Spacing Are Crucial for Ensuring Food Security

Improving rice yield and optimizing rice quality are of great significance for ensuring food security. In modern rice production, mechanical transplanting has become the dominant transplanting method. Precise regulation of plant spacing and row spacing contributes to the formation of different transplanting densities, which further exerts effects on photosynthetic spikelets filling, yield formation and quality development of rice. Two-year field experiments were conducted with two conventional japonica rice cultivars of contrasting yield levels under four transplanting hill spacings at a uniform row spacing of 30 cm. The results showed that rice cultivars with higher seed-setting rate with an increase ranging from 1.44 to 1.91% and larger grain weight with an increase ranging from 13.17 to 13.40% presented more prominent yield potential. In addition, high-yield rice cultivars possessed more excellent photosynthetic carbon metabolism characteristics, which effectively improved the spikelets filling process of rice kernels. Superior photosynthetic carbon metabolism characteristics were conducive to increasing head rice rate and reducing chalkiness, while maintaining the duration of spikelets filling benefited the improvement of rice taste value. Narrowing the transplanting plant spacing reduced the physiological enzyme activities in rice leaves and grains, weakened photosynthetic carbon metabolism and hindered spikelets filling, which further decreased head rice rate and protein content but increased chalkiness. Notably, rice taste value also showed an increasing trend. The taste value of superior spikelets (SSs) of the two rice cultivars increased by 1.97–5.11% and 0.98–2.60% respectively, and that of inferior spikelets (ISs) increased by 1.37–3.64% and 1.62–4.12% respectively. Reducing transplanting plant spacing also significantly increased the number of effective panicles, resulting in an increase in population spikelet number. The final yield of the two rice cultivars increased by 5.38–11.62% and 5.23–11.03% respectively.