The double-season rice system plays an important role in ensuring food security in China. However, changes in the grain yield and quality of recently released rice varieties are still not fully understood, especially early indica rice (Oryza sativa). In this study, we collected the yield and quality traits of 224 early indica rice varieties released in China’s Hunan and Jiangxi provinces from 2000 to 2020. The results showed that rice grain yield, but not quality, was significantly improved in early indica rice from 2000 to 2020, and the improvement in grain yield was mainly the result of an increased spikelet number per panicle. Quality traits such as head rice rate, chalky rice rate, and amylose content remained stable while the milled rice rate and degree of chalkiness showed adverse changes during early indica rice breeding. These results suggested that improving grain yield, but not quality, has been prioritized during early indica rice breeding in the past few decades. Further analysis showed that the degree of chalkiness had the largest coefficient of variation among the studied quality traits in the 224 early indica rice varieties, indicating that there is still great potential to decrease the degree of chalkiness through rice breeding. Interestingly, the results showed that lower degrees of chalkiness were associated with lower amylose contents and chalky rice rates but with a higher length-to-width ratios and a gel consistency. Therefore, focusing on the degree of chalkiness seems a promising strategy to synergistically improve the quality traits of early indica rice. Overall, our results have value for guiding future research on high-yield and high-quality breeding in early indica rice.

The southern rice-growing region plays a crucial role in ensuring national food security in China. However, rice production in this area is often affected by unfavorable weather conditions such as rainy and dim days, which significantly impact rice yield. Therefore, we conducted two field experiments to explore and compare the effects of climate variations and simulated shading on rice yield and quality. The results indicated that (a) both interannual climate variation and simulated shading had adverse effects on rice yield and quality, (b) the impact of interannual climate variation on yield was less severe compared with simulated shading, but it had a more significant negative effect on rice quality, and (c) different cultivars/quality groups of rice exhibited variations in response to weak solar radiation, with high-quality rice being more susceptible. The findings suggest that in the production of high-quality rice, it is important to select cultivars that are resilient to interannual climate variation and to develop supporting cultivation techniques to cope with growing incidence of weakened solar radiation in the future. Breeders can try to tap into potential weak-light-resistance genes and cultivators can try to use different cultivation methods to determine the optimal water and fertilizer regimes.

Nitrogen (N) management is an important strategy for improving the yield, grain quality, and N use efficiency of rice (Oryza sativa). Exploring appropriate N application rates is essential for high-quality rice production in China, especially in the context of the large extension of these varieties in recent years. Field experiments were conducted to study changes of grain N content and their correlations to yield and yield components using twenty high-quality rice varieties grown at three N application rates (105, 165 and 225 kg ha⁻¹) in 2019 and 2020. Additionally, a micro-plot experiment based on ¹⁵N isotope tracing technique was also conducted with two contrasting high-quality rice varieties of Y-liangyou 911 and Yeiangyoulisi under two N application rates (165 and 225 kg ha⁻¹) in 2021, with grain N accumulation, its subdivision, and N utilization investigated. We found that the grain N content of high-quality rice increased with the increase of N application rates, while there was no consistent response in grain yield. There was a significant quadratic relationship between grain yield and grain N content, while panicles m⁻² and grain-setting rate had a positive and a negative correlation with grain N content, respectively, in both 2019 and 2020. Across three N application rates and two years, the coefficient of variation (CV) of grain N content ranged from 10.36% to 21.26% among twenty varieties, of which Y-liangyou 911 had the smallest CV, and six varieties, including Yexiangyoulisi, had the largest CV. The micro-plot experiment showed that, in comparison with N₁₆₅, the grain N content, grain N accumulation, and N recovery rate increased under N₂₂₅ in both varieties, but a significant increase in grain yield was only observed for Y-liangyou 911. Grain N accumulation derived from panicle N fertilizer and its ratios to total grain N accumulation and the N recovery rate was significantly higher than those derived from basal and tillering N fertilizers. Nevertheless, increasing N application rates had much greater effects on the grain N accumulation derived from basal and tillering fertilizers and on ratios to total grain N accumulation in Yexiangyoulisi than those in Y-liangyou 911. Our results suggested that adopting a moderate N application rate (165 kg ha⁻¹) is conducive to maintaining an appropriate grain N content and achieving higher grain yield and N use efficiency as well as better quality of high-quality rice. Besides, moderately reducing basal and/or tillering N fertilizers is necessary for those varieties with a larger CV of grain N content.

An increased planting density and decreased nitrogen (N) rate combination may obtain a stable yield and enhance N utilization. However, the effects of an increased planting density and decreased N rate combination on the yield and nutrient utilization in different ecological regions are unclear. The aim of this research was to assess the interactive impacts of the N rates and planting densities on double cropping rice yields and nutrient utilization in two ecological regions in field experiments during 2018 and 2019. The results showed that, at Shanggao, increased planting densities of 67% and 200% compensated for the biomass, nutrient uptake and yield losses from N application reductions of 20% and 27% and increased the nutrient utilization of the early and late seasons. However, at Xingguo, compared with the N₂D₁ treatment (165 kg ha⁻¹ with 57 plants per m²), the late rice yield under the N₁D₂ treatment (120 kg ha⁻¹ with 114 plants per m²) decreased by 6.71% and 5.02% in 2018 and 2019, respectively. The photosynthetic rate and nutrient uptake were likely related to the positive interaction on the double cropping rice yield in the two ecological regions. Our results indicate that dense planting is a feasible cultivation strategy to decrease N inputs for double cropping rice, but the low soil nutrient supplies negatively affect stable yields in different ecological regions.

High-quality (i.e., higher appearance and eating quality) rice (Oryza sativa L.) is being increasingly and widely planted in China with the improvement of people’s living standards and the achievement of rice breeding efforts in recent years. However, the agronomic and physiological performances of high-quality indica rice (HQIR) under different nitrogen (N) application conditions in southern China are little known. Two-year consecutive field experiments were conducted with two HQIR and two ordinary-quality indica rice (OQIR) varieties under moderate and high-N application rates, with yield and yield components, biomass, N uptake, and their related traits, being investigated. We found that grain yields of HQIR were slightly decreased, but grain yields of OQIR were significantly increased by 7.0–9.6% under a high N rate, compared with a moderate N rate within two years. Thereby, OQIR produced a 5.7–14.7% and 18.7–25.6% higher grain yield than HQIR under moderate and high N rates, respectively. The different responses of grain yield to N application rates were mainly due to a decreased grain setting rate in HQIR and increased spikelets m⁻² in OQIR under a high N rate. Furthermore, a high N rate significantly reduced pre-anthesis AE (apparent exportation of pre-anthesis stem and leaf blade dry matter) and improved the grain-leaf area ratio, while it did not increase post-anthesis dry matter, compared with a moderate N rate in HQIR, which might result in carbon-metabolic deterioration, an imbalance of the source–sink relationship and, subsequently, a lower supply of carbohydrate in panicle. Our results suggest that a moderate N rate (165 kg N ha⁻¹) is beneficial for the HQIR varieties to balance the maximum grain yield and high quality in southern China.