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.

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.

While nitrogen is essential for rice production, excessive application promotes lodging, threatening yield stability. In this study, we evaluated the impact of three nitrogen application rates (105, 165, and 225 kg ha⁻¹) on yield formation and lodging resistance in two elite late-season indica rice varieties—Meixiangzhan 2 (MXZ2) and Taiyou 871 (TY871). Our findings demonstrate that (1) elevated nitrogen increased productive panicle number but reduced grains per panicle and percentage of filled grains. (2) High nitrogen inputs substantially elevated the lodging risk, with the lodging index increasing by 20.4% and 45.7% in MXZ2, and by 15.4% and 38.3% in TY871, at 165 and 225 kg ha⁻¹, respectively. (3) Reduced mechanical strength—associated with impaired morphological structure (e.g., increased plant height and height of gravity center), diminished structural carbohydrate content in sheaths and culms, and expanded pith cavity area—collectively contributed to the increased lodging susceptibility with elevated nitrogen. (4) MXZ2, though lower-yielding, exhibited greater lodging resistance than TY871, owing to its superior culm anatomy and sheath strength. (5) Interannual climate variation—particularly low temperature and light intensity in 2020—amplified the negative effects of high nitrogen. Our results provide insights into nitrogen-driven trade-offs between yield and lodging, supporting tailored nitrogen management strategies for indica rice under varying environmental conditions.

Water availability is one of the most limiting factors in crop production, and understanding crop water use efficiency is especially important in environments where changes in crop performance are driven by water availability. The stable isotope δ¹³C has been proposed to evaluate the water management and physiological water use efficiency (WUE) for upland crops. However, this technique has rarely been used for rice, especially for water-saving management systems such as the Ground Cover Rice Production System (GCRPS), which can significantly improve WUE. Therefore, a two-year lowland rice field experiment was conducted to evaluate the effects of sampling time, plant organ and yearly climatic changes on δ¹³C and its relationship to WUE. Our study revealed the following: (1) Significant differences in shoot and leaf δ¹³C were found between paddy and GCRPS plants only at the maximum tillering stage but not at the flowering stage. (2) The shoot δ¹³C at maximum tillering showed a significant positive correlation with irrigation (WUE_i) and total water use efficiency (WUE_t), but there was no such relationship at the flowering stage. (3) Shoot δ¹³C was significantly higher in 2013 than in 2014, most likely related to higher solar radiation from transplanting to maximum tillering in 2013, as there were no significant differences between the years in other climatic factors and soil water content. Our results suggest that the δ¹³C of rice shoots sampled at the maximum tillering stage shows the best correlation with crop water use efficiency.