High temperature frequently occurs during rice’s early grain-filling period in the south of China, negatively affecting rice yield and quality and posing a major threat to local rice production. This experiment researched the influence of 3.5 °C warming during the first 20 grain-filling days on rice yield and quality and emphatically investigated the effects of the low-broadcast nitrogen fertilizer application level (LBN), high-broadcast nitrogen fertilizer application level (HBN) and foliar nitrogen fertilizer application (FN) at heading on the rice organ temperature, leaf photosynthesis, chlorophyll fluorescence, yield and grain quality, pasting and thermal properties under high temperature in 2020 and 2022, with a widely planted japonica rice variety, “Wuyunjing31”, in order to explore the practical mitigation measures for reducing the adverse impact of high temperature on rice productivity. The results showed that high temperatures during grain filling increased the rice plant temperature, damaged the chlorophyll fluorescence system and decreased the net photosynthesis rate. This led to a decline in the seed-setting rate and grain weight, resulting in a 7.0% and 13.9% yield loss in 2020 and 2022, respectively. In addition, high temperature caused a decline in the head rice rate and an increase in chalk occurrence and pasting temperature, thereby deteriorating rice grain quality. Under high temperatures, HBN enhanced the rice yield by 3.6% and 13.0% in 2020 and 2022, respectively, while FN enhanced the rice yield by 11.5% in 2022. The increase in yield was linked to the increased seed-setting rate and 1000-grain weight. LBN did not significantly affect the rice yield under high temperatures. The positive effects of nitrogen fertilizer measures on rice yield were associated with their role in lowering plant temperature and protection against the damage to the chlorophyll fluorescence system. All three nitrogen application measures generally improved rice milling quality and appearance quality under high temperature, with HBN generally showing the greatest impact. Under high temperature, LBN and FN tended to make the texture of cooked rice softer due to the decreased consistency, retrogradation enthalpy and retrogradation percentage, and this was closely associated with the decline in amylose content. In summary, nitrogen supplementation at the heading could efficiently mitigate the adverse impact of high temperature during the early grain-filling period on rice yield and quality.

Rice–crayfish rotation expanded rapidly in China, as it provides considerable profit and reduces pesticide application. This study investigated the impact of different mechanical transplanting methods and planting geometry on the yield and lodging resistance of an indica rice variety under a rice–crayfish rotation system in 2018 and 2019 in Hongze Lake district, using an excellent-quality variety Taoyouxiangzhan. Seedlings were mechanically transplanted using two mechanically transplanted carpet seedling (MTCS) with equal spacing (30 cm) at five spacings and mechanically transplanted pot seedling (MTPS) with wide and narrow rows (23 cm + 33 cm) at five spacings. The yield of MTPS was 8.3% higher across 2 years under the most optimum density than MTCS using the same density, mainly due to a significantly increased number of spikelets per panicle. Accompanied by the decrease in planting density, rice yield first increased and then decreased under each mechanically transplanting method, and reached the maximum in the A4 and B4 treatments (63.9 × 10⁴ seedlings ha⁻¹), respectively. Compared with MTCS, MTPS significantly increased the fresh weight of single stem, plant height, gravity center height, ear weight, length and area of the upper three leaves, so as to increase the bending moment at basal internodes by whole plant (WP), while it also increased the stem diameter, wall thickness and stem and leaf sheath dry weight per unit length of the second internode at the base, resulting in a significant increase in the force applied to break the base segment (F) and the bending moment of whole plant (M). The higher increase percentage in M than WP led to a decreased lodging index. The lodging index of both mechanically transplanting methods reduced with declined planting density in virtue of the improved breaking load of the basal internode, and it was primarily a result of increased stem diameter, wall thickness and plumpness of basal internode. The balance between the high yield and lodging resistance of Taoyouxiangzhan under a rice–crayfish rotation system can be realized at 63.9 × 10⁴ seedlings ha⁻¹, with MTCS at 15.7 cm plant spacing and MTPS at 16.8 cm. Generally, MTPS showed obvious advantages in yield and lodging resistance than MTCS.