Delayed sowing date improves the quality of mechanically transplanted rice by optimizing temperature conditions during growth season

Given the expected increase in global warming, rice production has been predicted to face increased frequency and intensity of high temperature (HT) stress in the future. Adjusting the sowing date is a low-cost and helpful strategy for overcoming the negative impact of HT; however, the effect of delayed sowing on rice quality is not well understood. In this study, field experiments with two varieties were conducted at Nanbu, Shehong, Anzhou, and Dayi in Sichuan Province in 2018 and 2019. Seeds were sown at conventional sowing date 1 (CS1), conventional sowing date 2 (sowing 10 days after CS1), delayed sowing date 1 (sowing 30 days after CS1), and delayed sowing date 2 (sowing 40 days after CS1). The amylose content (AC), protein content (PC), and temperature conditions during the rice growth season could explain 74.5% and 74.6% of the total variation in rice chalk grain rate (CGR) and chalkiness degree (CD), respectively. Delayed sowing date treatment markedly increased the daily mean (MT), maximum (MaT), minimum (MiT), and effective accumulated temperatures before heading but decreased these temperatures (including temperature difference) after heading. These changes contributed to an increase in AC and a decrease in PC, CGR, and CD of rice. The findings demonstrated that MT, MaT, and MiT values greater than 23.1 °C, 28.3 °C, and 19.4 °C before heading, and lower than 25.7 °C, 31.1 °C, and 22.2 °C after heading are necessary for improving the quality of mechanically transplanted rice. The study suggests that attaining optimal temperature conditions to improve the grain quality of mechanically transplanted rice in southwest China can be achieved by delaying the sowing date.