Effect of Low Temperature and Weak Light Combined Stress During Panicle Differentiation on Grain Yield and Physiological Property in Rice

Climate change threatens rice production by increasing the frequency of adverse weather conditions, such as continuous rainy and overcast days, which lead to combined low temperature and weak light stress (LTWL) during the rice growing stage. To investigate the impact of LTWL stress on rice grain yield and its physiological mechanisms, we conducted a 2-year study focusing on the panicle differentiation stage. Two rice cultivars were examined: conventional japonica rice and indica-japonica hybrid rice. The experimental treatments consisted of varying durations of LTWL exposure during panicle differentiation, namely T1 (0–7 days), T2 (0–14 days), T3 (0–21 days), T4 (8–14 days), and T5 (15–21 days) in 2021 and 2023, with the addition of T6 (22–28 days) in 2023. In addition, the normal temperature and sunlight treatment were conducted as the control (CK). The results revealed that, compared to the CK treatment, LTWL during panicle differentiation reduced rice grain yield by 6.25%–26.84% for NG9108 and by 3.05%–20.51% for YY2640. This yield reduction was primarily attributed to a decrease in the number of grains per panicle, with NG9108 experiencing a range of 4.60%–22.62% and YY2640 showing a range of 1.76%–20.14%, which resulted from reduced spikelet differentiation and increased spikelet degeneration. Among the 7-day LTWL treatments, the T5 treatment caused the most significant yield loss. Furthermore, as the duration of the LTWL stress increased, the decline in grain yield became more substantial. For the two types of cultivars, conventional japonica rice was more sensitive to LTWL treatments compared to the indica-japonica hybrid rice. Physiological analysis indicated that LTWL treatments enhanced internode elongation and increased leaf SPAD values. Additionally, the activity of antioxidant enzymes was elevated, suggesting a stress response to mitigate oxidative damage. However, LTWL stress also reduced leaf photosynthetic rates and root activity, which collectively contributed to the observed decline in grain yield during panicle differentiation.