The mechanism of carbon and nitrogen metabolism under low temperature and low light stress after heading in late indica rice.

Double-season late indica rice frequently experiences low temperature accompanying with low light stress during the grain filling stage in southern China, which alters the carbon and nitrogen metabolism of the rice grains, thereby impacting both grain yield and quality. However, the physiological mechanism is still unclear. A pot experiment using two late indica rice cultivars (high-quality and common-quality rice cultivars) was conducted under control (CK), low temperature (LT) and low temperature and light (LT + LL) to investigate the grain filling, photosynthetic characteristics, carbon and nitrogen metabolic enzymes and related gene expression. The results indicated that both LT and LT + LL treatments primarily reduced grain weight in the two cultivars compared to CK. This effect was particularly pronounced for LT + LL, which showed a significant difference in both superior and inferior grains of the common-quality rice cultivar. This reduction in grain weight was attributed to an average decrease in the photosynthetic rate of rice leaves by 23.5%, a decrease in Fv/Fm by 6.2%, and a decrease in the original grain filling rate by 10.3%. LT and LT + LL treatments decreased the activities of soluble starch synthase (SSS), granules bound starch synthetase (GBSS) and starch branch enzyme (SBE) in the early grain filling stage, and its related gene expression including OsAGPL2, OsAGPS2b, OsGBSSI and OsSSIIIa, and the influence degree was intensified by LT + LL, compared with LT. For nitrogen metabolism, the activity of glutamic oxalic aminotransferase (GOT) also significantly decreased by LT and LT + LL during grain filling in the cultivars, which was relatively lower under LT + LL, and only decreased the glutamic pyruvic transaminase (GPT) in the early grain filling stage but increased in the later period. However, the related gene expressions of OsGOT1B and OsGS1;3 were enhanced significantly by LT and LT + LL treatments, and reached the highest level under LT + LL treatment, implying the disordered process of nitrogen metabolism. The results suggest that low temperature decreased photosynthetic traits to hinder grain filling in the cultivars, mainly derived from the worsened carbon and nitrogen metabolism, and LT + LL combined stress aggravated the damage degree.