Effect of low-temperature stress on glucose and water status in rice germ and its relationship with seedling emergence

To investigate varietal differences in rice germ survival and seedling emergence under low-temperature stress and to analyze the mechanisms of low-temperature tolerance during seedling emergence, germinated seeds of four rice varieties with different cold tolerance were used as experimental material and exposed to low temperature and ambient temperature (control) conditions. Carbohydrate and moisture content of seed and germ, seed α-amylase activity, sucrose synthase, antioxidant enzyme activity, hydrogen peroxide (H2O2) and malondialdehyde (MDA) content of germ during seedling emergence were analyzed over time, along with their correlation with germ survival. The study found that the glucose content and antioxidant enzyme activity (SOD, POD, and CAT) in the germ of the cold-sensitive varieties Zhongjia 8 (S1) and Zhongjiazao 17 (S2) decreased significantly after 4 days of cold treatment, while H2O2 and MDA contents increased gradually, leading to phenotypical stagnation of germ growth and symptoms of death under low-temperature stress. In contrast, the glucose content in the germ of cold-tolerant varieties Nipponbare (T1) and Yunliangyoujiu 48 (T2) remained relatively high, and the enzyme activity of SOD, POD, and CAT, as well as H2O2 and MDA contents, remained stable after 8 days of cold treatment, allowing the germ to maintain upright growth. Analysis revealed that cold inhibited seed α-amylase activity but promoted germ sucrose synthase activity. Cold also delayed root development in cold-tolerant varieties T1 and T2 but inhibited root development in cold-sensitive varieties S1 and S2. Additionally, the root vigor of cold-sensitive varieties was significantly lower than that of cold-tolerant varieties, and the water content of seeds and germs in cold-sensitive varieties S1 and S2 decreased continuously, while that of cold-tolerant varieties T1 and T2 remained stable. Correlation analysis demonstrated that germ survival rate was significantly positively correlated with germ and seed glucose content, germ water content, CAT activity, root length, volume, and root vigor, but significantly negatively correlated with seed starch and germ sucrose content. Moreover, increasing water supply to S1 and S2 exogenously significantly improved their survival rate under low temperature. In conclusion, low-temperature stress not only inhibited seed α-amylase activity but also suppressed root growth and development, reducing the ability to transport water to seeds and germs. This, led to a decrease in the ability to degrade seed starch and germ sucrose during the seedling emergence process, resulting in inadequate glucose supply to germs, an imbalance in germ antioxidant metabolism, and reduced survival rate. However, exogenous increase in water supply could improve germ survival rate to some extent under low temperature.