Rice Genomics and Genetics 2024, Vol.15, No.1, 36-47 http://cropscipublisher.com/index.php/rgg 40 It is a common phenomenon that the rice seed setting rate decreases under high temperature and rainy weather. Especially after heavy rain days, the rice seeds in the lodging fields are immersed in water for a long time, and the rice absorbs water, which further promotes germination on the ear. The performance of different varieties in the same area is different. Some varieties are relatively resistant to high temperatures, and the yield reduction is obviously not much; the seed setting rate of varieties with large ears is more obvious than that of varieties with small ears. The high-temperature response of rice at different stages will be reflected in the development of growing points. If the early high-temperature drought is not treated, the growth will be slow, the leaves will twist and lose water, and then dry up and die. Post-harvest testing of rice in hot and dry weather during the heading period shows that the 1,000-grain weight is generally reduced. Varieties with high 1,000-grain weight have a significant reduction, while varieties with low 1,000-grain weight have a relatively small reduction. For example, the chalky grain rate and chalkiness of rice in Sichuan and Chongqing in 2022 are significantly higher than in 2021, and the polished rice rate is also significantly lower. Rice mills are less willing to purchase local rice, and there are also findings in the rice milling that year, 100 kg of rice can only yield about 50 kg of whole rice, and the taste has also deteriorated (Jin et al., 2022). High temperature stress adversely affects the development of rice reproductive organs such as inflorescences, spikelets, and pollen. High temperatures may lead to reduced inflorescences, abnormal pollen development, and abnormalities in the fertilization process. These changes directly affect the quantity and quality of grains, resulting in reduced yields. These physiological changes are interrelated and jointly affect the growth and yield of rice under high temperature conditions. Understanding these changes is crucial for developing high temperature stress management strategies and developing high temperature resistant rice varieties. 3 Changes in Photosynthesis under High Temperature Stress 3.1 Effect of high temperature stress on key enzymes Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is a key enzyme involved in the carbon fixation process in photosynthesis. It is widespread in plants, algae and some bacteria. The effect of high temperature stress on Rubisco enzyme can affect the growth and productivity of plants and other photosynthetic organisms, because Rubisco plays a key catalytic role in photosynthesis (Yuan et al., 2018). High temperature may cause the activity of Rubisco to decrease. This is because high temperature may cause protein denaturation and affect the three-dimensional structure of Rubisco, thereby reducing its affinity for the substrate (ribulose-1,5-bisphosphate). Rubisco's substrate specificity for oxygen and carbon dioxide may also be affected by high temperatures, causing the enzyme to mistakenly use oxygen for the reaction instead of carbon dioxide, producing an oxidation reaction instead of a carbon fixation reaction, which is called photorespiration. Rubisco is the main enzyme for carbon fixation in photosynthesis, and the decrease in its activity may lead to a decrease in carbon fixation efficiency. This will further weaken the efficiency of photosynthesis and affect the photosynthetic capacity of rice. Rubisco enzyme is the key rate-limiting enzyme in the photosynthetic dark reaction stage. When the temperature and CO2 concentration increase, the total activity and protein content of Rubisco enzyme are affected. Under the current atmospheric CO2 concentration, the decrease in Rubisco enzyme activity when the temperature increases has been verified in many plants, including rice (Yuan et al., 2018). The decrease in activity of Rubisco enzyme under high CO2 concentration is not its own inactivation (Rubisco enzyme is still active at 50 degrees) but the decrease in the activation of Rubisco activating enzyme (RCA) caused by changes in ATP energy state (Figure 2).
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