Rice Genomics and Genetics 2024, Vol.15, No.1, 36-47 http://cropscipublisher.com/index.php/rgg 42 3.3 Changes in physiological parameters related to photosynthesis at high temperatures Under high temperature stress conditions, the photosynthesis-related physiological parameters of rice undergo significant changes, and these changes are directly related to plant growth and yield. One key change is a decrease in photosynthetic rate. Research shows that in high-temperature environments, the photosynthetic rate of rice leaves is significantly reduced. This is due to insufficient supply of CO2 due to closed stomata, which limits the carbon fixation process. At the same time, stomatal closure caused by high temperature also leads to stomatal restriction, making it impossible for rice to maintain normal gas exchange, exacerbating the problem of reduced CO2utilization efficiency (Yuan et al., 2018). On the other hand, high temperature stress also caused changes in chlorophyll fluorescence parameters. For example, chlorophyll maximum photochemical efficiency (Fv/Fm) is often used to assess PSII health. Under high temperature conditions, the Fv/Fm value of rice leaves decreased, suggesting damage to the structure and function of PSII, which may be related to stomatal closure caused by high temperature and oxidative stress caused by heating of leaves (Table 1). Table 1 Changes in various physiological parameters of rice under high temperature stress Physiological parameters Changes under high temperature stress Photosynthetic rate Descending, affected by pore closure and insufficient CO2 supply Stomatal limitation Increase, leading to a decrease in CO2 utilization efficiency Chlorophyll fluorescence parameters FV/FM reduction, reflecting damage to SPII structure and function Impairment of PSII Structure and function may be affected by high temperature stress Photorespiration Increase as a response to the impact of insufficient CO2 supply Photosystem II (PSII) is also a key change under high temperature stress. Studies have found that in high temperature environments, the structure and function of PSII may be damaged, leading to the interruption of photosynthesis. This includes denaturation of proteins and loss of photosynthetic pigments, which in turn affects the stability of the electron transport chain. 4 Functions and Changes of Antioxidant System 4.1 The role of antioxidant enzymes in high temperature stress When rice faces high temperature stress, antioxidant enzymes play a key protective role, helping plants maintain intracellular redox balance and reduce the degree of oxidative damage. An important antioxidant enzyme is superoxide dismutase (SOD), which can catalyze the reduction of superoxide anions and prevent the accumulation of this harmful oxidative substance. Under high temperature conditions, the activity of SOD in rice usually increases significantly to cope with oxidative stress. Studies have found that the increase in SOD activity in rice leaves after high temperature treatment helps remove excess superoxide anions, thereby maintaining the integrity and function of cell membranes. Another important antioxidant enzyme is peroxidase (POD), which is involved in removing hydrogen peroxide and other substances in cells. Under high temperature stress, POD activity in rice often shows an upward trend, which helps slow down oxidative damage to cell membranes. Research shows that the increase in POD activity in rice under high temperature conditions synergizes with the antioxidant system to help maintain a stable redox state in cells and reduce the damage caused by oxidative stress. CAT (Calase) is another antioxidant enzyme whose main function is to break down hydrogen peroxide into water and oxygen. Studies have found that the activity of CAT in rice will also significantly increase under high temperature stress, which helps to remove reactive oxygen species such as hydrogen peroxide and protect cells from oxidative damage. Experimental data show that after high temperature treatment, the increase in CAT activity in rice is closely related to alleviating oxidative stress in leaves and maintaining the integrity of the chloroplast structure (Kang et al., 2021). Ascorbate peroxidase (APX) is also a key antioxidant enzyme in rice under high temperature stress. Studies have shown that high temperature treatment leads to an increase in APX activity in rice. Experimental data show that under high temperature stress conditions, the activity of APX in rice leaves increases significantly, which helps maintain intracellular redox balance and reduce oxidative damage caused by high temperature.
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