Plant Gene and Trait 2025, Vol.16, No.4, 152-161 http://genbreedpublisher.com/index.php/pgt 153 2 Physiological Effects of Heat Stress on Tea Plants 2.1 Imbalance in photosynthesis and stomatal regulation High temperatures can deteriorate the photosynthesis of tea plants. The most obvious manifestation is the decline in the carboxylation rate of Rubisco enzyme and the regeneration rate of RuBP. When it is hot, both the optical system II (PSII) and the optical System I (PSI) will be damaged. Among them, PSII is more prone to problems. At this time, photosynthetic parameters such as Fv/Fm, Y(I), Y(II) will decrease, while Y(NO), Y(NA) will increase instead, indicating that electron transfer slows down. In addition, high temperatures can also interfere with the opening and closing of stomata, making it more difficult for tea plants to carry out effective photosynthesis (Huang et al., 2024). 2.2 Enhanced oxidative stress and membrane damage When the temperature rises, reactive oxygen species (ROS) in tea plants accumulate rapidly, such as an increase in H₂O₂ and O₂⁻. This can cause lipid peroxidation, and malondialdehyde (MDA) will also increase, making the cell membrane unstable (Seth et al., 2021). In heat-resistant varieties, the activities of some antioxidant enzymes, such as SOD and POD, are enhanced, which can help eliminate ROS and reduce oxidative damage (Huang et al., 2024; Zheng et al., 2024). However, for varieties that are more sensitive to high temperatures, the activity of antioxidant enzymes will decrease instead, and cell membranes are more prone to damage. 2.3 Disruption of secondary metabolite biosynthesis High temperatures can affect some secondary metabolites in tea plants. Research has found that substances such as polyphenols and catechins change their accumulation patterns at high temperatures. Some heat-tolerant tea tree varieties can enhance their resistance by activating genes like FLS to increase flavonoids in their bodies (Figure 1) (Huang et al., 2024). However, some genes, such as CsUGT75C1 which controls anthocyanin synthesis, are expressed less at high temperatures, resulting in a decrease in anthocyanins (Shen et al., 2019). If exogenous GABA is applied to tea plants, the contents of polyphenols and catechins can increase, and the activities of related enzymes will also improve, thereby enhancing the antioxidant capacity (Ren et al., 2021). Figure 1 Identification of DEGs involved in flavonoids biosynthesis (Adopted from Huang et al., 2024) Image caption: (A) The heat map of differentially accumulated flavinols in ‘FWH’, ‘FWM’ and ‘FWS’ varieties; (B) the DEGs in flavonoids biosynthesis pathway. CHS chalcone synthase, CHI chalcone flavanone isomerase, F3H flavanone 3P-hydroxylase, DFR dihydroflavonol 4-reductase, FLS flavonol synthase, FNS flavone synthase (Adopted from Huang et al., 2024) 3 Heat-Tolerant Traits in Tea 3.1 Morphological traits: leaf thickness, pubescence, leaf angle Heat-resistant tea trees usually have some distinct physical features. Their leaves are relatively thick, covered with a lot of downy hairs, and the Angle of the leaves is also quite moderate. Thick leaves can reduce water evaporation and help tea trees retain moisture in drought and high temperatures. The pubescence on the leaves can reflect sunlight, reduce the temperature of the leaves and prevent sunburn. The angle of the leaf can affect sunlight exposure and air circulation, and also help regulate leaf temperature (Huang et al., 2024).
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