Molecular Plant Breeding 2025, Vol.16, No.1, 44-54 http://genbreedpublisher.com/index.php/mpb 50 Table 3 Comprehensive evaluation of heat resistance of four ivy varieties Variety Comprehensive membership function value Ranking Sark 0.639 1 Ingelise 0.539 2 Wonder 0.399 3 Golden Ivalace 0.302 4 High temperature can induce the degradation of chlorophyll, and heat-resistant varieties have higher and relatively stable chlorophyll content. Studies have shown that the reduction of chlorophyll content by high temperatures is mainly achieved by inhibiting the activity of enzymes related to chlorophyll synthesis, such as the first enzyme in the pyrrole biosynthesis pathway, 5-aminolevulinate dehydratase (ALAD), whose activity significantly decreases at high temperatures (Mohanty et al., 2006). Another study suggests that the decrease in chlorophyll caused by high temperatures may be due to the occurrence of peroxidation of chloroplasts and thylakoid membrane liposomes (Kaushal et al., 2016). In the entire high temperature stress of this experiment, the chlorophyll a and total chlorophyll content of the heat-resistant varieties ‘Sark’ and ‘Ingelis’ were higher than those of the thermosensitive variety ‘Golden Ivalace’. The relationship between the changes in chlorophyll a and total chlorophyll content of ‘Wonder’ and its heat resistance is relatively more complex. On the third day of heat treatment, it is higher than the medium resistance ‘Ingelise’, but on the seventh day, it is lower than ‘Ingelise’. It can be seen that the use of chlorophyll content to evaluate plant stress resistance varies among different plants, varieties, and time periods of high temperature stress. There have been similar reports by previous researchers, such as Shen and Zhao (2018) who found that most Rhododendron varieties experienced varying degrees of decrease in chlorophyll a, chlorophyll b, and total chlorophyll content after high temperature stress, but some varieties experienced an increase in chlorophyll b after high temperature stress. Throughout the entire high-temperature stress process in this experiment, the trend of total chlorophyll content in different varieties was similar to that of chlorophyll a content, showing an increase on the first day of stress followed by a continuous decrease, while chlorophyll b showed no significant change pattern. The effects of high temperature stress on plant photosynthesis mainly involve photosystem II (PSII), ribulose-1,5-diphosphate carboxylase/oxygenase (Rubisco), cytochrome B559 (Cytb559), and plastiquinone (PQ) (Mathur et al., 2014), among which PSII is the most sensitive component to high temperature stress in photosynthesis (Hasanuzzaman et al., 2013). Fv/Fm reflects the maximum photoelectrochemical efficiency when the PSII center is fully open, while ETR reflects the efficiency of electron transfer and the apparent photosynthetic electron transfer rate. When not under stress, Fv/Fm is relatively stable, but under high temperature stress, Fv/Fm significantly decreases (Tu et al., 2013; Song et al., 2014). Four types of ivy grew differently after high temperature stress, and the Fv/Fm values of all tested ivy decreased to varying degrees compared to the control group. The Fv/Fm value of the thermosensitive ‘Golden Ivalace’ decreases first. At different time points of high-temperature treatment, the decrease in Fv/Fm values of heat-resistant varieties ‘Sark’ and ‘Ingelise’ is smaller than that of thermosensitive varieties ‘Golden Ivalace’ and ‘Wonder’, which also confirms that the more Fv/Fm decreases, the greater the damage to PSII, and the weaker the high-temperature resistance. The amplitude of Fv/Fm changes reflects the plant's ability to respond to stress and is related to the plant's stress resistance. When plants are subjected to high temperature stress, if the intracellular antioxidant enzymes still cannot fully respond to the oxidative stress response induced by high temperature stress, a large amount of ROS will accumulate in the cell, leading to lipid peroxidation, and MDA is a marker of lipid peroxidation (Tu et al., 2013). The more MDA accumulates, the greater the degree of damage. Song et al. (2014) found that the content of MDA in poplar trees did not change significantly after 3 and 4 hours of heat treatment, but increased significantly after 24 hours. Generally speaking, the content of MDA is negatively correlated with plant stress resistance, and the MDA content of heat-resistant materials is significantly lower than that of non heat-resistant materials under high temperature stress (Wilson et al., 2014). We found that MDA accumulated in all four ivy varieties under high
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