Molecular Plant Breeding 2025, Vol.16, No.1, 44-54 http://genbreedpublisher.com/index.php/mpb 46 Figure 1 Changes of chlorophyll content of ivy under heat stress Note: Different lowercase letters indicate significant differences between different treatments (P<0.05) Under high temperature stress, the maximum photochemical efficiency Fv/Fm of the four ivy varieties showed a decreasing trend overall, with significant differences in the degree of decrease among different varieties (P<0.05) (Figure 2A). When not subjected to high temperature treatment, there was no significant difference in Fv/Fm values among the four types of ivy, ‘Sark’, ‘Ingelise’, ‘Golden Ivalace’, and ‘Wonder’. When subjected to high temperature stress for one day, the Fv/Fm value of ‘Golden Ivalace’ slightly decreased. On the third day of heat treatment, the Fv/Fm values of all varieties significantly decreased compared to the control group (P<0.05). Among them, the Fv/Fm values of ‘Golden Ivalace’ and ‘Wonder’ decreased by 65.9% and 67.8%, respectively, while ‘Sark’ and ‘Ingelish’ only decreased by 15.0% and 16.7%, respectively. On the 5th day of heat treatment, the Fv/Fm values of all varieties decreased more compared to the control, but the Fv/Fm values of ‘Sark’ and ‘Ingelise’ were still higher than those of ‘Golden Ivalace’ and ‘Wonder’. On the 7th day, the Fv/Fm values of ‘Sark’, ‘Ingelise’, ‘Golden Ivalace’, and ‘Wonder’ decreased by 34.8%, 49.5%, 88.7%, and 69.4%, respectively, compared to the control group. The apparent photosynthetic electron transfer rate (ETR) of four ivy varieties increased slightly during high temperature stress and then decreased significantly. On the first day, the ETR of all four ivy varieties showed varying degrees of increase, reaching its highest value on the third day, and significantly decreasing compared to the control on the fifth and seventh days (Figure 2B) ‘Sark’ and ‘Ingelise’ showed a decrease of 62.3% and 62.5% compared to the control, while ‘Golden Ivalace’ and ‘Wonder’ showed a decrease of 70.0% and 76.5% compared to the control. The ETR level of ‘Sark’ at each stage was significantly higher than that of other varieties (P<0.05). As the duration of stress increases, high temperature affects the electron transfer ability of the PS Ⅱ reaction center in ivy.
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