Physiological response to high temperature and heat tolerance evaluation of different lines in Nanfeng tangerine

Abstract：【Objective】In order to explore the physiological response of different lines in Nanfeng tangerine to high temperature and evaluate their heat resistance, the present experiment was undertaken. 【Methods】With the main cultivars of Nanfeng tangerine, Yangxiao-26 and Nanfeng-28, as the experimental materials, the effects of 24 h and 48 h treatments at 42 ℃ on the leaf tissue structure, stomatal morphology, photosynthetic fluorescence parameters, energy transfer, reactive oxygen species and antioxidant enzyme activity of plants were studied. Based on the results, an entropy weighted TOPSIS heat tolerance model was established to determine the heat resistance degrees of Yangxiao-26 and Nanfeng- 28.【Results】With the extension of high temperature and time, the thickness changes of the epidermis and palisade tissue of the leaves of Yangxiao-26 and Nanfeng-28 were not significant, but the values ofYangxiao-26 were greater than those of Nanfeng-28. After 48 h high-temperature treatment, the thicknesses of the epidermis and sponge tissue significantly decreased by 14.63% and 14.29% in Yangxiao- 26, respectively, while they were 13.47% and 15.75% in Nanfeng-28, respectively. With 24 h high temperature treatment, there was no significant differences in the ratio of palisade tissue to spongy tissue between Yangxiao-26 and Nanfeng-28, compared to the untreated group, but the difference was significant with 48 h high temperature treatment. At room temperature, the stomatal areas of Yangxiao-26 and Nanfeng-28 were 62.71 μm and 54.17 μm, respectively. After 48 h high-temperature treatment, the stomatal area of both significantly decreased by 84% and 93%, respectively. High temperature had a significant impact on the length and width of stomata in Yangxiao-26 and Nanfeng-28, both of which significantly decreased with the duration of high temperature treatment. The stomatal density ranges of Yangxiao-26 and Nanfeng- 28 were 62.27-69.41 and 61.31- 64.06, respectively, which indicated that they were not affected by high temperature treatment. The stomatal closure percentage of Yangxiao-26 and Nanfeng-28 decreased significantly with the extension of high temperature time. After 48 h high temperature treatment, the stomatal closure percentage of both decreased by 58% and 81%, respectively. Before high- temperature treatment, the stomatal length, width, density and closure percentage of Yangxiao-26 were all greater than those of Nanfeng-28. At room temperature, there was no significant difference in leaf Pn between Yangxiao- 26 and Nanfeng- 28; With the extension of high temperature time, the Pn of both showed a decreasing trend. After 48 h high temperature treatment, the Pn rates of both decreased by 57% and 82% respectively, compared to those without high temperature treatment, and at this time, the Pn of Yangxiao-26 was significantly higher than that of Nanfeng-28. The variation pattern of Gs and Pn was almost consistent, and Gs in both Yangxiao-26 and Nanfeng-28 decreased with the extension of high temperature time. The variation pattern of Ci and Ls in Yangxiao-26 and Nanfeng- 28 under different treatment durations under high temperature conditions was opposite. With 24 h high temperature treatment, the Ci of Yangxiao-26 and Nanfeng-28 decreased compared to that without high temperature treatment, while the value of Ls increased compared to that without high temperature treatment. With 48 h high-temperature treatment, the Ci values of Yangxiao-26 and Nanfeng-28 increased compared to those without high-temperature treatment, while the Ls value decreased compared to that without high-temperature treatment. However, there was no significant difference in Ci and Ls at both 24 h and 48 h between Yangxiao-26 and Nanfeng-28. During the entire high-temperature treatment period, the maximum photochemical quantum yield (Fv/Fm), energy absorbed per unit light cross-section (ABS/ CSm), energy captured for reducing QA (TRo/CSm), energy captured for electron transfer (ETo/CSm), and dissipated energy (DIo/CSm) values in the leaves of Yangxiao-26 were consistently higher than those of Nanfeng-28; With the increase of high temperature duration, the contents of superoxide anion (O2 - ), hydrogen peroxide (H2O2) and Malondialdehyde (MDA) in the leaves of Yangxiao-26 and Nanfeng-28 increased continuously, and the contents of O2 - , H2O2 and MDA in the leaves of Yangxiao-26 were always lower than those of Nanfeng-28. With the increase of high-temperature treatment time, the ascorbic acid peroxidase (APX) activity in the leaves of Yangxiao-26 and Nanfeng-28 significantly increased. The superoxide dismutase (SOD) activity in the leaves of Yangxiao-26 increased with the extension of high temperature treatment time, while the SOD enzyme activity in the leaves of Nanfeng-28 first increased and then decreased with the extension of high temperature treatment time. With 24 h of high temperature treatment, there was no significant difference in leaf SOD enzyme activity between Yangxiao- 26 and Nanfeng-28, but after 48 hours of treatment, the SOD enzyme activity in Yangxiao-26 leaves was significantly lower than that of Nanfeng-28. The order of catalase (CAT) activity in the leaves of Yangxiao-26was 48 h＞24 h＞0 h, while in the leaves of Nanfeng-28, the order of CAT enzyme activity was 24 h＞ 0 h＞48 h. Moreover, with high temperature treatment for 24 h, the CAT enzyme activity of Yangxiao- 26 was significantly higher than that of Nanfeng-28, while with high temperature treatment for 48 h, the CAT enzyme activity of Yangxiao- 26 was significantly lower than that of Nanfeng- 28. Based on the above physiological parameters, a heat tolerance evaluation model based on entropy weighted TOPSIS was constructed, and it was found that the heat tolerance of Yangxiao-26 (0.678 4) was higher than that of Nanfeng-28 (0.412 9).【Conclusion】The main reasons for higher heat resistance of Yangxiao-26 under high temperature conditions are its more stable leaf tissue, high photosynthetic energy conversion efficiency, less damage to cell membrane and high activity of SOD and CAT enzymes.