Physiological response to high temperature and heat tolerance evaluation of different lines in Nanfeng tangerine
[Objective]In order to explore the physiological response of different lines in Nanfeng tan-gerine 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 experi-mental 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 anti-oxidant 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 of Yangxiao-26 were greater than those of Nanfeng-28. After 48 h high-temperature treatment, the thick-nesses 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 tem-perature treatment, there was no significant differences in the ratio of palisade tissue to spongy tissue be-tween Yangxiao-26 and Nanfeng-28, compared to the untreated group, but the difference was signifi-cant with 48 h high temperature treatment. At room temperature, the stomatal areas of Yangxiao-26 and Nanfeng-28 were 62.71μm and 54.17μm, respectively. After 48 h high-temperature treatment, the sto-matal area of both significantly decreased by 84％and 93％, respectively. High temperature had a signif-icant impact on the length and width of stomata in Yangxiao-26 and Nanfeng-28, both of which signifi-cantly 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％, respective-ly. 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 treat-ment. 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-), hy-drogen peroxide (H2O2) and Malondialdehyde (MDA) in the leaves of Yangxiao-26 and Nanfeng-28 in-creased 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 su-peroxide 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 signifi-cantly lower than that of Nanfeng-28. The order of catalase (CAT) activity in the leaves of Yangxiao-26 was 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 (0.6784) was higher than that of Nanfeng-28 (0.4129).[Conclusion]The main reasons for higher heat resistance of Yangxiao-26 un-der high temperature conditions are its more stable leaf tissue, high photosynthetic energy conversion ef-ficiency, less damage to cell membrane and high activity of SOD and CAT enzymes.
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