Effects of high temperature on leaf physiological indexes of Summit in seedlings sweet cherry

Abstract:【Objective】Temperature is one of the important environmental factors in plant growth and development. According to the report of the IPCC, the global average temperature will rise by 0.3 ℃ every decade. The latest assessment report of IPCC Working Group I in 2021 pointed out that the global temperature rise will reach or exceed 1.5 ℃ by predicting the average temperature change in the next 20 years. High temperature has gradually become one of the important abiotic stress factors, which seriously harm the growth and development and inhibit the physiological metabolism of plants. Sweet cherry is mainly cultivated in Tai’an, Yantai and Dalian in northern China. The south of China become warm earlier and faster in spring than the north, so it can take the advantage of earlier marketing. Be-sides, sweet cherry cultivation has gradually expanded to the south due to the leisure picking market`s promoting in recent ten years. The summer temperature in Zhejiang province is relatively high, and the average annual extreme high temperature weather above 40 ℃ from July to September is 4.1 days. For sweet cherry has adapted to the cold climate in the north, the summer temperature in Hangzhou is too high, and the high temperature above 30 ℃ is closely related to the occurrence of double pistils, which affects the fruit setting rate of the next year. High temperature has become one of the limiting factors for the development of sweet cherry in southern China. This study aimed to analyze the response of sweet cherry to high temperature stress from the physiological level, which not only provided reference for the establishment of the evaluation system of the heat tolerance of sweet cherry in the next program, but also established a theoretical foundation for the screening and breeding of high temperature resistant varieties (lines) of sweet cherry in the south of China in the future.【Methods】One-year-old sweet cherry seedlings (Summit) were used as the experimental materials. Artificial climate chamber and field natural high temperature were used as high temperature treatment, respectively, which were set as group A, B and C. The artificial climate chamber was used to simulate high temperature in group A and B, and the natural high temperature in the field was used in group C. The three treatments (7 states) were labeled as A-25 ℃ (T1), A-45 ℃ (T2), B-25 ℃ (T3), B-45 ℃ (T4), C-0 d (T5), C-3 d (T6), C-5 d (T7), respectively. Firstly, the physiological indexes (such as the activities of SOD, POD, CAT, APX and GR, and the MDA content, superoxide anion (O2 ^·- ) producing rate, the soluble sugar, soluble protein and proline contents) and chlorophyll fluorescence indexes (such as Fv/Fm, Fv/Fo, ETR and NPQ) of leaves in three treatments were investigated. Then, the correlation analysis and principal component analysis were used to determine the important physiological index. The cluster analysis was performed using K-means method to analyze and screen the physiological evaluation indexes of sweet cherry response to high temperature.【Results】The activities of SOD, POD, GR, CAT and APX of the leaves in group A and C increased after high temperature stress treatment, but the activities of CAT and APX of the leaves in group B decreased. The content of proline in group A, B and C was 3.07, 4.03 and 2.94 times of that before high temperature, respectively, and the O2 ^·- producing rate was 2.37, 2.48 and 2.25 times of that before high temperature, respectively. In group A and C, the soluble protein content in sweet cherry leaves increased and the soluble sugar content decreased significantly, while in group B, the soluble protein content decreased significantly, which was 54.18% of the pre-treatment content and the soluble sugar content increased by 1.32%. The values of fluorescence parameters such as Fv/Fm, Fv/Fo, ETR and NPQ decreased in three treatments, and the same was as the activity of PSⅡ reaction center. In group C, when the temperature decreased and the plants entered the recovery stage, the NPQ in sweet cherry leaves still showed a downward trend, while the values of Fv/Fm, Fv/Fo and ETR rose to some extent, but were still lower than the levels before high temperature treatment. The ultrastructure of mesophyll indicated that high temperature caused damage to chloroplast and mitochondria in sweet cherry. Correlation analysis showed that there was a correlation between some enzyme activities (CAT and APX) and some fluorescence parameters (ETR and Fv/Fo), or between the activity of GR and the contents of various osmotic regulatory substances (proline, soluble protein and soluble sugar) in the leaves of sweet cherry. There was no significant relationship among the activities of POD, the content of MDA, Fv/Fm and other indexes. Principal component analysis was used for factor analysis of 11 physiological indexes of sweet cherry. The first two factors were selected as the main factors, and the variables with large characteristic vector- values were selected as the representative variables. PC1 selected three indexes (the activities of APX, SOD and GR) which were defined as enzyme activity parameter factors. PC2 selected two indexes (ETR and NPQ) which were defined as fluorescence parameter factors. Based on cluster analysis, 11physiological indexes were grouped into 3 categories. Combined with correlation analysis, the activities of APX and GR, and ETR were selected as physiological evaluation indexes of sweet cherry response to high temperature stress. Finally, using these three indexes under K-means iterative calculation in dynamic cluster, the sweet cherries of two treatments (7 different states) in the experiment were divided into 3 categories. The first category contained T4, whose response to high temperature was characterized by high activities of GR and low activities of APX. The second category included T1, T2, T3 and T5, which were characterized by high ETR index and low activities of GR. The third category included T6 and T7, which were characterized by high activities of APX and low ETR index. The clustering results were corresponding to the heat map analysis of physiological indexes of sweet cherry with 7 different states. Both the activities of APX and GR and ETR could be used as representative indexes of the effect of high temperature on sweet cherry in this paper.【Conclusion】High temperature treatment caused oxidative damage to sweet cherry, and both chloroplast and mitochondrial structure were damaged. In order to maintain the stability of antioxidant enzyme system and reduce the damage of membrane permeability, the oxidative damage of sweet cherry was caused by high temperature treatment, and the production rate of O2 ·- and the content of MDA increased, while the activity of PSⅡ reaction center decreased. The activities of APX and GR, and ETR could be used as representative indexes of the effect of high temperature on sweet cherry. The results provided reference for the establishment of the evaluation system of the heat tolerance of sweet cherry in the next program.