Physiological and transcriptomic analysis reveals the responses and difference to high temperature and humidity stress in two melon genotypes

Due to the frequent occurrence of continuous high temperature and heavy rain in summer, extreme high temperature and high humidity environment is caused, which has serious harm to crop growth. High temperature and humidity (HTH) stress have become the main environmental factors of combined stress in summer. The response of morphological indexes, physiological and biochemical indexes, gas exchange parameters and chlorophyll fluorescence parameters were measured and combined with chloroplast ultrastructure and transcriptome sequencing to analyze the reasons for the difference of tolerance to HTH stress in sensitive JIN TAI LANG and tolerant JIN DI varieties. The results showed that with the extension of stress time, the SOD,POD and APX activities of the two melon varieties increased rapidly, leaf water content increased, and the tolerant varieties showed stronger antioxidant capacity. Among the sensitive cultivar, Pn, Fv/Fm, photosystem II and photosystem I chlorophyll fluorescence parameters were severely inhibited, and decreased rapidly with the extension of stress time, while the humidity and heat tolerant cultivars slightly decreased. The cell membrane and chloroplast damage in sensitive cultivars was more severe, and Lhca1, Lhca3, and Lhca4 proteins in photosystem II and Lhcb1-Lhcb6 proteins in photosystem I were inhibited compared with those in tolerant cultivar. These conclusions may be the main reason for the different tolerance of the two cultivars. These findings will provide new insights for the response of other crops to HTH stress, and also provide a molecular basis for the following researches on the mechanism of HTH resistance of melon.