Physiological response and evaluation of melon (Cucumis melo L.) germplasm resources under high temperature and humidity stress at seedling stage

Melon is a favourite fruit and is widely planted in China. High temperature and humidity (HTH) stress describes the main limiting factors for melon due to the continuous hot and rainy weather in summer. The physiological response of melon to HTH stress and an evaluation method for identifying varieties resistant to HTH have not been reported. In this study, seedlings of 56 melon germplasm resources were used to measure the growth indexes and physiological and biological indexes under high temperature and humidity stress and to evaluate the tolerance of different varieties to HTH. Compared with the control, the plant height (PH) grew rapidly, and stem diameter (SD), leaf length (LL), and leaf width (LW) were inhibited in all melon varieties. The photosynthetic parameters Pn and Fv/Fm decreased compared with the control. Most of the varieties showed increases in chlorophyll (Chl), soluble protein (SP) and MDA. According to the membership function value and cluster analysis methods, three varieties, M32, M52 and M56, were selected for their better HTH tolerance. Through correlation analysis, it was determined that Fv/Fm, Pn, Chl, LL, LW, PH, MDA and SD can be used as the identification indexes of early tolerance to HTH. In conclusion, this work provides basic materials for melon breeding and basic theories for the molecular mechanism of HTH tolerance in melon.