Effects of molybdenum on growth and fruit quality of small fruit melon (Cucumis melo L.) cultivated under high-temperature stress

Recurrent and prolonged heat waves during summer have significantly impacted the growth and quality of cultivated melons in China. Molybdenum (Mo), a trace element crucial for the plant’s photosynthesis process under normal planting conditions, is posited to not only improve plant stress tolerance but also enhance fruit quality and nutritional content. In this study, melon cv. ‘Japanese Sweet Treasure’ was used as the experimental material. Various concentrations of ammonium molybdate solution (0, 0.05, 0.1, 0.2, 0.4 mg·L–1) were foliar sprayed to examine their impact on the growth, photosynthetic characteristics, carbohydrate metabolism, and fruit quality of melons under high-temperature stress. The results indicated that plants sprayed with Mo exhibited enhanced plant parameters, including height, stem diameter, root volume, root activity, and physiological characteristics of melons under high-temperature stress, in comparison to the control (CK). The most significant improvements were observed in plants treated with 0.1 mg·L–1 Mo (T2). This treatment also showed higher improvement in melon net photosynthetic rate (Pn), transpiration rate (Tr) and total chlorophyll relative to other treatments. It also significantly promotes carbohydrate metabolism in melon plant leaves under high-temperature stress, enhancing their antioxidant enzyme activity. Melon plants exhibit a respective increase in sucrose, soluble sugars, superoxide dismutase (SOD), and peroxidase (POD) compared to the control. Melon sprayed with 0.1 mg·L–1 Mo showed significantly higher levels of vitamin C, soluble proteins, and soluble solids in fruits compared to other treatments, with a respective increase of 27.9% in individual fruit weight and 20.1% in per-plant yield compared to the CK. In conclusion, spraying 0.1 mg·L–1 Mo effectively mitigates damage caused by high-temperature stress during melon cultivation. It enhances the photosynthetic capacity of melon leaves, promotes carbohydrate metabolism in plant leaves, and thereby strengthens stress resistance. This comprehensive improvement leads to enhanced quality and yield of melon fruits.