Natural biostimulants foliar application as sustainable mitigation strategy of drought stress damage on the melon crop (Cucumis melo L.)

The main aims of this study were to evaluate the potential biostimulant effect of aqueous extracts of Ilex paraguariensis A.St.-Hil. (IP) to improve the tolerance of honeydew melon plant (Cucumis melo L. var. inodorus) to drought stress and to establish the dose with the best performance to mitigate the negative effects of stress. The experiment was carried out on potted plants and five treatments were established: (1) Control: unstressed plants, 100–90% available water (AW); (2) DS: drought stress treatment, 30–20% AW; (3 to 5) drought stress and foliar spraying of IP extract (three doses): DS+IP1, DS+IP2, DS+IP3, respectively. This study revealed that drought stress negatively affects all growth parameters and those parameters most affected were leaf area (LA) and dry biomass production with reductions of 50 and 46%, respectively. Furthermore, oxidative damage increased by 41% malondialdehyde (MDA) levels and significantly decreased plant water status and the content of phenolic compounds. However, the application of IP extracts at least partially attenuated the negative effect of stress. IP2 was the most effective dose since it presented similar LA values as the control and decreased the dry biomass by only 14%. Furthermore, MDA levels increased by only 23% and activated the antioxidant enzymes ascorbate peroxidase and superoxide dismutase to a greater extent by 95 and 11%, respectively, compared to control. Moreover, IP treatments improve pigment biosynthesis and reinforce non-enzymatic antioxidant defense mechanisms by providing external phenolic compounds that counteract the reactive oxygen species increase caused by stress conditions. These findings propose IP extracts as valuable biostimulants in agriculture, mitigating the effects of water scarcity on melon crops.