Boron (B) is a microelement required in vascular plants at a high concentration that produces excess boron and toxicity in many crops. B stress occurs widely and limits plant growth and crop productivity worldwide. Salicylic acid (SA) is an essential hormone in plants and is a phenolic compound. The goal of this work is to explore the role of SA in the alleviation of excess B (10 mg L⁻¹) in watermelon plants at a morphological and biochemical level. Excess boron altered the nutrient concentrations and caused a significant reduction in morphological criteria; chlorophyll a, b, and carotenoids; net photosynthetic rate; and the stomatal conductance and transpiration rate of watermelon seedlings, while intercellular carbon dioxide (CO₂) was significantly increased compared to the control plants (0.5 mg L⁻¹ B). Furthermore, excess boron accelerated the generation of reactive oxygen species (ROS), such as hydrogen peroxide (H₂O₂) and induced cellular oxidative injury. The application of exogenous SA significantly increased chlorophyll and carotenoid contents in plants exposed to excess B (10 mg L⁻¹), in line with the role of SA in alleviating chlorosis caused by B stress. Exogenously applied SA promoted photosynthesis and, consequently, biomass production in watermelon seedlings treated with a high level of B (10 mg L⁻¹) by reducing B accumulation, lipid peroxidation, and the generation of H₂O₂, while significantly increasing levels of the most reactive ROS, OH⁻. SA also activated antioxidant enzymes, such as superoxide dismutase (SOD), peroxidase (POD), and ascorbate peroxidase (APX) and protected the seedlings from an ROS induced cellular burst. In conclusion, SA can be used to alleviate the adverse effects of excess boron.

Strigolactone (SL) plays essential roles in regulating plant growth, development, and stress response. This study was conducted to evaluate the effect of exogenous SL on watermelon resistance against herbicides penoxsulam (PXL) and bensulfuron-methyl (BSM). These herbicides were found to negatively impact watermelon root growth and photosynthetic pigments, probably due to the ultrastructural damage and cell death in leaf and root tissues under herbicide stresses. The activation of SL-related gene expression suggested that the SL pathway may mitigate herbicide toxicity in watermelon. The exogenous SL dose-dependently reversed the PXL- or BSM-induced antioxidant activity, suggesting that SL may participate in maintaining antioxidant enzyme homeostasis under herbicide stresses. The up-regulation of herbicide metabolization and detoxification-related genes (cytochrome P450 and acetolactate synthase) by exogenous SL also in part explained how this phytohormone alleviates herbicide toxicity in watermelon. Our findings will provide valuable information underlying the regulatory effects of SL on herbicide tolerance in Cucurbitaceae crops.

Drought stress has detrimental effects on crop productivity worldwide. A strong root system is crucial for maintaining water and nutrients uptake under drought stress. Wild watermelons possess resilient roots with excellent drought adaptability. However, the genetic factors controlling this trait remain uninvestigated. In this study, we conducted a bulk segregant analysis (BSA) on an F₂ population consisting of two watermelon genotypes, wild and domesticated, which differ in their lateral root development under drought conditions. We identified two quantitative trait loci (qNLR_Dr. Chr01 and qNLR_Dr. Chr02) associated with the lateral root response to drought. Furthermore, we determined that a small region (0.93 Mb in qNLR_Dr. Chr01) is closely linked to drought adaptation through quantitative trait loci (QTL) validation and fine mapping. Transcriptome analysis of the parent roots under drought stress revealed unique effects on numerous genes in the sensitive genotype but not in the tolerant genotype. By integrating BSA, fine mapping, and the transcriptome, we identified six genes, namely L-Ascorbate Oxidase (AO), Cellulose Synthase-Interactive Protein 1 (CSI1), Late Embryogenesis Abundant Protein (LEA), Zinc-Finger Homeodomain Protein 2 (ZHD2), Pericycle Factor Type-A 5 (PFA5), and bZIP transcription factor 53-like (bZIP53-like), that might be involved in the drought adaptation. Our findings provide valuable QTLs and genes for marker-assisted selection in improving water-use efficiency and drought tolerance in watermelon. They also lay the groundwork for the genetic manipulation of drought-adapting genes in watermelon and other Cucurbitacea species.