Foliar Salicylic Acid Modulates Watermelon Responses to Deficit Irrigation at Different Phenological Stages

Water scarcity caused by irregular rainfall and high evapotranspiration rates in the Brazilian semi-arid region is one of the main factors limiting watermelon cultivation, underscoring the need for irrigation strategies capable of mitigating the adverse effects of water deficit. This study aimed to evaluate the effects of foliar salicylic acid application on the induction of water-deficit tolerance in watermelon plants subjected to water restriction at different phenological stages under semi-arid conditions. The experiment was conducted using a randomized block design in a split-plot arrangement, with five irrigation management strategies based on crop evapotranspiration (ETc) and four salicylic acid (SA) concentrations, with three replications and three plants per plot. Water deficit adversely affected the morphophysiological traits of the plants and the physical and chemical attributes of ‘Crimson Sweet’ watermelon fruits, with water restriction during the vegetative and flowering stages causing the most severe effects. Foliar application of salicylic acid at concentrations ranging from 1.2 to 2.6 mM increased relative water content by 9.4%, reduced electrolyte leakage by 11.60%, and enhanced CO2 assimilation by 18.82%, instantaneous water-use efficiency in 128.10%, and instantaneous carboxylation efficiency by 46.92%. Salicylic acid concentrations within this range also improved plant water status, gas exchange, photosynthetic pigment content, growth, and the physical and chemical attributes of the fruits. In contrast, concentrations above 2.6 mM reduced gas exchange, photosynthetic pigment accumulation, chlorophyll a fluorescence, and the physical and chemical quality of ‘Crimson Sweet’ watermelon fruits. Thus, salicylic acid may be an alternative to modulate the tolerance of watermelon plants under water deficit during phenological phases.