Optimizing Foliar Application of Abscissic Acid to Improve Drought Tolerance of Melon Transplants

Successful field establishment of vegetable transplants often depends on their ability to overcome post-planting drought stress. Abscisic acid (ABA) is known to act as a stress signal which triggers stomatal closure to reduce water loss. Greenhouse studies were conducted to determine the effects of ABA foliar application on growth and physiology in melon (Cucumis melo L., cv. Caravelle) transplants subjected to a short desiccation period. In the first study, plants were treated with seven concentrations of ABA (0, 62.5, 125, 250, 500, 1000, and 2000 mg∙L^-1) followed by 3-day desiccation. Net CO₂ assimilation rate (A_CO2) and stomatal conductance (g_s) decreased exponentially with ABA 1-d after treatment, indicating that stomatal closure was rapidly regulated by exogenous ABA. However, this effect became gradually less noticeable thereafter. When A_CO2 and g_s were plotted against log-transformed ABA concentrations, their responses at the end of desiccation were best described by a quadratic function, with maximum A_CO2 and g_s occurring at 500 mg∙L^-1. This was probably because stomatal closure by drought stress exceeded that by exogenous ABA at 0-250 mg∙L^-1. In addition, ABA maintained higher leaf water potential and reduced electrolyte leakage, which was most significant at 500-2000 mg∙L^-1. Consequently, recovery of A_CO2 and g_s after rewatering was enhanced at those concentrations. Plants treated with ABA at 500 mg∙L^-1 showed the greatest increases in shoot (38%) and root (45%) dry matter compared to untreated plants. In the second study, plants were treated with three concentrations of ABA (0, 125, and 500 mg∙L^-1) and grown with or without 5-day desiccation. Overall, A_CO2 and g_s responses to ABA were as observed in the first study. Plants exhibited a quick recovery from ABA within 3 d, regardless of desiccation. These results suggest that ABA foliar application at 500 mg∙L^-1 is effective in improving drought tolerance of melon transplants by temporarily limiting transpirational water loss.