Seed Priming and Foliar Application of Salicylic Acid is Equally Beneficial in Mitigating Drought Stress in Cucumber

Drought, a global challenge, severely restricts growth and development of agronomic and horticultural crops. Cucumber (Cucumis sativus L.) is a water-intensive crop and water scarcity at critical growth stages significantly reduces its fruit yield and quality. Several plant growth regulators are applied as a mitigation strategy, and salicylic acid (SA) is one such option. However, the potential role of SA under different application methods in alleviating drought stress in cucumber has not been thoroughly investigated. The objective of this study was to evaluate the effects of SA applied as a seed priming material or foliar spray on cucumber plants under drought stress. Laboratory and polyhouse experiments were conducted. Salicylic acid was applied in five doses (0 [control], 0.25, 0.50, 0.75, and 1.00 mM) either as a seed priming material (in laboratory study and experiment 1 in the polyhouse) or as a foliar spray (experiment 2 in the polyhouse) under four soil moisture regimes (40%, 60%, 80%, and 100% field capacity [FC]). Throughout the crop growth period, a portable soil moisture meter was used to regularly monitor soil moisture regimes for maintaining the appropriate FC levels. The effect of decreasing soil moisture was evident on all evaluated parameters and caused up to 84% reduction in fruit yield at 40% FC compared with that at 100% FC in both polyhouse experiments. A consistent trend of better results with 0.75 mM dose of SA regardless of application methods or crop growth stages was observed, which resulted in a significant increase in all germination parameters, growth, and fruit yield, as well as physiological traits. The same dose (0.75 mM) caused an increase of 19–69% and 31–69% in shoot dry matter in comparison to the control plants across soil moisture regimes applied as a seed priming material and foliar spray, respectively. More than threefold increase in fruit yield and water productivity was observed for this dose at 60% FC in both polyhouse experiments in comparison to the control plants. Compared with the control plants, the same dose also caused a significant increase in leaf relative water content and membrane stability index irrespective of application methods or soil moisture regimes. The highest dose of 1.00 mM largely remained ineffective and the same was also generally true for the other doses applied in the present studies. Application of SA at 0.75 mM is recommended either as a seed priming material or foliar spray to grow cucumber in drought-prone areas.