Effect of exogenous chitosan, salicylic acid, and their combination on some physiological parameters of Citrullus colocynthis (L.) under drought stress

Drought stress is alarmingly on the rise over the past decades. Growth regulators including salicylic acid (SA) and chitosan are being successfully used to protect plants against biotic and abiotic stresses. To investigate the effect of SA, chitosan, and their combination on biochemical traits and cucurbitacin contents in Citrullus colocynthis (L.) under different levels of drought stress, four irrigation levels (control (100% Field capacity: FC), 75, 50, and 25% FC) were used along with three different treatments by administering SA and chitosan (150 mg L-1), and their combinations. Drought stress significantly increased the amount of sugar, proline, lipid peroxidation, and the activity of antioxidant enzymes such as superoxide dismutase (SOD) and catalase (CAT) while decreasing growth parameters, protein, leaf relative water content (RWC), chlorophyll, and carotenoid contents. Although SA could increase some biochemical traits including sugar, chlorophyll, and carotenoid contents only in severe level of drought stress, chitosan and the combined treatment exerted beneficial effects in almost all levels of irrigation. Furthermore, combination of SA and chitosan induced more protective effects compared to only chitosan treatment in increasing proline and the activities of SOD and CAT. Combined treatment was also effective in increasing cucurbitacin B, C and L contents but not cucurbitacin E. Combination of SA and chitosan showed the major impact on improving physiological parameters and cucurbitacin contents of C. colocynthis and therefore could be a potential candidate to protect the plant against adverse effects of drought stress.