Foliar application of sodium nitroprusside and chitosan induced antioxidant system and alleviated drought stress in spinach

Spinach (Spinacia oleracea L.) is a leafy green vegetable known for nutrition and health benefits. However, it is highly susceptible to drought, compromising its growth, yield, and nutritional quality. To alleviate drought stress, application of elicitors is a recognized strategy. This work aimed at exploring the impact of chitosan (CS) and sodium nitroprusside (SNP) on the oxidative status and antioxidant response of spinach. The SNP (0 and 50 µM) and CS (0 and 30 mg L^-1) were sprayed on the 3-leaved spinach seedlings under drought (0, 5 and 15% PEG 6000) in two steps, with 2-day intervals. The 21-day-old age seedlings were then harvested to evaluate their morpho-physio-molecular responses by different methods based on spectrophotometry, HPLC and qRT-PCR. The SNP and CS increased the activity of key antioxidant enzymes and the production of osmolytes (glycine betaine and proline), phytohormones, and radical scavengers (tocopherol). Co-application of SNP and CS caused a synergistic increase in tocopherol, NO, abscisic acid, methyl jasmonate and an antagonistic decrease in brassinosteroid, gibberellin, and auxin. The qRT-PCR analysis showed higher expression of glycine betaine biosynthesis genes (CMO and BADH) in the spinach treated with SNP and CS under drought. The glycine betaine content was highly correlated with oxidative status in the spinach. The foliar application of both elicitors mitigated adverse effects of drought and promoted the spinach growth in normal condition. Cross-talk between phytohormones, signaling molecules, and osmolytes regulated the antioxidant response, turned out significant changes in the spinach morphology under each treatment.