Chitosan Treatment Effectively Alleviates the Adverse Effects of Salinity in <i>Moringa oleifera</i> Lam via Enhancing Antioxidant System and Nutrient Homeostasis

Salt stress is a significant and common abiotic stress that negatively affects plant growth and development. Chitosan is a biodegradable and non-toxic stimulant of plant growth, which produces new routes to ameliorate the adverse effects of abiotic stresses. The possible profits of chitosan in salt stress alleviation have not been reported yet in <i>Moringa oleifera</i>, an important nutritional and medicinal tree. Therefore, the aim of the current study was to investigate the effect of chitosan treatment on salt-stressed moringa and its underlying physiological and biochemical mechanisms. Moringa plants were grown under 0, 25, 50 and 75 mM NaCl, while chitosan was applied at a 1% concentration as a foliar spray treatment. Growth attributes were considerably impaired, due to the salt stress treatment; however, chitosan application significantly reversed such an effect. Relative to the control, the fresh and dry weights of leaves were reduced by 51.44 and 48.74% in 75 mM-treated plants, while after chitosan treatment they were 33.61 and 35.72%, respectively. Additionally, chitosan treatment retarded chlorophyll and carotenoids reductions, enhanced the carbohydrate content, proline content, and phenol content and induced the activities of catalase, superoxide dismutase and ascorbate peroxidase in salt-stressed plants. Thus, chitosan application alleviated the oxidative injury, observed by lower malondialdehyde and H₂O₂ levels, thereby preserving membrane stability and improving antioxidant capacity and salt tolerance. In 50 mM NaCl-treated plants, chitosan treatment increased the activities of CAT, SOD and APX enzymes by 2.63, 2.74 and 2.31-fold relative to the control, respectively. Furthermore, chitosan application prevents the disturbance in ion homeostasis, and therefore not only enhanced the contents of N, P, K, Mg and Fe but also decreased Na content under salinity. Collectively, chitosan treatment overcame the adverse effects of salinity in moringa by activating the antioxidant machinery and preventing disturbance in ion homeostasis.