Melatonin alleviates salinity-induced impairments by regulating plant growth and physiological indices of cauliflower (Brassica oleracea L.) seedlings.

Melatonin (MT) is a natural, multifunctional molecule with amphiphilic properties, enabling it to cross cellular membranes rapidly, and it also contributes to plant resistance against abiotic stresses. However, the possible complex mechanisms by which MT mitigates salt toxicity and oxidative damage in cauliflower (Brassica oleracea L.) remain unclear. To fill this gap and clarify the pathway to salt stress resistance, the present study investigated the effects of exogenous 50 μM MT on growth, physiological, biochemical, and phyto-hormonal responses of cauliflower seedlings subjected to 200 mM NaCl-induced salinity stress. Our results revealed that salinity stress triggered a significant reduction in leaf and root biomass, chlorophyll and carotenoid pigments, gas exchange parameters, and K⁺ and Mg²⁺ ions indicators, while Na⁺ levels and hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) concentrations were significantly elevated, suggesting that cauliflower plants were adversely affected by salt-induced oxidative stress. However, exogenous MT application alleviated the reductions in growth, biochemical parameters, and physiological functions, promoting melatonin content and reducing reactive oxygen species (ROS) accumulation and lipid peroxidation by enhancing photosynthetic efficiency and promoting the accumulation of osmoprotectants under salt stress. Moreover, MT suppressed salt-induced oxidative stress by declining oxidative indicators via enhancing antioxidants activities such as superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) enzymes as well as significantly increasing abscisic acid (ABA) levels in the leaves of cauliflower plants under saline stress. In conclusion, we propose that exogenous MT application significantly enhances the physiological and biochemical profiles of cauliflower plants by improving organic osmolytes and mitigating salt-induced oxidative stress. Likewise, the correlation analysis presented strong evidence and confirms a direct contribution of MT+NAA in the growth, physio-biochemical, and phyto-hormonal traits under severe saline stress. This finding suggests that exogenous melatonin application could offer valuable strategies for cauliflower cultivation in saline environments.