Antioxidant enzymes and molecular responses of purslane (Portulaca oleracea L.) subjected to mannitol under salt stress conditions

Salinity stress severely limits plant growth and productivity through oxidative damage and metabolic disturbances. This study investigated the effects of mannitol application on growth, antioxidant enzyme activities, lipid peroxidation, and expression of phenylpropanoid pathway genes in Portulaca oleracea under salinity stress. A factorial experiment based on a completely randomized design (CRD) with four replications was conducted using four mannitol concentrations (0, 10, 20, and 30 mM) and two salinity levels (0 and 120 mM NaCl). Salinity significantly reduced shoot dry weight (SDW), catalase (CAT), peroxidase (POX), and polyphenol oxidase (PPO) activities, while increasing superoxide dismutase (SOD), ascorbate peroxidase (APX), and malondialdehyde (MDA) content. Mannitol application effectively alleviated salinity-induced damage by enhancing antioxidant defense responses and reducing oxidative stress. The highest SDW was observed at 30 mM mannitol, with a 67.11% increase compared with untreated plants. Under saline conditions, 30 mM mannitol significantly increased SOD and CAT activities by 67.20% and 96.77%, respectively, while 20 mM mannitol produced the highest POX activity. In contrast, MDA accumulation decreased by 28.42% at 30 mM mannitol, indicating reduced membrane lipid peroxidation. Gene expression analysis revealed significant upregulation of CHS and PAL genes following mannitol treatment, with the highest expression levels recorded at 30 mM mannitol. A strong positive correlation (r = 0.908) between CHS and PAL expression indicated coordinated regulation of secondary metabolism under stress conditions. Overall, exogenous mannitol improved salinity tolerance in purslane by stimulating antioxidant enzyme activities, reducing oxidative stress, and enhancing the expression of stress-responsive genes associated with secondary metabolite biosynthesis.