Salt stress impedes plant growth and development due to several factors, including the generation of cellular oxidative stressors. This study aimed to assess the impacts of seawater-induced salinity on the plant development, physio-biochemical responses, and antioxidant capacity of Catharanthus roseus grown in a variety of seawater (4, 8, and 12 dS/m) for varying durations (60, 90, and 120 days). The experiment was laid out in a randomized complete block design with five replications. The results demonstrated that C. roseus successfully endured moderate salinity (8 dS/m) by maintaining plant height, number of leaves, branches, relative water content, and chlorophyll content with a minimum drop in dry biomass (25%) in a time- and dose-dependent approach. Furthermore, greater proline and soluble sugar contents suggested that C. roseus possessed enhanced osmoprotective capabilities to counteract osmotic stress caused by salinity. Conversely, all growth indicators decreased significantly at high salinity (12 dS/m). Increased levels of antioxidant enzyme activity catalase and ascorbate peroxidase, phenol and flavonoid, 2,2-diphenyl-1-picrylhydrazyl and 2,2-azino-bis-3-ethylbenzothiazoline-6-sulphonic acid indicate a coordinated function for antioxidant components in regulating reactive oxygen species (ROS) at low (4 dS/m) and moderate (8 dS/m) salinities. In contrast, excessive salinity (12 dS/m) led to a burst of ROS, as seen by elevated levels of hydrogen peroxide, malondialdehyde, and electrolyte leakage that greatly reduced total dry matter (72%), especially on days 120. The ion studies on plants subjected to salinity revealed that most Na+ remained in the roots. In contrast, most K+, Ca2+, and Mg2+ are deposited more firmly in the leaves than in the roots. The findings imply that C. roseus may tolerate moderate salinity (8 dS/m) owing to its enhanced antioxidant defense system and osmolytes, which trigger antioxidant enzymes and maintain ionic balance.
