Salinity Tolerance and Antioxidant Response in Watermelon Seedlings Pre-Treated with Abiotic Stress Attenuators

Salinization of agricultural areas is one of the main abiotic factors responsible for the reduction of seed germination and vigor. In this context, the use of stress attenuators applied to seeds may contribute to mitigating the effects of salinity and improving the physiological and antioxidant performance of seedlings. This study aimed to evaluate the effects of stress attenuators on the tolerance and antioxidant activity of watermelon Citrullus lanatus (Thunb.) Matsum & Nakai cultivars under saline conditions. The study was conducted in two stages. In the first stage, a 3 × 6 factorial scheme was used to evaluate three salinity levels (0, −0.2, and −0.4 MPa) and six watermelon cultivars. In the second stage, in a 2 × 6 factorial scheme, two cultivars (sensitive and tolerant) were subjected to the combination of salinity (−0.4 MPa) and attenuators: hydropriming, gibberellic acid, salicylic acid, and hydrogen peroxide. Physiological and biochemical traits were evaluated, including hydrogen peroxide content, lipid peroxidation, and the activity of the enzymes, superoxide dismutase, catalase, and ascorbate peroxidase. Salinity reduced germination and seedling vigor, with Crimson Sweet, Charleston Gray, and Charleston Super being the most sensitive cultivars, whereas Congo and Omaru exhibited greater tolerance, and Fairfax also showed good performance under saline conditions. The selection of cultivars for the second stage was based not only on physiological tolerance but also on agronomic and commercial relevance, including post-harvest resistance traits. Seed treatment of Crimson Sweet with salicylic acid and hydrogen peroxide increased antioxidant enzyme activity, with increases of up to 103% in ascorbate peroxidase activity, and reduced oxidative damage, with reductions of 44% in hydrogen peroxide and 49% in malondialdehyde levels. In Fairfax, gibberellic acid contributed to osmotic adjustment, promoting increase of up to 76% in total soluble sugars, while pre-germinative treatment with salicylic acid and hydrogen peroxide promoted higher enzyme activity, contributing to the reduction of oxidative stress.