Foliar Application of a Sargassum spp.-Based Biostimulant Improves Fruit Quality and Antioxidant Enzyme Activity in Melon

Seaweed-derived biostimulants are promising tools for improving crop performance and fruit quality in sustainable horticulture. This study evaluated the effects of foliar application of a liquid Sargassum spp. extract on the commercial quality, nutraceutical attributes, and antioxidant response of melon fruit (Cucumis melo L. cv. ‘Cruiser’). Four extract concentrations (0, 1.5, 3.0, and 4.5%, v/v) were applied in a randomized complete block design with six replications. Compared with the control, the 1.5% treatment increased (p ≤ 0.05) fruit firmness (33.5%), total soluble solids (25.4%), flesh thickness (16.5%), and ascorbic acid content (51.2%). Total phenolic and flavonoid contents also increased by 22.9% and 22.4%, respectively (p ≤ 0.05). Antioxidant enzyme activities increased in a concentration-dependent manner (p ≤ 0.05), highlighted by a 207.1% increase in catalase activity at 4.5% with catalase. In contrast, the highest concentration reduced carbohydrate content by 49.3% (p ≤ 0.05), while fruit yield remained unaffected (p > 0.05). Overall, 1.5% provided the best balance between commercial and nutraceutical quality while avoiding the adverse biochemical responses observed at 4.5%. These findings suggest the potential use of Sargassum spp. extract as a biostimulant for sustainable melon production under the conditions of this study.