Improving Photosynthetic Efficiency and Biochemical Adaptations in Tomatoes under Drought Stress through Seaweed Aqueous Extract Application

Abstract Pursuing natural products to enhance the growth and quality of fruit and vegetable crops is crucial to ensuring sustainable food production. In this sense, this study aimed to investigate the effects of Kappaphycus alvarezii seaweed extract (SEA) on the physiological and biochemical responses of Solanum lycopersicum cv. Micro-Tom under three levels of water availability: field capacity (100% FC), moderate stress (50% MS), and severe stress (30% SS). Plants were cultivated under controlled conditions (24 ± 1 °C). Ten plants were used per treatment (n = 10), totaling 60 plants arranged in a completely randomized design (CRD). The treatments included a control group (C) and groups receiving 5% seaweed extract (SEA) application. The SEA was applied weekly, starting before induction of the water deficit and continuing throughout the stress period. The effects on the stomatal density, stomatal conductance, quantum yield, and morphological and biochemical parameters were evaluated. SEA-treated plants showed marked physiological and biochemical improvements under both mild and severe stress. Quantum yield increased by 7–18% compared to controls, indicating enhanced PSII photochemical efficiency. In parallel, significant increases were observed in total chlorophyll (up to 23%), soluble sugars (31%), and total flavonoids (60%) relative to those in untreated plants. These results confirm that bioactive compounds derived from seaweed can mitigate damage to the photosynthetic machinery, osmotic adjustment, and antioxidant metabolism, thereby promoting improved plant performance under limited water availability.