Productivity and Quality Characteristics of Tomato Fruits ( Solanum lycopersicum ) Are Improved by the Application of a Green Seaweed ( Ulva ohnoi )

In the last decade, interest in the use of seaweed and seaweed-derived products in horticulture has grown due to their great potential as biostimulants for increasing yields and improving food quality in multiple crops. A greenhouse experiment was conducted to investigate the effects of the application of the green seaweed Ulva ohnoi (either as a seaweed suspension [SWS] or seaweed extract [SWE]) on the yield, size, shape, and nutritional quality (i.e., proximate composition and dietary antioxidant content) of tomato fruits ( Solanum lycopersicum L. cv. Rio Fuego). A total of 36 tomato plants were potted individually and organized into three experimental groups: SWS (plants drenched with 250 mL of seaweed suspension [2.0%]), SWE (plants drenched with 250 mL of seaweed extract [0.2%]), and control (plants irrigated with water). Each treatment included three replications. The fruits harvested (66%) from SWS-treated plants were produced during the earliest harvest stages. In contrast, the fruits harvested from SWE-treated plants (82%) and control plants (77%) were produced during the late and very late harvest stages. Notably, SWS application significantly enhanced the number of fruits harvested per plant, average fruit weight, yield (kg/plant), number of seeds per fruit, and fruit size. Furthermore, tomato fruits from plants treated with either SWS or SWE exhibited higher percentages of protein, fat, crude fiber, dry matter, and total soluble solids, as well as lower acidity and reduced total carbohydrate content, compared to the control. The antioxidant metabolites in tomatoes, including lycopene, β-carotene, flavonoids, and phenolic acids, increased following the application of SWS and SWE, while anthocyanin and ascorbic acid contents increased only in SWS-treated plants. These results demonstrate that both forms of U. ohnoi application have biostimulating effects on tomato. In particular, the use of SWS shows great potential as a strategy to enhance tomato fruit productivity and quality in sustainable horticultural systems.