Boron Toxicity Alters Yield, Mineral Nutrition and Metabolism in Tomato Plants: Limited Mitigation by a Laminaria digitata-Derived Biostimulant

The use of unconventional water sources, such as those from marine desalination plants, is challenging for agriculture due to boron concentrations exceeding 0.5 mg L−1, which can impact crop yield and quality. To ensure sustainability, it is crucial to understand crop responses to high boron levels and to develop strategies to mitigate its toxic effects. This study evaluated the impact of irrigation with a nutrient solution containing 15 mg L−1 of boron on tomato plants (Solanum lycopersicum L.). To modulate the physiological effects of boron toxicity, two biostimulant products based on an extract from the brown alga Laminaria digitata and other active ingredients were applied foliarly. Agronomic, nutritional, and metabolic parameters were analyzed, including total yield, number of fruits per plant, and fruit quality. Additionally, mineral analysis and metabolomic profiling of leaves and fruits were performed, focusing on amino acids, organic acids, sugars, and other metabolites. A control treatment was irrigated with a nutrient solution containing 0.25 mg L−1 of boron. The results showed that a boron concentration of 15 mg L−1 significantly reduced total yield by 45% and significantly decreased fruit size and firmness. Mineral and metabolomic analyses showed significant reductions in Mg and Ca concentrations, significant increases in P and Zn levels, excessive boron accumulation in leaves and fruits, and significant changes in metabolites associated with nitrogen metabolism and the Krebs cycle. Biostimulant application did not significantly improve agronomic performance, likely due to high boron accumulation in the leaves, although significant changes were detected in leaf nutritional status and metabolic profiles.