Exploiting Laminaria digitata L. Aqueous Extract for Tomato Biostimulation: A Physiological and Metabolomic Approach

Water deficit negatively affects crop yield and quality. Biostimulants, such as brown seaweed extracts, offer a sustainable solution to mitigate these effects. This study evaluated the impact of Laminaria digitata-aqueous extract (LE) on tomato performance and its potential to alleviate drought stress by examining morphological, physiological, and metabolic responses. Two-week-old tomato plants were foliar-sprayed with LE (0.0, 0.1, and 1.0 g L^-1) and split into well-watered (WW) and water-limited (WL) groups. WW plants received regular irrigation, while WL faced a one-week drought. Then, half of the WL plants were rewatered and allowed to recover for 24 h (REC group). In general, LE effects were influenced by irrigation conditions, mainly impacting physiological and metabolomic parameters. Under WW conditions, LE decreased chlorophyll content, improved energy conversion by regulating photosystem antenna size, enhanced photochemical efficiency (ΦPSII) and gas exchange parameters (g_s, C_i, P_N), and promoted photosynthesis through stomatal modulation and RuBisCO activity. Conversely, under WL conditions, LE (especially 0.1 g L^-1) decreased gas exchange parameters, but increased water use efficiency by inducing stomatal closure without impairing CO₂ assimilation. LE-treated plants exhibited ROS detoxification and phytohormone downregulation, which in turn negatively affected the content of certain secondary metabolites (e.g., catechin). Phytohormone modulation may result from reduced ROS levels or crosstalking with LE compounds, including mannitol, alginates, laminarans, minerals, and phlorotannins, which likely act synergistically to improve physiological regulation. Overall, LE application improved drought tolerance by enhancing photosystem regulation, phytohormone modulation, and antioxidant capacity in tomato plants, highlighting its potential use as a biostimulant for sustainable agriculture.