Is the application of bioactive anti-stress substances with a seaweed-derived biostimulant effective under adequate growth conditions?

The use of biostimulants in agriculture is currently emerging as a resource to increase crop productivity and quality. Additionally, the concurrent application of other bioactive materials or bioregulators alongside biostimulants can prove valuable in mitigating oxidative stress induced by various abiotic stresses. However, in some cases, these compounds applied at inadequate doses produce toxic effects under non-stressful conditions. Therefore, the objective of this study was to analyze the effects of applying various doses of a seaweed-derived biostimulant containing additional bioactive compounds (Cytolan (R) Stress) under optimal growth conditions. Lettuce plants were grown under control conditions without biostimulants (control plants) and with two biostimulants Cytolan (R) (seaweed-derived biostimulant with proline) and Cytolan (R) Stress applied foliarly at three doses (150, 300, and 500 mL/hL). Growth and quality parameters and physiological processes within primary and secondary metabolism were analyzed. The results identified the 300 mL/hL dose of Cytolan (R) Stress as the best treatment. Thus, it improved biomass production, induced photosynthetic activity, increased phytohormone, amino acids (AAs), and mineral nutrient profiles, and stimulated N assimilation. Besides, it substantially enhanced the antioxidant capacity and the concentration of various antioxidant compounds, leading to increased lettuce quality and potential stress tolerance. However, the 500 mL/hL Cytolan (R) Stress caused phytotoxic effects, underscoring the importance of applying the biostimulant at the proper dose.