Biostimulatory and Inhibitory Effects of Natural Extracts on Vigna radiata: Concentration-Dependent Responses

Natural extracts derived from plants and algae are increasingly recognized for their ability to modulate plant growth and physiological processes due to their rich content of bioactive compounds. Depending on their composition and concentration, these extracts may act as biostimulants, enhancing germination, stress tolerance, and antioxidant activity, or may exhibit inhibitory effects. This study aimed to determine the effects of extracts from Ascophyllum nodosum, Fucus vesiculosus, and Sideritis scardica, prepared using different solvents, on germination parameters, early growth, chlorophyll content, chlorophyll a fluorescence parameters, secondary metabolite levels (phenolic compounds and flavonoids), and antioxidant activity in mung bean (Vigna radiata L.). The results showed that aqueous and hydroethanolic extracts were generally safe for germination, maintaining high germination capacity, whereas 10% ethanolic extracts exhibited pronounced inhibitory effects, reducing germination energy to as low as 8%. Secondary metabolism was markedly affected, particularly by S. scardica extracts, with dose-dependent changes observed in total phenolic and flavonoid contents. Despite these biochemical alterations, chlorophyll fluorescence parameters remained stable, indicating the absence of photoinhibitory stress. Overall, the results indicate that the biological activity of the extracts depends strongly on solvent type and concentration, which determine whether they act as biostimulants or inhibitors by modulating early growth, metabolism, and physiological status. These findings provide a basis for the development of optimized plant- and algae-based biostimulant formulations.