Effect of biostimulants on soybean (Glycine max L. Merr.) tolerance to hydrothermal stress under organic farming in Ukraine

Climate change-induced drought and extreme temperatures pose major challenges to global soybean (Glycine max L. Merr.) production. Therefore, sustainable strategies to mitigate hydrothermal stress are crucial, particularly in organic farming systems. A field experiment (2022–2024) was conducted in Ukraine to assess the effects of biostimulants on soybean yield under contrasting hydrothermal conditions. The tested biostimulants contained arbuscular mycorrhizal fungi (AMF) Glomus intraradices (MycoFix), nitrogen-fixing rhizobia Bradyrhizobium japonicum (Anderis) and phytohormonal metabolites from Streptomyces violaceus (Violar). Biostimulant effectiveness was assessed by abscisic acid (ABA) and relative water content (RWC), stomatal resistance (Rs) and yield of organic soybean cv. Adamos. The experimental design included 5 treatments from an untreated control to a combination of 3 biostimulants. Integrated application of MycoFix + Anderis + Violar reduced stress induced ABA accumulation by 35.8–40.0 % vs. the control. This effect was associated with improved plant water status, reflected in higher RWC (94.8–97.2 % vs. 73.2–77.6 %) and lower Rs (1.87–2.39 vs. 3.04–4.53 s×m2/mol), resulting in a 40.6–58.9 % increase in seed yield (3.21–3.82 vs. 2.02–2.63 t/ha). Correlation analysis showed strong relationships between ABA and Rs (r=0.970), ABA and RWC (r=-0.871) and RWC and yield (r=0.992), confirming the integration of hormonal, physiological and agronomic responses. Principal component analysis (PCA) showed that biostimulant treatments explained 93.6 % of total variance, far exceeding temporal climatic variation (4.4 %) and confirming their role in plant adaptation. These findings highlight the potential of multi-component biostimulant systems to improve soybean stress tolerance in organic farming under climate change.