Effect of Foliar Treatment with Organic Acids and Inorganic Nutrients on Yield Retention and Secondary Metabolite Profiles under Heat Stress in Soybean

Foliar application of inorganic nutrients and organic acids can affect the metabolic profile of soybean leaves and enhance soybean productivity. In this study, we investigated whether foliar application of 12 inorganic nutrients [N, P, K (10 g L⁻¹ each); Ca, Mg, B, Zn, Fe, Cu, Mo, Si (200 g L⁻¹ each); Se (20 g L⁻¹)] and five organic acids [acetic, ascorbic, citric, formic, and malic acids (200 mM each)] could alter secondary metabolite pathways and improve yield under heat stress. Treatment with Fe, Ca, and Mg induced accumulation of 3,4-dihydroxybenzoic acid, with subsequent increases in downstream isoflavones, while application of Se, Mo, Zn, P, and acetic acid led to a decrease in biochanin A, along with correlated changes in other metabolites including caffeic acid and daidzin. N and Si treatment independently increased cinnamic acid content. Ca and P treatment enhanced the accumulation of glycitin and genistein, while acetic acid treatment increased the accumulation of glycitin and daidzin. Treatment significantly altered secondary metabolite profiles in soybean leaves and alleviated yield loss under heat stress, although individual metabolite concentrations showed no significant correlations with this effect. Heat-stress tolerance mediated by metabolites appears to arise from comprehensive changes in the metabolic profile rather than the effect of a single metabolite. Overall, foliar applications reshaped leaf metabolite networks and improved yield retention under heat stress, with the largest profile shifts under P, Ca, and Mg. Follow-up physiological and enzymatic assays are recommended to establish causality for heat stress mitigation and metabolite enhancement. These findings suggest soybean leaves as a valuable source of beneficial secondary metabolites and indicate new options for soybean use and cultivation value.