Soybean Seed Phenol, Lignin, and Isoflavones and Sugars Composition Altered by Foliar Boron Application in Soybean under Water Stress

Previous research showed that foliar boron (B) application at flowering or seed-fill growth stages altered seed protein, oil, and fatty acids. The objective of this research was to investigate the effects of foliar B fertilizer on seed phenolics (phenol, lignin, and isoflavones) and sugars concentrations. A repeated greenhouse experiment was conducted on soybean [(Glycine max(L.) Merr.)] under watered and water-stressed conditions. Soybean plants were divided into different sets, and each set was subjected to one of the following treatments: W = plants were watered with no foliar B; WB = plants were watered and received foliar B; WS = plants were water-stressed with no foliar B; WSB = plants were waterstressed and received foliar B. Foliar B was applied at rate of 0.45 kg/ha twice at flowering and twice at seed-fill stages. The results showed that total phenol and lignin concentrations were higher in seed collected from water-stressed plants compared with those collected from watered plants whether B was applied or not. The higher total phenol and lignin concentration in seed collected of water-stressed plants may be due to B-deficiency in plant tissues. Application of B resulted in higher concentrations of total seed B and isoflavones under watered and water-stressed plants. Higher cell wall B was higher in water-stressed plants than in watered plants, having an opposite trend to total B. Application of B resulted in higher seed sucrose in watered and water-stressed plants, but raffinose and stachyose were significantly higher under water-stressed plants. The research demonstrated that foliar B fertilizer altered seed phenol, lignin, isoflavones, and sugars, suggesting that B involved in phenolics and sugar metabolism. The higher cell wall B in waterstressed plants than in watered plants supports previous research that B has mainly a structural role. The higher sucrose resulting from foliar B in watered plants is desirable as sucrose contributes to seed quality. The increase of raffinose and stachyose concentrations in seed of water-stressed plants is undesirable as raffinose, and especially stachyose may be involved in water stress/drought tolerance. The current knowledge would help soybean breeders select for higher phenolic compounds and desirable sugars for higher seed qualities under drought conditions.