Effects of Foliar Boron Nutrition on Seed Sugars in Soybean Under Drought Stress Conditions.

Boron is an essential nutrient for plant growth, development, yield, and seed quality. Its deficiency is soil or limited uptake due to abiotic stress factors such as drought results in yield loss and poor seed quality. Limited information is available on the effects of foliar boron (B) application on soybean seed sugars. The objective of this research was to investigate the effects of foliar boron nutrition on seed sugars (sucrose, raffinose, stachyose, glucose, and fructose) under drought stress conditions.  In a repeated greenhouse experiment, soybean cultivars were grown in either irrigated (Control) or non-irrigated (drought stress,  DS) conditions, and either foliar B applied or not. Foliar B, in the form of boric acid (H₃BO₃), was applied twice at a rate of 1.1 kg ha^-1. The first application was during flowering stage ((R1-R2) and the second application was during seed-fill stage (R5-R6). The treatments were: watered plants without B (W-B), watered plants with B (W+B), water-stressed plants without B (WS-B); water-stressed plants with B (WS+B). Soil water potential for watered plants was kept between −15 and −20 kPa and water-stressed plants was kept between −90 and −100kPa. The results showed that seed sucrose, glucose, and fructose (desirable sugars) were higher in W-B and W+B compared with WS-B and WS+B. Raffinose and stachyose (undesirable sugars) were higher in seed of plants grown under drought stress conditions. The lower seed sucrose, glucose, and fructose under drought stress conditions was accompanied with limited B movement from leaves to seed, indicating possible involvement of B in seed sugar accumulation. This research demonstrated that foliar B application increased B accumulation in seed and altered seed sugars, indicating a possible involvement of B in soybean seed sugar quality.