Uptake and redistribution of 10 B-labeled foliar boron sources in soybean: assessing complexed sources

Foliar boron (B) fertilization in soybean [Glycine max (L.) Merr.] may be limited by the restricted mobility of B within the plant, which can reduce its delivery to tissues formed after spraying. Complexed B sources have been proposed as alternatives to improve the efficiency of foliar fertilization, but quantitative evidence on their uptake and redistribution in soybean remains limited. This study evaluated whether complexed B sources improve the uptake and redistribution of foliar-applied B under contrasting substrate B supply. Soybean plants were grown in pots under background-B or high-B substrate supply and received foliar applications of 10B-labeled boric acid, B-monoethanolamine (B-MEA), B-polyol, or deionized water (control). Treatments were applied at R2, and plants were harvested at R6. Dry mass, total B, B derived from foliar fertilizer (Bdff), foliar uptake, redistribution, mobilization, and Bdff partitioning were determined in roots, sprayed parts, and new parts. Foliar uptake of 10B, estimated as the recovery of foliar-applied B in plant tissues, ranged from 14% to 31% of the B applied by spraying and was higher under background-B than high-B supply. B-polyol promoted the highest recovery of applied 10B relative to B-MEA, whereas B-MEA showed the lowest recovery. However, the higher uptake promoted by B-polyol did not increase Bdff concentration, mobilization, or recovery in new parts compared with boric acid. Foliar B sources did not consistently increase dry mass or B accumulation in new parts. Therefore, polyol complexation improved B recovery in plant tissues but did not overcome the limited within-plant mobility of B in soybean.