During the soybean seed filling stage, 2 mechanisms existed for photosynthate transfer: phloem unloading in the seed coat to regulate sucrose efflux, and sucrose uptake by cotyledon cells (Liu et al., 2010). Since all photosynthate entering the soybean cotyledon must enter through the seed coat, ABA might offer a driving force for photosynthate phloem unloading in the seed coat. Of course, ABA may also stimulate phloem assimilate unloading in controlling cotyledon enlargement during the seed growth period. This is because higher ABA concentration decreases the ATPase proton extrusion from the sieve tube elements, thus enhancing the efflux of sucrose into the apoplast by H+-sucrose co-transport (Tanner, 1980).