Water Entry in Dry Soybeans at Imbibition Observed by Dedicated Micro-Magnetic Resonance Imaging

Water entry into dry soybeans (Glycine max) and delivery paths inside the testa at initial stages of imbibition were investigated. Time-lapse images of water uptake by the beans were tracked using a dedicated micro-magnetic resonance imaging (MRI) system equipped with a small 1.0-Tesla (T) permanent magnet. Images were continuously measured by the three-dimensional (3D) gradient-echo method at 5-min intervals with a T1-weighted condition, where water incorporated in the seed was considered to be the tracer of water migration. There was no barrier against water intruding into the seed coat for soybeans, i.e., activation of the water channel was not needed. Water was taken up from the raphe and rapidly transported to the hypocotyl at the base of the radicle pocket through paths formed in the dorsal testa, while water migrated via another path through the hilar tissue by the hilum and reached the radicle tip. Water incorporation inside the embryonic sac seemed biologically regulated because water did not penetrate into the space between the protruding hypocotyl-radicle axis and the cotyledons, or between expanding cotyledons, for considerable periods; which protects the seed from soaking damage. The raphe is considered to be the water entry for soybeans. Water is transported over the seed coat through vascular traces near the raphe. The intensely contrasted images in a low-field MRI might clearly exhibit the direction of water migration at imbibition for dry soybeans. The light and small MRI equipment is thought to be a useful means for seed researches.