Bound Water in Soybean Seed and Its Relation to Respiration and Imbibitional Damage

In an attempt to understand the initial stage of seed imbibition-the wetting stage-we have examined water binding in dry soybean cotyledon tissue using water sorption isotherm curves. The sorption isotherms show three levels of water affinity: a region of strongly bound water at moisture contents below 8%, a region of weakly bound water at moisture contents between 8 and 24%, and a region of very loosely bound water at contents greater than 24%. The enthalpies of the water binding for the three sectors were -6 to -12.5, about -2.5, and about -0.5 kilocalories per mole water, respectively.The degree of physiological activity in the tissue reflects the level of water binding. O(2) consumption is first detectable in the second region of water affinity (8-24% water), and increases dramatically with increasing water content above about 24%. Damage due to imbibing water is greatest when initial seed moisure contents are in the region of strongest water binding. Damage is lessened and finally absent when seed moisture contents are increased to the second and then to the third level of water affinity.