Seed imbibition: a critical period for successful germination

Imbibition, the uptake of water by the dry seed, involves absorption of water by cell wall and protoplasmic macromolecules, i.e., proteins and polysaccharides, wherein water molecules are held by electrostatic forces such as hydrogen bonds. The movement of water into the seed is due to diffusion and capillary action with water moving from a region of higher to lower water potential. Of the three components of seed water potential, i.e., osmotic potential, matric potential, and turgor pressure, it is the matric potential of cell walls and their contents which is primarily responsible for imbibition. Permeability of the testa, or seed coat, is a major factor controlling the rate of water uptake. Although necessary, imbibition is a period of peril: rapid uptake of water may cause imibibitional injury, cold temperatures may cause chilling injury, anaerobic conditions may lead to accumulation of toxic chemicals, and leaching of cellular constituents into the soil may stimulate microbial attack. Low vigor legume seeds with permeable seed coats are especially susceptible to imbibitional injury. The imbibition period offers opportunity as well as hazard. Seeds may be primed for increased vigor by imbibing and then drying back. High membrane permeability during early imbibition may facilitate insertion of germination-promoting and anti-pathogen chemicals into seed tissues. This review covers: The physical process of imbibition, the seed coat as a protection against imbibitional injury, membrances during imbibition relative to seed vigor, leachates as indicators of vigor, and use of the imbibition period to improve vigor.