Water Relations of Tomato Seed Germination

Water uptake during the germination of UC 82B tomato seeds was triphasic. Seed Ψ measurements indicated that phase I imbibition occurred because of the large Ψ gradient between the seed and the imbibition solution (water). During phase II the seed Ψ was in equilibrium with the water. Phase III water uptake recommenced with the onset of radicle emergence without changes in Ψ or Ψπ. Changes in embryo water content were also triphasic. During phase II the embryo Ψ remained at - 1.5 MPa, not in equilibrium with the imbibing solution. At radicle emergence it was - 0.8 MPa and rose to -0.3 MPa as the radicle elongated. There was no evidence of a lowering of embryo Ψπ nor of a build up of Ψp prior to radicle emergence. Water uptake studies with excised embryos indicated that, within the seed, the enclosing tissues prevented the embryo from taking up water. It is suggested that embryo water content is restricted by the constraint on embryo expansion caused by the enclosing endosperm tissue. Lowering of embryo Ψπ to build up Ψp is not necessary for radicle emergence. The control of germination may lie in the mechanism which leads to weakening of this mechanical restraint of the endosperm.