To explain the differing germination behaviour of seeds of wild type, gibberellin-deficient (gib1) or abscisic acid-deficient (sitw) mutants of tomato (Lycopersicon esculentum Mill. cv. Moneymaker), growth and water relations of fruit tissues, seeds and embryos were determined during development. The objective was to determine whether the hormones influenced water relations of fruit and seed tissues and as a consequence germinability. Despite the up to 70% lower fruit weight of the mutants, the fresh and dry seed weights were similar to that of the wild type. Water relations of pericarp and locular tissues in wild type and gib1 fruits were characterized by a slowly increasing ψπ that reached a constant value ψ of -0.60 MPa at 35-40 days after anthesis, resulting in a complete loss of turgor. However, sitw fruit tissues maintained their turgor throughout development, mainly as a result of considerably lower ψπ values but similar ψ values compared with the other genotypes. The ψ of wild type and gib1 seeds decreased from -0.50 to -0.80 MPa and ψπ from -0.80 to -1.00 MPa between day 30 and 40. From day 40 onwards, ψp was similar in both genotypes, approximately 0.20 MPa. As in the fruit tissues, sitw seed water relations were also characterized by higher turgor values than the other genotypes. Up to day 40, ψp was ca 0.40 MPa, dropped temporarily to zero, and increased again to approximately 0.40 MPa at day 50. Embryo water relations of both mutants deviated from the wild type in that ψp of the gib1 and sitw embryos remained at 3.5 MPa and 2.5 MPa, respectively, from day 40 onward, whereas in wild type embryos ψp decreased from 3.0 MPa at day 35 to approximately 1.0 MPa at 50 days after anthesis. This was mainly due to an increasing ψπ which was absent in the mutants. Throughout development there was equilibrium between ψ of pericarp, locular tissues and seeds but between embryo and seed or fruit tissues ψ gradients of up to 1.5 MPa were calculated in the wild type, and up to 1.0 MPa in the mutants. Thus, precocious germination is prevented by the action of the fruit's osmotic environment and ABA on the seed tissues surrounding the embryo and not the embryo itself. Embryos have a ψ which is low enough to overcome the solute potential of the fruit tissues throughout development.