The germination of seeds of tomato Lycopersicon esculentum(L.) Mill. cv. Moneymaker has been compared with that of seeds of the gibberellin-deficient dwarf-mutant linega-1, induced in the same genetic background. Germination of tomato seeds was absolutely dependent on the presence of either endogenous or exogenous gibberellins (GAs). Gibberellin A4+7was 1000-fold more active than commercial gibberellic acid in inducing germination of thega-1seeds. Red light, a preincubation at 2°C, and ethylene did not stimulate germination ofga-1seeds in the absence of GA4+7; however, fusicoccin did stimulate germination independently. Removal of the endosperm and testa layers opposite the radicle tip caused germination ofga-1seeds in water. The seedlings and plants that develop from the detippedga-1seeds exhibited the extreme dwarfy phenotype that is normal to this genotype. Measurements of the mechanical resistance of the surrounding layers showed that the major action of GAs was directed to the weakening of the endosperm cells around the radicle tip. In wild-type seeds this weakening occurred in water before radicle protrusion. Inga-1seeds a similar event was dependent on GA4+7, while fusicoccin also had some activity. Simultaneous incubation of de-embryonated endosperms and isolated axes showed that wild-type embryos contain and endosperm-weakening factor that is absent inga-1axes and is probably a GA. Thus, an endogenous GA facilitates germination in tomato seeds by weakening the mechanical restraint of the endosperm cells to permit radicle protrusion
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