| Abstract
| - The stimulative action of applied gibberellins (GA) on seed germination has been reported for a large number of species in the past three decades. Applied GA often replaces the need for environmental stimuli like specific temperature pretreatment or light. Therefore, it has been suggested that endogenous GAs are essential intermediates in the stimulation of germination. Endogenous GAs have been identified in seeds of a limited number of species, but most of the evidence for a regulatory role is entirely circumstantial. The use of isogenic mutants with lesions in the GA biosynthesis has presented direct evidence of a key role for endogenous GAs in the regulation of germination. GA deficiency absolutely prevents germination in seeds of Arabidopsis thaliana and tomato. Two different mechanisms of action exist The first one involves hydrolysis of reserve food. In seeds of tomato a factor, probably GA, diffuses prior to germination from embryo to endosperm where it induces hydrolysis of gaJactomannan-rich cell walls that are part of the mechanical resistance to the protrusion of the radicle. A second mechanism of GA action consists of a direct effect on the growth potential of the embryo. In Arabidopsis the stimulation of germination by light depends on the ability of the seeds to synthesize GA, but light also enhances the sensitivity of the seeds to GA. Dry storage and pre-incubation at 2°C also increased the responsiveness of Arabidopsis seeds to GA
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