Gibberellins and the photosensitivity of isolated embryos from non-stratified apple seeds.

The phytochrome system is responsible for the photosensitivity of dormant, isolated apple embryos in culture. Maximum photosensitivity occurs on the second day of culture and it is unaffected by gibberellins (GAs) in concentrations below 10(-4)M. Higher concentrations of GA decrease the photosensitivity.The endogenous quantities of GA4 and GA7 were determined in embryos grown at white light, in darkness and in darkness following an exposure to red light. The GA7 level remained unaffected by the light conditions, whereas the amount of GA4 was three times higher in light or red-light-treated cultures than in the dark grown ones.Similar experiments were done using AMO-1618, an inhibitor of GA biosynthesis, which is also a strong inhibitor of apple seed germination. In this case the level of both GA4 and GA7 was light-independent. These experiments suggest that the phytochrome system participates in the regulation of GAs biosynthesis by mediating one of the last steps of GA4 formation.