Interactions of phytochrome and exogenous gibberellic acid on germination of Lamium amplexicaule L. seeds.

In henbit (Lamium amplexicaule L.) seeds, micromolar levels of gibberellic acid (GA3) introduced at the onset of imbibition markedly stimulated germination. Sensitivity to GA3 stimulation is rapidly lost if the seeds are pretreated in darkness at 15° for 1 day or more on a water substrate. Loss in responsiveness to GA3 is hastened by pretreatment in far-red or by dark pretreatment at higher (25°) temperatures and is reduced by pretreatments at 5°. A portion of the lost responsiveness to GA3 at 15° or higher is attributable to onset of a secondary dormancy, but the results of far-red treatment at 5°, where secondary dormancy is not a factor, clearly indicate that some phytochrome in the far-red absorbing form increased the effectiveness of gibberellic acid action.