Morphogenetic Responses to Gibberellic Acid of a Radiation-induced Mutant Dwarf in Groundsel, Senecio vulgaris L.

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Abstract
A dwarf mutant of Senecio vulgaris L. induced by gamma radiation, which differs from the wild-type by a single recessive gene, grew as tall as the wild-type plant, and almost five times as tall as dwarf control plants when treated at weekly intervals from the seedling stage with 10 p.p.m. gibberellic acid. The wild-type groundsel responded only slightly to similar treatment. Some evidence was obtained that the response to GA was relativily greater when plants were grown during the late autumn. Dwarfness in the mutant is due to fewer as well as to shorter internodes; the rate of leaf initiation is reduced, but the onset of the reproductive phase and the duration of the life-cycle are not affected by the mutation. Gibberellic acid leads to a conspicuous elongation of the internodes intreated dwarf plants, but not always to a significant increase in their number. An increase in the dry weight of treated plants was observed, and this was shown to be due to an increase in photosynthetic area and not to increased photosynthetic efficiency. Comparative observations of shoot morphogenesis in wild-type and dwarf groundsel. and in dwarf treated with gibberellic acid, have shown that the type of shoot development induced by gibberellic acid in dwarf plants does not correspond exactly to the normal mode of development in wild-type plants. Increased mitotic activity in the subapical tissue of the mutant following gibberellic acid treatment resulted in greater and earlier elongation of the internodes. However, the typical form and dimensions of the wild-type plant were not wholly restored.

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