Effect of Environment on the Responses of Dwarf Groundsel to Gibberellic Acid

Abstract

THE striking effects of gibberellic acid on internodal elongation in dwarf mutants and rosette plants of a number of species are now well established, and the effects of this substance on a single gene dwarf mutant, induced by γ-radiation in groundsel, Senecio vulgaris L., have recently been described¹. The observations now reported show that the responses of this mutant to gibberellic acid may be considerably affected, both quantitatively and qualitatively, by environmental conditions. In fact, the phenotypic expression of both wild-type and mutant groundsel is greatly influenced by the environment: in certain unfavourable conditions the plants are stunted and the vegetative phase is prolonged. When the wild-type (tall) and mutant (dwarf) groundsel are grown in a greenhouse during the normal growing season, that is, from early spring to late summer, the transformation of the vegetative apex to an inflorescence apex occurs not later than 2 weeks after the emergence of seedlings from the soil, and fruiting begins when plants are 7–10 weeks old; but the apices of tall and dwarf plants grown in a growth cabinet under ‘daylight’ fluorescent tubes, 400 foot-candles, with a day-length of 10 hr., and a temperature of 22 ± 2° C, remained vegetative for 11 weeks.