Gibberellins in Relation to Flowering and Stem Elongation in the Long Day Plant Silene armeria

Abstract

Two long days induced some flowering and 4 or more long days caused 100% flowering in Silene armeria. On long days microscopically detectable flower primordia were first seen after 6 days, which is at least 1 day before the start of stem elongation. Both gibberellin A₃ and A₇ caused flowering on short days, but the results were variable and flowering was never 100%. Three different gibberellins were detected in Silene extracts. The pattern of gibberellins extracted from plants on short and long days was qualitatively the same, but on long days gibberellin content was up to 100% higher than on short days. Only small amounts of diffusible gibberellins were obtained from Silene shoot tips (including very young leaves) on short days. However, on long days the diffusible gibberellins increased by as much as 10-fold after 4 to 6 long days but then declined somewhat after 10 long days. The gibberellins extracted from the shoot tips at the completion of the diffusion period also increased under long days, although the increase was not as large as for the diffusible gibberellins. An A₅-like gibberellin present in extracts was not detected in diffusates.

Treatment with AMO 1618 (2-isopropyl-4-dimethylamino-5-methylphenyl-1-piperidine-carboxylate methyl chloride) completely inhibited stem elongation on long days but had no effect on flowering. In addition, treatment with AMO 1618 caused at least an 80% decrease in the level of extractable gibberellin, while the diffusible gibberellin was reduced below the limits of detection in the d5 corn bioassay. When endogenous gibberellin levels were suppressed by pretreatment with AMO 1618 on short days, gibberellin A₃ caused more stem elongation in plants moved to long days than in plants left on short days. Thus the sensitivity of Silene plants to gibberellin with respect to stem growth is affected by photoperiod. It is concluded that in Silene endogenous gibberellins are a controlling factor for stem elongation but apparently are not required for flower formation.