Influence of daylength on gibberellin metabolism and stem growth in Silene armeria

Summary When radioactive gibberellin A₅ (³H-GA₅) was applied to the apices and surrounding young leaves of the long-day plant Silene armeria, it was partially converted to at least two other acidic substances. One of them was similar to GA₃ in chromatographic, but not in biological properties. The other metabolite was more polar than GA₃ and inactive in the dwarf d-5 corn assay.The rate of ³H-GA₅ conversion was influenced by the photoperiod under which Silene plants were grown. Exposure to 2 long days significantly increased ³H-GA₅ metabolism over that in control plants kept under short days. The increased conversion of ³H-GA₅ persisted for at least a few days after transferring Silene plants back from long to short days. Likewise, stem growth induced by long photoperiods continued for a considerable period of time under subsequent short days.Application of the growth retardant AMO-1618 to Silene reduced the levels of two endogenous GA-like substances, one of them with GA₅-like properties, more under long than under short days. These results indicate that long photoperiods, which induce flower formation and stem elongation in Silene, increase the turnover of endogenous gibberellins.