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

When radioactive gibberellin A5 ((3)H-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 (3)H-GA5 conversion was influenced by the photoperiod under which Silene plants were grown. Exposure to 2 long days significantly increased (3)H-GA5 metabolism over that in control plants kept under short days. The increased conversion of (3)H-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.