The induction of flowering by chilling of Pharbitis nil Choisy, strain Kidachi, short day plant, grown in continuous light, first became apparent when the plants were 10d old and the first foliage leaf was emerging from the plumule. Thereafter, as the shoot developed with more leaves the flowering response became stronger.
Gibberellin A₃ (GA₃) stimulated flowering when it had been applied to the shoot apex before or during chilling. After chilling GA₃ caused little or no promotion of flowering, depending on the older plants or young ones. These results indicate that early floral processes at the apex initiated by chilling are mostly stimulated by GA₃. For older plants, subsequent floral processes could be promoted as well even after the end of chilling. GA₃ also elicited flowering response in defoliated plants which failed to respond to chilling. Thus, the role of the leaves would be partly performed by GA₃. These results suggest not only a perception site of the shoot apex to chilling but an important role of gibberellin factor in leaves on onset of floral processes at the apex exposed to chilling.