Abstract
The effect of exposing on floral response of developing Pharbitis plants to high tempera-tures (20° to 35°C) before or after being induced by chilling at 15°C was examined.
1. Exposure to high temperature before or after floral induction was inhibitory to flower-ing, but the effect was less pronounced on older plants than on younger ones.
2. Application of gibberellin A3 (GA3) to the shoot apex reversed the inhibitory effect of the high temperature treatment.
3. Exposure to diurnal thermoperiods of chilling and heating indicated that the flowering occurred at a certain minimum of hours of chilling for the plants of a given age. With an increase in plant age, the threshold hours of chilling became shorter for flowering to occur and the plants became less sensitive to reversal by high temperature.
4. The loss of sensitivity to high temperature of the older plants could be attributed to maturation of the shoot apex.
