ABA in photoperiodic induction of Pharbitis nil.

The endogenous content of ABA in the cotyledons of Pharbitis nil is high during the light phase before an inductive 16-h-long dark period. During the night, however, at the beginning, the level of ABA is relatively low with the tendency to increase during the second half of an inductive dark period. The dual effect of exogenous ABA on the Pharbitis nil flowering has been observed. ABA applied to the cotyledons on subthreshold photoperiod (12-h-long night) stimulates flower bud formation. On the other hand, however, ABA applied during an inductive (16-h-long) dark period, as well as applied to the medium of cultured plantlets, inhibits flowering. Thus, the flowering effect of ABA is clearly dependent on the state of flower induction which is different in plants growing on various photoperiods.