Gibberellins in the control of photoperiodic flower transition in Pharbitis nil

The role of gibberellins in the photoperiodic flower induction of short‐day plant Pharbitis nil has been investigated. It has been found that the endogenous content of gibberellins in the cotyledons of P. nil is low before and after a 16‐h‐long inductive dark period. During the inductive night the content of gibberellins is high at the beginning of darkness and about the middle of the dark period. Exogenous GA3 when applied to the cotyledons of non‐induced plants does not replace the effect of the inductive night but it can stimulate the intensity of flowering in plants cultivated on suboptimal photoperiods. GA3 could also reverse the inhibitory effect of end‐of‐day far‐red light irradiation on P. nil flowering. 2‐Chloroethyltri‐methylammonium chloride (CCC) applied to the cotyledons during the inductive night also inhibited flowering. GA3 could reverse the inhibitory effect of CCC. The obtained results strongly suggest that gibberellins are involved in the phytochrome controlled transition of P. nil to flowering. Their effect could be additive to that of photoperiodic induction.