Our earlier experiments on flowering in the short day plant
Pharbitis nil involved far- red/dark (FR/D)
interruptions of 90 min duration at various times during a continuous light,
constant temperature period before a single inductive dark period. They
revealed a rhythm with a period of 12 h, hence semidian. We concluded that the
phasing of this semidian rhythm determined the length of darkness required for
floral induction. This conclusion has since been challenged so we sought other
pretreatments which reveal the semidian rhythm. Interruptions at
12°C–17°C for 45–90 min at various times prior to the
inductive dark period were as effective as FR/D in eliciting the semidian
rhythm, with significant effects on flowering persisting for at least three
cycles in constant conditions in continuous light. The rhythmic response to
12°C pretreatments was 3 h out of phase with that to FR/D
pretreatments.
Flowering responses to the semidian rhythm exposed by 12°C pretreatments
were additive to and independent of those to a circadian rhythm. Some evidence
was obtained of reversal of the inhibition or promotion of flowering by
FR/D or 12°C by exposure immediately afterwards to the other
pretreatment at times of their opposite effect. Pretreatments at 12°C,
like those with FR/D, either reduced (if promotive) or extended (if
inhibitory) the length of the dark period required for floral induction in
this short day plant.