The semidian rhythm in flowering response ofPharbitis nilin relation to dark period time measurement and to a circadian rhythm

Abstract

When seedlings of Pharbitis nil Choisy, cv. Violet, are exposed to a single inductive dark period at 27°C, brief interruptions with red light (R) can be promotive after 2–3 h of darkness but increasingly inhibitory to flowering up to the 8–9th h of darkness. This rhythmic response to R interruptions can be advanced in phase by > 1 h when the preceding light period is interrupted with far‐red (FR) 2 h before darkness (FR ‐2 h) or with FR – 15 h, whereas FR –8 h or FR–22 h retard the rhythm. These shifts in the R interruption rhythm are paralleled by equal shifts in the length of the dark period required for flowering. Brief FR interruptions of darkness displayed a similar rhythm which was also advanced by FR –2 h and retarded by FR –8 h. We conclude therefore that the semidian rhythm in the light, which we have previously described, continues through at least the first 12 h of darkness, is manifested in the R interruption rhythm, and determines the critical night length. A circadian rhythm with a marked effect on flowering was also identified, but several lines of evidence suggest that the circadian and semidian rhythms have independent additive effects on flowering and do not appear to show phase interaction.

Evidence that photoperiodic, dark time measurement in Pharbitis nil involves a circadian rather than a semidian rhythm

“…The second conclusion is that FRD iven 8 h or more before the end of the photoperiod has no ffect on the phase of the circadian rhythm running in darkess. This is in contrast to the conclusions drawn from ig.2 of Heide et al (1988) that FRD at -2, -8,-15 and 22 altered the timing of the R night-break response, lowever, careful examination of their data shows that onlŷ^R D at -2 h brought about an advance in both the descendng and ascending parts of the response curve; for the other ireatments the ascending (escape) part of the curve was shifted, but the descending part was altered very little, if at all. It is possible that some component of the induction process additional to the rhythm was affected.…”

“…(1) FRD interruptions of the photoperiod with a near CNL show a semidian rhythm. FRD affects both CNL and the time of maximum sensitivity to a R night break (Heide et al 1988).…”

“…In a later paper (Heide, King and Evans 1988), results were presented showing that, when a dark period was interrupted with FR, flowering was maximally inhibited when FR was given after 8-10 h of darkness. From this it was concluded that the semidian rhythm continues in darkness and that both the timing of the flowering response to R interruptions of an inductive dark period (the night-break response) and the measurement of the CNL are a function of this semidian rhythm (Heide etal.…”

Dark and Circadian Regulation of mRNA Accumulation in the Short-Day Plant Pharbitis nil

“…The experimental system we have used is P. nil Choisy strain Violet (Japanese moming glory), a qualitative SD species that has been well characterized with regard to the physiology and biochemistry of photoperiodic floral induction (Imamura, 1967;Takimoto, 1967;Vince-Prue and Gressel, 1985). The photoperiodic timing mechanism of this SDP (Takimoto and Hamner, 1965;Spector and Paraska, 1973;Lumsden et al, 1982;Vince-Prue and Lumsden, 1987;Heide et al, 1988) and the role of phytochrome in its photoperiodic responses Vince-Prue et al, 1978;Takimoto and Saji, 1984;Vince-Prue, 1989;Thomas, 1991) are also perhaps the most extensively characterized of a11 photoperiodic species. Unlike other SD photoperiodic species, Pharbitis becomes fully responsive to photoperiodic treatment to induce flowering within a few days after germination (Marushige and Marushige, 1963a;Imamura and Marushige, 1967), and flowering in seedlings is induced by exposure of the cotyledon to a single dark period (Imamura and Takimoto, 1955).…”

The Photoperiodic Control of Flowering: Progress Toward Understanding the Mechanism of Induction

“…The complicated nature of the timing mechanism is illustrated by the short-day plant, Pharbitis nil, for which endogenous rhythms are well characterized Hamner, 1964, 1965;Spector and Paraska, 1973;Evans et al, 1986;Heide et al, 1986Heide et al, , 1988Lumsden et al, 1982;VincePrue, 1975). For this short-day plant, physiological experiments have shown that the timing of the CNL and response to NB is controlled mainly by an endogenous rhythm initiated or reset at the onset of the main light period.…”