Cycle Length Experiments

Another approach which has revealed a circadian rhythm in photoperiodic timekeeping is to examine the flowering response under different cycle lengths: these are sometimes called resonance experiments. It has been shown that the flowering response may change rhythmically with increasing duration of the dark period which follows a short photoperiod. For example, when a constant short photoperiod was combined with dark periods of various durations up to 64 h, flowering in the SDP Glycine and Impatiens was inhibited in cycle lengths of 36 and 60 h, while optimum flowering occurred in cycle lengths of 24, 48 and 72 h (Fig. 2.8 and Nanda et al, 1969b). With a single dark period interrupting continuous light, the SDP Chenopodium rubrum also showed a circadian rhythm (with a periodicity of about 30 h) in the flowering response to the duration of darkness. This rhythm was shown to originate in the leaf (the site of photoperiodic perception) and required only one leaf to be exposed

SDP: Glycine max Biloxi Number of flowering nodes /

LDP: Hyoscyamus niger Number of plants bolting .

SDP: Glycine max Biloxi Number of flowering nodes /

LDP: Hyoscyamus niger Number of plants bolting .

Cycle length / h

FIG. 2.8. Effect of cycle length on the flowering response of short-day and long-day plants. Photoperiods of 8 hours (Glycine) or 6 hours (Hyoscyamus) were combined with dark periods of various durations to achieve the cycle lengths indicated on the abscissa (e.g. 24 h cycle = 8 or 6 h light + 16 or 18 h dark). Glycine plants received 7 cycles and Hyoscyamus plants 42 cycles. Data from Hamner and Takimoto, 1964 (Glycine) and Hsu and Hamner. 1967 (Hyoscyamus).

Cycle length / h

FIG. 2.8. Effect of cycle length on the flowering response of short-day and long-day plants. Photoperiods of 8 hours (Glycine) or 6 hours (Hyoscyamus) were combined with dark periods of various durations to achieve the cycle lengths indicated on the abscissa (e.g. 24 h cycle = 8 or 6 h light + 16 or 18 h dark). Glycine plants received 7 cycles and Hyoscyamus plants 42 cycles. Data from Hamner and Takimoto, 1964 (Glycine) and Hsu and Hamner. 1967 (Hyoscyamus).

(King, 1975). A similar rhythm in the response to the duration of the dark period has been observed in LDP. In Hyoscyamus, when a 6 h photoperiod was combined with dark periods of various durations, a high level of flowering occurred in cycle lengths of 12, 36 and 60 h, with minimum flowering in 24, 48 and 72 h cycles (see Fig. 2.8). Thus, a circadian rhythm in the flowering response to cycle length has been observed in both LDP and SDP (including the alga Acrochaetium ) and, as with the night-break experiments, there is a clear difference between the two photoperiodic categories in the timing of their responses to light (see Fig. 2.8). It is evident that, if flowering is to occur in these plants, the external light/dark pattern must be synchronised in some way with an internal circadian oscillation. The results illustrate particularly well that, contrary to the conclusions drawn from early experiments in 24 h cycles (see Fig. 1.5), the absolute length of the dark period is not always the controlling factor in photoperiodic induction. The LDP Hyoscyamus failed to flower when a 6 h photo-period was combined with 18 h of darkness (a 24 h cycle) but flowering was restored if the duration of the dark period was either increased or decreased (see Fig. 2.8). It is apparent that, in Hyoscyamus, a SD inhibits flowering in a 24 h cycle not because the night is too long, but because the light/dark cycle is unfavourable. However, although SDP also show rhythmic responses to cycle length, they do not flower with less than a critical duration of darkness.

Not all photoperiodically sensitive plants show clear-cut rhythms in cycle length. Xanthium appears to be totally non-rhythmic under these conditions (Fig. 2.9),

Xanthium floral stage i

Xanthium floral stage i

Duration of dark period / h

FIG. 2.9. Flowering responses of two short-day plants, Xanthium strumarium and Pharbitis nil, to a single dark period of various durations. Xanthium plants were grown in long days prior to the experiment. Pharbitis plants were grown in continuous light and then received 8 hours of darkness followed by 12 hours of light before transfer to the experimental dark period; plants that were transferred from continuous light to the various durations of darkness behaved like Xanthium. After Vince-Prue, 1975 (Data for Xanthium from Moore et al., 1967 and for Pharbitis from Takimoto and Hamner, 1964).

Pharbitis Number of flower buds

Duration of dark period / h

FIG. 2.9. Flowering responses of two short-day plants, Xanthium strumarium and Pharbitis nil, to a single dark period of various durations. Xanthium plants were grown in long days prior to the experiment. Pharbitis plants were grown in continuous light and then received 8 hours of darkness followed by 12 hours of light before transfer to the experimental dark period; plants that were transferred from continuous light to the various durations of darkness behaved like Xanthium. After Vince-Prue, 1975 (Data for Xanthium from Moore et al., 1967 and for Pharbitis from Takimoto and Hamner, 1964).

although as already mentioned, it exhibits other features associated with circadian rhythms. Pharbitis shows yet another type of response. When a single inductive dark period followed a long period in continuous light, the response was non-rhythmic and resembled Xanthium-, if, however, an inductive dark period was preceded by a non-inductive dark-light cycle, the response to increasing duration of darkness showed a stepwise increase with a periodicity of about 24 h (see Fig. 2.9).

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