The decisive effect of night length on flowering indicates that measuring the passage of time in darkness is central to photoperiodic timekeeping. Most of the available evidence favors a mechanism based on a circadian rhythm (Bunning 1960). According to the clock hypothesis, pho-toperiodic timekeeping depends on an endogenous circa-dian oscillator of the type involved in the daily rhythms described in Chapter 17 in relation to phytochrome. The central oscillator is coupled to various physiological processes that involve gene expression, including flowering in photoperiodic species.
Measurements of the effect of a night break on flowering can be used to investigate the role of circadian rhythms in photoperiodic timekeeping. For example, when soybean
FIGURE 24.20 The time when a night break is given determines the flowering response. When given during a long dark period, a night break promotes flowering in LDPs and inhibits flowering in SDPs. In both cases, the greatest effect on flowering occurs when the night break is given near the middle of the 16-hour dark period. The LDP Fuchsia was
FIGURE 24.20 The time when a night break is given determines the flowering response. When given during a long dark period, a night break promotes flowering in LDPs and inhibits flowering in SDPs. In both cases, the greatest effect on flowering occurs when the night break is given near the middle of the 16-hour dark period. The LDP Fuchsia was given a 1-hour exposure to red light in a 16-hour dark period. Xanthium was exposed to red light for 1 minute in a 16-hour dark period. (Data for Fuchsia from Vince-Prue 1975; data for Xanthium from Salisbury 1963 and Papenfuss and Salisbury 1967.)
FIGURE 24.21 Rhythmic flowering in response to night breaks. In this experiment, the SDP soybean (Glycine max) received cycles of an 8-hour light period followed by a 64hour dark period. A 4-hour night break was given at various times during the long inductive dark period. The flowering response, plotted as the percentage of the maximum, was then plotted for each night break given. Note that a night break given at 26 hours induced maximum flowering, while no flowering was obtained when the night break was given at 40 hours. Moreover, this experiment demonstrates that the sensitivity to the effect of the night break shows a circadian rhythm. These data support a model in which flowering in SDPs is induced only when dawn (or a night break) occurs after the completion of the light-sensitive phase. In LDPs the light break must coincide with the lightsensitive phase for flowering to occur. (Data from Coulter and Hamner 1964.)
plants, which are SDPs, are transferred from an 8-hour light period to an extended 64-hour dark period, the flowering response to night breaks shows a circadian rhythm (Figure 24.21).
This type of experiment provides strong support for the clock hypothesis. If this SDP were simply measuring the length of night by the accumulation of a particular intermediate in the dark, any dark period greater than the critical night length should cause flowering. Yet long dark periods are not inductive for flowering if the light break is given at a time that does not properly coincide with a certain phase of the endogenous circadian oscillator. This finding demonstrates that flowering in SDPs requires both a dark period of sufficient duration and a dawn signal at an appropriate time in the circadian cycle (see Figure 24.15).
Further evidence for the role of a circadian oscillator in photoperiod measurement is the observation that the pho-toperiodic response can be phase-shifted by light treatments (see Web Topic 24.4).
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