The involvement of a circadian oscillator in photoperi-odism poses an important question: How does an oscillation with a 24-hour period measure a critical duration of darkness of, say, 8 to 9 hours, as in the SDP Xanthium? Erwin Bunning proposed in 1936 that the control of flowering by photoperiodism is achieved by an oscillation of phases with different sensitivities to light. This proposal has evolved into a coincidence model (Bunning 1960), in which the circadian oscillator controls the timing of lightsensitive and light-insensitive phases.
The ability of light either to promote or to inhibit flowering depends on the phase in which the light is given. When a light signal is administered during the light-sensitive phase of the rhythm, the effect is either to promote flowering in LDPs or to prevent flowering in SDPs. As shown in Figure 24.21, the phases of sensitivity and insensitivity to light continue to oscillate in darkness in SDPs. Flowering in SDPs is induced only when exposure to light from a night break or from dawn occurs after completion of the light-sensitive phase of the rhythm. In other words, flowering is induced when the light exposure is coincident with the appropriate phase of the rhythm. This continued oscillation of sensitive and insensitive phases in the absence of dawn and dusk light signals is characteristic of a variety of processes controlled by the circadian oscillator.
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