Carl Johnson johnsonc at CTRVAX.VANDERBILT.EDU
Mon Nov 17 12:24:14 EST 1997

>In our class discussion of the photoperiodic control of flowering, after I
>presented the experiments showing (by means of night interruption) that
>plants are timing the length of the night, rather than the length of the
>day, a student asked **why** (in an evolutionary sense) it is the night
>that is timed.

        The internal circadian (daily) biological clock is the timer used
by organisms (plants and animals) to measure the duration of the
photoperiod. Circadian clocks are reset by light/dark signals. For light
pulses such as those used in the night-interruption experiment, pulses
given at night have the largest effect on the circadian clock in causing
phase shifting (interpretation: light in the day is expected, therefore no
phase shift is required; light at night indicates that the clock is out of
phase, therefore provoking a phase shift).
        Although the night interruption experiment has classically been
interpreted as indicating that the night interval is the one measured, this
interpretation is probably an over-simplification. Light and dark signals
phase a circadian clock that is the photoperiodic timer, and the duration
of those light and dark signals interact with the clock to promote a
physiological response.
        --Carl Johnson
        Vanderbilt University

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