Photoperiodism Monitoring Day Length

As we have seen, the circadian clock enables organisms to determine the time of day at which a particular molecular or biochemical event occurs. Photoperiodism, or the ability of an organism to detect day length, makes it possible for an event to occur at a particular time of year, thus allowing for a seasonal response. Circadian rhythms and pho-toperiodism have the common property of responding to cycles of light and darkness.

Precisely at the equator, day length and night length are equal and constant throughout the year. As one moves away from the equator toward the poles, the days become longer in summer and shorter in winter (Figure 24.16). Not surprisingly, plant species have evolved to detect these seasonal changes in day length, and their specific photoperi-odic responses are strongly influenced by the latitude from which they originated.

FIGURE 24.16 (A) The effect of latitude on day length at different times of the year. Day length was measured on the twentieth of each month. (B) Global map showing longitudes and latitudes.

FIGURE 24.16 (A) The effect of latitude on day length at different times of the year. Day length was measured on the twentieth of each month. (B) Global map showing longitudes and latitudes.

Photoperiodic phenomena are found in both animals and plants. In the animal kingdom, day length controls such seasonal activities as hibernation, development of summer or winter coats, and reproductive activity. Plant responses controlled by day length are numerous, including the initiation of flowering, asexual reproduction, the formation of storage organs, and the onset of dormancy.

Perhaps all plant photoperiodic responses utilize the same photoreceptors, with subsequent specific signal trans-duction pathways regulating different responses. Because it is clear that monitoring the passage of time is essential to all photoperiodic responses, a timekeeping mechanism must underlie both the time-of-year and the time-of-day responses. The circadian oscillator is thought to provide an endogenous time-measuring mechanism that serves as a reference point for the response to incoming light (or dark) signals from the environment. How changing photoperiods are evaluated against the circadian oscillator reference will be discussed shortly.

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