The duration of light and photoperiodic responses

Abstract

Exposure to alternating periods of light and darkness is a feature of the environment of most plants and animals. It is hardly surprising, therefore, to find that responses to the timing of these light/dark cycles are a nearly ubiquitous characteristic of life and that many of these responses represent useful adaptations to a fluctuating environment. A large number of biochemical events and several aspects of behaviour (such as ‘activity periods’ in certain animals and’ sleep movements’ in leaves) have been found to occur at a particular time of the day or night. For example, in the unicellular alga, Gonyaulax polyedra, there are two different manifestations of bioluminescence: low-intensity glowing which peaks towards the end of the night and high-intensity flashing which peaks in the middle of the night (Fig. 1c). In order to be able to locate an event at a particular time of day, the organism must possess some kind of timekeeping mechanism and must also be able to discriminate between light and darkness. Thus, daily timekeeping requires both a clock and a photoreceptor. When examined in detail it appears that in most, if not all cases the underlying clock is an endogenous oscillator which has certain characteristic properties in relation to light/dark cycles (Fig. 1). The photoreceptor, in contrast, varies widely between organisms.