The perception of light—dark transitions

Transitions between light and darkness are particularly important where these serve as Zeitgebers to synchronize circadian rhythms. A special case is photoperiodism, which depends on the accurate detection of light—dark transitions and on the coupling of this information to a timing mechanism that appears to be based on the circadian clock. Results from laboratory experiments are considered in relation to the natural changes experienced at dawn and dusk, and evidence is presented that the light—dark transitions that couple to the timing mechanism in short-day plants are perceived through changes in irradiance rather than through changes in light quality. It has been generally accepted that the light—dark transition is sensed by a decrease of Pfr levels in darkness, whereas dark—light is sensed by the rapid formation of Pfr in the light. However, Pfr in light-grown plants appears to be rather stable and so changes in Pfr level after transfer to darkness may not be a sufficiently accurate method of detecting the light—dark transition in photoperiodism. The paper reviews some of the evidence from photoperiodic experiments and concludes that the plant may discriminate between light and darkness through the continuous or intermittent formation o f ‘new’ Pfr.