Introduction. Examples

Most organisms have evolved to co-ordinate their activities with the day-night cycle caused by caused by the Earth's rotation. Rather than simply responding to the daily light-dark transitions, it would appear that organisms have evolved real clocks to time biological processes. "Circadian" rhythms (from 'circa'-about, 'dies'-a day) are the result of the best-characterised of these biological clocks, which times events that occur once per day. Even in the absence of environmental time cues, such rhythms maintain a period close to 24 hours. The circadian clock has been shown to regulate various aspects of metabolism, physiology and behaviour, in humans as in other organisms.

The circadian timing mechanism had been difficult to determine, until the identification of circadian rhythm mutants and their cognate genes in Drosophila, Neurospora and now in other organisms. Molecular and genetic studies indicate that the 24-hour period arises from a negative feedback loop controlling the transcription of the clock genes, though several elements of the loop probably have yet to be identified. Circadian rhythms seem very similar in all species, though it is unclear whether this reflects a common mechanism and hence, a common evolutionary ancestor, or whether an outwardly similar process has arisen several times in evolution.

In plants, the circadian clock controls processes including leaf and petal movements, the opening and closing of stomatal pores, the discharge of floral fragrances and many metabolic activities, especially those associated with photosynthesis. The circadian clock also influences seasonal cycles that depend on day-length, including the regulation of flowering. This photoperiodic system appears to depend on the circadian clock to measure the duration of the day or night, thus monitoring the passage of the seasons.


Arabidopsis thaliana, a model species for plant genetics, exhibits visible circadian rhythms in leaf movement ( see a video by Andrew Millar / Department of Biological Sciences, Warwick University )

   The video is made up of images, taken 3 hours apart, over a period of 24 hours.

and less obvious rhythms in the expression of many genes, notably chlorophyll a/b binding protein genes (CAB genes). The bioluminescent reporter gene, luciferase, has been used to visualise the circadian regulation of CAB gene expression, creating a glow rhythm in the CAB::luciferase transgenic plants. This rhythm can be monitored in single seedlings by low-light video imaging, which has allowed the identification of circadian rhythm mutants in Arabidopsis ( ).

The cellular clock of plant cells is well adapted to the 24 hour day-night cycle of the planet. Circadian rhythms arise in various aspects of photosynthesis, including carbon assimilation, stomatal conductance, and in levels of essential elements of photosynthesis, such as Photosystem II and RuBP. Changes in light intensity and carbon dioxide concentration can affect circadian rhythms of photosynthesis.

The graph below shows data from a single kidney bean leaflet for the circadian rhythms in carbon assimilation and stomatal
conductance under constant high light (500 mmol per m2 per second photon flux density) and constant ambient CO2 (35 Pa).

Under conditions of constant high light, the circadian rhythms in carbon assimilation and stomatal conductance quickly flatten out after a couple of days. The period is slightly longer than 24 hours. Notice that on the first day, the values of carbon assimilation (sy = 17 mmol CO2 per m2 per second) and stomatal conductance (sy = 0.52 mol H2O per m2 per second) were much higher than the values under moderate light conditions (sy = 7 and sy = 0.15, respectively). Under high light conditions, these values decrease over time, along with a decreasing amplitude. This is quite different from constant levels of moderate light, where the circadian rhythms persist for several days with the same amplitude and period as a 24 hour night-day cycle ( )