Circadian rhythms
The word circadian comes from circa diem (Latin) "about a day". Animals and plants have endogenous rhythms that control their metabolic activity on a c. 24 hour cycle. In animals they regulate responses such as sleep, glandular activity liver function, and urine secretion. In plants, they regulate activities such as photosynthesis and flowering.
Circadian rhythms are controlled by a biological clock, also termed an endogenous oscillator. The adaptiveness of this system is that it enables the organism to anticipate the needs of the day in good time to prepare for them. For plants, this may mean knowing when it's worth opening flowers: at night if you are bat-pollinated, during the day if you are bird- or bee-pollinated. The oscillator usually runs with an approximately 24 hr period.
If plants are grown under constant conditions (e.g. continuous darkness) they will free-run, showing a 'subjective day' and 'subjective night', i.e. flowers will still open and close on a 24 hour basis, even if kept in continuous light, with no 'actual' night.
If plants are kept in a constant environment for a long time (days to months, depending on species and response being considered), the circadian rhythms eventually fade away.
Zeitgeber (German) "time giver". The clock is normally entrained to an exact 24 hour period by signals from the environment. Changing the phase significantly can upset circadian rhythms: e.g. jet lag when moving between time zones.
Plants have endogenous circadian rhythms. Many are concerned with preparing the plant for photosynthesis before it actually gets light, e.g. stomata open just before dawn. Many of these rhythms are entrained by light, and are termed photonasties. Plant movements, such as leaf folding in Phaseolus beans, continue on circadian cycle even in a constant environment. The leaves are held horizontal (for maximum light interception) during subjective day, and vertical (to reduce heat-loss) during subjective night. Other circadian rhythms control the opening and closing of flowers to protect pollen from the dew, and to ensure flowers are only open when pollinators are around. However, many of the rhythms are metabolic, and not easily seen without special equipment. The single (but huge) celled alga Acetabularia, if placed into continuous light, show peaks of photosynthesis during the subjective day. If placed in continuous darkness it shows peaks of respiration during the subjective night. The cells switch scarce metabolic resources between functions. We cannot see the biological clock directly, but we can study its characteristics by looking at the functions it controls. The natural free-running period is genetically controlled and varies between species and between individuals. For a given individual, it tends to be constant to within a few minutes.
In plants, the most important zeitgebers are changes between light and dark at dawn and dusk, and changes in temperature between day and night. However, the free running period of the rhythm is almost independent of absolute temperature: this is important in environments where there are large temperature variations: these should entrain the clock, but not drastically alter its free-running period (from 24 to 6 hours for example). There may be a temporary "hiccough" in metabolic rhythms immediately following a sudden temperature change as temperature acts as a zeitgeber.
Each living plant cell has its own independent biological clock. They are normally all synchronised together by receiving common signals from the environment. In mammals, this is not the case: circadian rhythms are detected by the eye and pineal body in the brain, and is transduced to the rest of the body by the hormone melatonin. Arabidopsis TOC mutants suggest a mechanism for the biological clock in plants:
TOC: timing of chlorophyll-a/b binding protein (CAB is needed for
photosynthesis). Presumably chosen mostly so you can make bad jokes
about tick-tock. CCA: circadian clock associated protein. LHY: long
hypocotyl mutant
In Drosophila, cryptochrome mutants (cry) have faulty clocks. In Arabidopsis, cry mutants oscillate, but cannot entrain. Phytochrome mutants cannot entrain either. Both phytochrome and cryptochrome are clearly involved in light-entrainment, but the mechanism is not yet known.
Flowering is also under the control of light. 'Short-day plants' flower after the day length decreases below a certain threshold e.g. wheat. 'Long-day plants' flowers after the day length increases above a certain threshold, e.g. Arabidopsis. This is easily explained by a receptor that promotes flowering which has circadian rhythms of sensitivity. Long day plants (shown below) flower when this receptor is stimulated by daybreak falling ever earlier. The width of the arrow shows the length of time the receptor is stimulated for. Note the lengthening days, and increasing stimulation. The timescale has been altered to make the graph easier to interpret.
Explaining short-day plants is left as an exercise for the reader (or just check the answers out). The receptor in short-day plants appears to be phytochrome (from the action spectrum). In long-day plants, both phytochrome and cryptochrome are involves. It's all a bit complicated, and not very well understood!
Test yourself
- Why is a biological clock evolutionarily advantageous?
- Describe the process of light-entrainment in terms of the TOC1 and LHY/CCA1 genes.
- Explain short-day flowering in terms of a circadian oscillator.
Bibliography
- Taiz, L. and Zeiger, E. (2002). Plant Physiology. 3rd edition. Sinauer Associates Incorporated, Sunderland, Massachusetts. 559-590. "The control of flowering"
- Webb, A. A. R. (2003). The physiology of circadian rhythms in plants. New Phytologist 160:281-303. http://dx.doi.org/10.1046/j.1469-8137.2003.00895.x
This page has been peer reviewed by 1 person.