Let us make an in-depth study of the circadian rhythms in plants. After reading this article you will learn about 1. Endogenous Versus Exogenous Rhythms 2. Occurrence of Circadian Rhythms in Plants 3. Terminology 4. Establishment of Endogenous Nature of a Rhythm and 5. Biological Clock.
Introduction to Circadian Rhythms:
Like all other living organisms, the plants are and have always been exposed to strong and rhythmic environmental changes caused by planetary movements. For instance, the rotation of Earth on its axis gives rise to cycle of day and night, the revolution of Earth around the Sun gives rise to succession of seasons and the complicated movements of Moon in relation of Earth and Sun give rise to lunar months and tidal cycles.
It is but natural, that these environmental rhythmicities or periodicities find their counterparts in biological rhythms which control many behavioural and physiological activities of living organisms including plants. When the period of biological rhythmicities matches with those of the cycles of day and night, such rhythms are called as circadian rhythms (Circa = about; diem = a day).
Rhythmic behaviour in plants under natural conditions in known to scientist for over two thousand years. The so-called sleep movements (or up and down movements) of leaves in certain plants such as bean (Phaseolus multiflorus) is one such example and amongst the earliest observations on circadian phenomena.
In 1729, the French astronomer De Mairan carried out a very important experiment in which he transferred the plants to the continuous darkness of a cellar (a room under a house) and observed that the leaves showed rhythmic up and down movements for a number of days. He concluded that these were not dependent on the daily cycle of light and darkness even though they were normally synchronized with the latter. According to him these movements were controlled by an internal mechanism and were, therefore endogenous.
Since then, many distinguished biologists have investigated and confirmed this phenomenon including the famous Plant Physiologist Wilhem Pfeffer and Charles and Francis Darwin who devoted a full book to “The Power of Movements in Plants” (1880). Erwin Bunning (1971) who studied circadian leaf movements for over four decades believed that these leaf movements were “only an expression of plants kindness towards botanists, in allowing them to discover and record circadian rhythms within the plant”.
The findings of De Mairan marked the beginning of one of the most fascinating and mysterious areas of study (i.e., periodicities or rhythmicities in organisms) for plant physiologists and other biologists which in recent years has been termed as biological chronometry or bio-chronometry.
Besides circadian rhythms, the organisms have also developed many other endogenous rhythmicities during their long evolutionary history. The rhythmicities or periodicities whose periods match to those of lunation are called as circalunar rhythms (period ~ 29 days), to those of tide are called as circatidal rhythms (period ~ 12.4 or 24.8 hrs), to those of seasons are called as circaannual rhythms (period ~ a year) or to those of time between successive spring-low waters are called as circa semilunar rhythms (period ~ 14.7 days).
Endogenous non circadian rhythms with comparatively very short periods from a few minutes to some hours are called ultradian rhythms. However, among all these, circadian rhythms are best known. Based on the observations of Bunning and Tazawa (1957) an illustration of circadian rhythm in primary leaves of Phaseolus multiflorous is given in Fig. 22.1.
Endogenous Versus Exogenous Rhythms:
The discovery by De Mairan led to the firm establishment of distinction between the concepts of endogenous and exogenous rhythms. Exogenous rhythms found in many plants occur under natural conditions but they fail to persist in uniform environment since they are controlled solely by some environmental parameter.
For example, the rhythmicity observed in discharge of spores in cultures of the fungus Pilobolus crystallinus under natural conditions is lost if the cultures are transferred to uniform environmental conditions. On the other hand, as mentioned earlier, endogenous rhythms are not dependent on environmental parameters although they may be synchronized with them.
It is now generally believed that the plants (and other organisms) showing endogenous circadian (or other types) rhythms have time measurement system or biological clock inside their cells which measures the passage of time in much the same way as a pendulum.
Occurrence of Circadian Rhythms in Plants:
Circadian rhythms have been found in some members of almost all major groups of plants except Bryophyta and Gymnosperms. Some of the examples of circadian rhythms in plants are given in Table 22.1.
Terminology / Definitions:
In order to discuss the circadian (or other types) rhythms in plants, it is necessary first to get acquainted with common terms usually encountered in bio-chronometry.
Circadian rhythm:
Endogenous biological rhythm with an approximate period of 24 hrs. and not exactly 24 hrs. Usually it is between 21-28 hrs.
Oscillation:
Rhythmic movement such as up and down movement of leaves of certain plants comparable with the swinging movement of a pendulum.
Period:
The period of the rhythm is the time after which a definite phase of oscillation reoccurs, that is, for example, the time between successive peaks or troughs.
Phase (Ø):
It is the instantaneous state of an oscillation within a period.
Phase shift (Ư):
A single displacement of an oscillation along the time axis following a perturbation (disturbance). It may either involve an advance (+ Ư) or a delay (-Ư).
Amplitude:
It is the extent to which the observed response varies from the mean. In case of a pendulum, it is half the length of the swing.
Entrainment:
It is the synchronization of a biological oscillation (rhythm) to a zeitgeber (a rhythmically fluctuating environmental parameter) so that they both have the same period.
Free running period (r):
The period of an endogenous oscillator or rhythm under uniform environmental conditions i.e., in the absence of a zeitgeber e.g., at constant temp, and in continuous darkness or continuous light.
Zeitgeber:
This is a German word meaning time-giver. It is the forcing oscillation which entrains a biological oscillation e.g., the environmental cycles of light and temperature, tide, moonlight and season. In other words, it is a rhythmically fluctuating environmental parameter – a synchronizer.
Transient:
One or more temporarily shortened or lengthened period occurring as a result of perturbation by a light or temp, pulse.
Damping:
In most circadian rhythms in plants kept under uniform environmental conditions, there is gradual decrease in their amplitude with time. This is called as damping. Some of the characteristics of endogenous circadian rhythms are illustrated in Fig. 22.2.
Establishment of Endogenous Nature of a Rhythm:
A rhythm must satisfy the following set of five rules or conditions which were first proposed by Pittendrigh (1954), before it could be said to be endogenous:
1. The period of rhythm should not be exactly 24 hrs. (It is because the period of free running rhythm should not correspond exactly to known oscillations in environment).
2. The rhythm should continue under environmental conditions which are kept as constant as is physically possible.
3. The phase of the rhythm should be able to be shifted and the new phase subsequently retained under uniform environmental conditions.
4. The rhythm should be initiated by a single stimulus.
5. The phase of the rhythm should be delayed under anaerobic conditions (hypoxia).
The first three of the above five conditions or rules are especially important and if any one of these is not satisfied, a circadian rhythm is unlikely to be a true endogenous one. On the other hand, if the third condition mentioned above can be fulfilled by a clear demonstration of phase-shift, the endogenous nature of the rhythm is almost undoubtedly established.
Biological Clock:
The plants showing endogenous circadian rhythms have time measuring system or ‘biological clock’ inside their cells which measures the passage of time in much the same way as a pendulum. The nature and functioning of biological clock is not yet clearly understood.
There are, however, several evidences for the possible participation of:
(i) Membrane physiology, cytoplasmic organelles or
(ii) Nucleus and protein synthesis or
(iii) Higher frequency biochemical oscillations in the cells.
Accordingly, different models have been given by scientists to explain the working of circadian oscillator. For example, in Chronon model, the 24 hrs. period is ascribed to the time taken for the transcription of a hypothetical linear sequence of DNA, cistron by cistron.
According to a membrane model, this period is attributed to the slower processes involved in the lateral fusion of proteins with the lipid bilayer. According to another model, the circadian period is believed to be the result of interactions between higher frequency biochemical oscillations within the cell.
The location of the ‘biological clock’ too is not clear in the cells. Because circadian rhythms are not observed in cell-free extracts or isolated cell-organelles, it is likely that the biological clock does not lie within the cell but the whole cell itself probably acts as the biological clock.