Rhythms in Plant Life

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Rhythms in Plant Life

The natural environment is always changing, sometimes predictably, but more often not. Unlike animals, plants cannot move away or seek shelter from unfavorable conditions, so it is crucial that they are able to adapt quickly to such changes. Unpredictable changes include daily temperature, rainfall, and amount of light, and plants have developed a range of responses to deal with these changes. However, some aspects of the environment change regularly, such as the seasons; the monthly waxing and waning of the moon; the cycle of the tides coming in and out; and, of course, the daily changing of light and dark. It is therefore not surprising that, like most other living organisms, plants have evolved so that their behavior or development changes in synchrony with these predictable changes in the environment.

Types of Rhythms

A rhythm is a process that changes regularly and continuously. It can best be represented as a wave, as with light or radio waves, or on a graph where response is plotted against time. The distance between successive peaks or troughs of the wave is then referred to as the period of the rhythm. Rhythms in plants have a range of periods. For example, the circular growth of some stems has a period of less than one hour, but the flowering in some bamboos has a period of seven years. The most widespread rhythms are those with a period of about twenty-four hours, referred to as circadian rhythms (from the Latin circa, meaning "about," and diem, meaning "day"). Some examples of processes that show circadian rhythms are photosynthesis, stomatal movements, root pressure, nitrogen fixation, bioluminescence, cell division, leaf movements, flower opening, and fragrance emissions. Circadian rhythms, which match the daily twenty-four-hour cycle of day and night, have almost certainly been selected for during evolution, and it is thought that they are the visible expression of a biological clock in plants.

Characteristics of Circadian Rhythms

Perhaps the first observation of a circadian rhythm associated with a plant was made by Androsthenes, scribe to Alexander the Great, who noticed that the leaves of certain trees were elevated by day and drooped at night. More recently, an eighteenth century French astronomer, Jean de Mairan, observed that the leaves of certain "sensitive" plants, probably mimosa, continued to open and close even during long periods of darkness. In the first half of the twentieth century the German plant physiologist Erwin Bünning made detailed observations of the movement of bean leaves. He confirmed that the leaves continued to move up and down in constant darkness, and established that the period was 25.4 hours.

Bünning's work also established the most important property of these rhythms, which is that they are truly internal and thus generated by the plant. The rhythm continues running under constant environmental conditions (called a free running rhythm), which indicates that it is driven from within and not by a rhythm of the environment. Another important property is that the phase of the rhythm can be changed by light. This means that every day, at the onset of daylight (dawn) the rhythm is reset so as to coincide with the daily light-dark cycle in the environment. This phenomenon is known as entrainment and is crucial to the functioning of the biological clock.

Nature of Biological Clocks

The biological clock allows an organism to match its internal system with the time of day, so that in some sense it could be said to "know" what the time is. Inside every cell are processes that change rhythmically and that drive the observed rhythms. The actual mechanism of the clockwork is not known, although recent research in organisms such as fruit flies and fungi suggests that a cycle of gene transcription and protein synthesis is an important part. Another part of the clock is one or more photoreceptors through which light entrains or sets the clock to match up with the daily light cycle. Having many internal processes matched with the daily light cycle allows the plant to anticipate changes that occur during the day, such as switching on genes associated with photosynthesis before the onset of daylight. It also allows them to carry out incompatible processes at different times, such as nitrogen fixation and photosynthesis in unicellular cyanobacteria .

Having a biological clock that is reset by light and dark means that plants can measure the length of day and/or night. This also allows them to tell what season it is by whether days are getting longer or shorter. Many developmental responses are triggered by changes in the length of day, a type of behavior termed photoperiodism. While some changes occur in response to shortening of the daylength, others occur as days get longer. These two types of photoperiodic response were first recorded for the induction of flowering and led to the classification of plants as short-day plants or long-day plants. Those plants that do not respond to daylength are called day-neutral plants.

There are several advantages to having developmental responses controlled by daylength. For flowering, it means that members of a population will flower at the same time, which increases the chances of outbreeding and thus genetic recombination. If a pollinating insect's behavior is also photoperiodically controlled, this further improves the chance of successful pollination. Another example of the survival value of seasonal timing of flowering is that woodland plants can flower and set seed before the dense leaf canopy is formed. Other changes that occur in response to daylength include the formation of storage organs such as bulbs or tubers, the onset of dormancy, and the development of cold hardiness in trees. These changes help plants survive through the winter and are triggered by the shortening daylength during autumn.

see also Photoperiodism; Senescence; Tropisms and Nastic Movements.

Peter J. Lumsden