Circadian rhythms in plants are phases of growth and activity that appear in regular, approximately twenty-four-hour, cycles.
Biological activities that cycle in approximately twenty-four-hour intervals are called circadian rhythms (from the Latin circa, meaning “about” and dies, meaning “a day”). Circadian rhythms allow plants to anticipate environmental cycles and to coordinate their activities with them.
Circadian rhythms are not simply responses to changing external conditions, as they continue even when a plant is placed under constant conditions. This continuation indicates that circadian rhythms are controlled by endogenous (internal) timing mechanisms, collectively referred to as the biological clock.
Plant circadian rhythms include cycles in gene regulation, enzyme activity, leaf movements, flower opening, and stomatal opening.Circadian rhythms also interact with photoperiodism in the control of major developmental processes, such as dormancy and the induction of flowering.
In 1729 the French astronomer Jean-Jacques Dortous de Mairan discovered the endogenous nature of circadian rhythms when he looked at the sleep movements of leaves of the sensitive plant, Mimosa, known as nyctinastic leaf movements. Mimosa leaves fold closed at night and open during the day.
It had been thought that these leaf movements occurred in response to external cycles of light and darkness. De Mairan examined the plants under constant environmental conditions and discovered that the nyctinastic movements of the leaves continued.
This was the first description of a biological activity with an endogenous circadian rhythm. Current models for how plants accomplish circadian rhythms are divided into three parts: entrainment, biological clock, and output pathways.
The synchronization of circadian rhythms to the cycles of the outside world is accomplished via input pathways and is referred to as entrainment. In nature, circadian rhythms are entrained primarily by light or temperature cycles to have periods of twenty-four hours.
It is essential that circadian rhythms be entrained, because without synchronization of the biological clock with environmental cycles, the advantages of circadian rhythms would be lost.
The biological clock is also referred to as the central oscillator and the pacemaker. It is endogenous and self-sustaining. Although circadian rhythms are entrained by external stimuli, they continue in the absence of external cycles.
Under artificially constant conditions, circadian rhythms do not maintain twenty-four-hour periods but revert to free-running periods that are usually between twenty-one and twenty-seven hours. The molecular mechanisms of the biological clock remain unknown, but they are thought to include autoregulatory feedback mechanisms.
An interesting feature of the biological clock is that the free-running period is generally insensitive to changes in temperature. Most chemical and biological processes are affected by temperature changes; higher temperatures make them go faster, and lower temperatures make them go slower.
That the biological clock is able to compensate for temperature changes and maintain time keeping functions is important to plants experiencing extreme changes in temperature.
The output pathways, or “hands” of the biological clock, are the measurable rhythms exhibited by the plant. Known circadian rhythms range from the subcellular level to the cell and tissue level to the developmental level.
Subcellular circadian rhythms include cycles in gene regulation (at the levels of transcription, transcript abundance, translation, and post-translational modification), calcium signaling, and enzyme activity. One well-characterized rhythm is the rate of carbon dioxide assimilation in plants with CAM (crassulacean acid metabolism) photosynthesis.
Such plants open their stomata at night to allow for gas exchange, fixing carbon dioxide into an organic acid that is stored in the vacuole. During the day, the plants close their stomata (presumably to conserve water) and continue photosynthesis using carbon dioxide released from the organic acids.
The circadian rhythm of carbon dioxide assimilation in CAM plants is controlled by rhythmic changes in the activity of the enzyme (PEP carboxylase) that fixes carbon dioxide into the organic acid.
Cell and Tissue Levels
Cell- and tissue-level circadian rhythms include those controlled by cycles in cell expansion and contraction, such as the obvious rhythms of leaf and petal movements and the opening and closing of stomata.
Leaf movements are brought about by a cycling in the expansion and contraction of specialized cells in a region at the base of the leaf that is called the pulvinus. Nyctinastic leaf movements presumably allow a plant to maximize light interception for photosynthesis.
Many plants open their flowers in the morning and close them at night. Other plants open their flowers in the afternoon (such as the four o’clocks, Mirabilis jalapa), in the evening (evening primrose, Oenothera biennis), or even at night (the bat-pollinated cactus Cereus).
These cycles are important for timing pollen availability with the activity of insect, bird, and mammal pollinators. It is essential that flowers of the same species be open at the same time of day or night to promote outcrossing that results in increased genetic variation.
Circadian cycles of stomatal opening and closing allowa plant to balance carbon dioxide uptake with water loss. Plants with CAM photosynthesis open their stomata at night, in contrast to plants that carry out C3 and C4 photosynthesis, which open their stomata during the day. Other known tissue-level circadian rhythms include hypocotyl elongation, nectar secretion, and hormone synthesis.
Developmental processes that depend on interactions with circadian rhythms and the biological clock include the photoperiodic control of flowering and dormancy.
These photoperiodic responses rely on the ability of a plant to measure relative amounts of light and darkness within each twenty-four-hour period. It remains unknown whether one biological clock controls both photoperiodism and circadian rhythms.