Biological Clocks

<h1>Text Preview</h1> <p>Most forms of life have an internal means of measuring time, and plants are no exception. Plants respond to 24-hour cycles, called circadian rhythms. Circadian rhythms control gene expression, leaf and petal movements, the release of floral fragrances, the opening and closing of stomata, and many of the metabolic activities associated with photosynthesis.</p> <p>For example, stomata generally open during the day to allow carbon dioxide in for photosynthesis and close at night to minimize water loss. How do stomata know when to open and when to close? Stomatal openings and closings respond to numerous external cues such as light and carbon dioxide concentration. </p> <p>In addition, an internal clock keeps track of the 24-hour cycle, so that stomatal opening and closing occurs on a 24-hour cycle even when the plant is grown in continuous light. </p> <p>While some of the molecular details of stomatal opening and closing are well known, such as the involvement of the hormone abscisic acid, the molecular basis of the internal clock remained a mystery until just recently. </p> <p>Scientists had suspected for over ten years that oscillations in intracellular calcium concentrations played a role in the plant’s internal clock mechanism. Calcium is a common second messenger in many signal transduction systems. In June of 2001, researchers at the University of California, San Diego, and in Munich, Germany, reported that they cracked the code imbedded in the calcium signals of a plant.</p> <p>Using a mutant form of the plant Arabidopsis, they discovered that specific frequencies of calcium oscillations signal the opening and closing of stomata for extended periods of time, such as in their closure at night. </p> <p>Circadian rhythms also control the movement of leaves. The French astronomer Jean Jacques Ortous de Mairan was intrigued by the daily opening and closing of the leaves of a plant, so he did an experiment. He started with the hypothesis that the plant opened its leaves during the day in response to light. So he placed the plant in the dark.</p> <p>Based on his hypothesis, he expected the leaves to stay closed while the plant was kept in the dark. To his surprise, he observed that the leaves still opened on a daily basis even though they received no light. This experiment, done in 1729, is the first experiment known to study the circadian rhythms of a plant.</p> <p>Some flowers, like morning glories, bloom only during the day. Others, like evening primrose, bloom exclusively at night. The release of floral scents also depends on the time of day. Many flowers have coevolved with the animals that pollinate them. Flowers pollinated by nocturnal creatures, such as moths and bats, tend to be open at night; flowers that depend on daytime pollinators are open during the day. </p> <p>The response of a plant to the relative lengths of day and night is called photoperiodism. A single exposure to light during an extended dark period may shift any circadian rhythm. For example, florists control photoperiodism and trick greenhouse plants to bloom out of season by exposing them to artificial light.</p> <p>Blue-green pigments found in plants, called phytochromes, become activated when light is present. The active form of phytochrome influences plant hormones, which then influence plant development, such as flowering.</p> <p>Seeds also contain phytochromes. If a tiny seed germinates very deep in the soil, it won’t have enough energy to grow to reach the surface. So seeds don’t germinate until the phytochrome they contain is activated by light. Alternatively, if a seed is under a deep forest canopy, it won’t germinate until an opening in the canopy forms. The opening allows sunlight to reach the seed’s phytochrome and activate it.</p> <p>The responses of plants to their environment are fairly predictable under favorable conditions. But how does environmental stress alter these responses? We’ll find out in the next section.</p> <p id="TextCopyright">Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education</p>