Charles Darwin is probably most famous for his ideas on evolution through natural selection. In his later years, he became fascinated with plant movements while trying to find an evolutionary relationship among them. He did hundreds of experiments to keep track of the movements of the different varieties of plant leaves. After experimenting, Darwin concluded that the plants were moving their leaves so as to expose the smallest possible leaf surface to the night temperatures. His book,The Power of Movements in Plants details the years of work. In the years following, scientists would debate whether the rhythm arose from forces external to the plant or was endogenous.

In 1920, a landmark paper was written by W.W. Garner and H.A. Allard in which they showed that tobacco plants would flower only if exposed to a certain number of hours of light. The term "photoperiodism" was used to designate the response of the organisms to relative length of day and night. Garner and Allard showed that plants could tell time! The ability to sense day length is an important ability for plants so that they grow, reproduce and develop during favorable time of the year. The changing times of dawn and dusk contain seasonal information as well as time of day information so that the organisms have, in effect, an internal clock and calendar[23].

In plants a photoperiodic clock not only controls flowering, but also induction and termination of dormancy in buds and bulbs, seed germination, and daily rhythms such as leaf movements, petal movements, and nectar secretion.

In more recent experiments with plants, Steve Kay et al have developed an interesting technique to measure rhythms in Arabodopsis thaliana . The researchers chose this plant for many reasons including its small size, short life cycle, and number of chromosomes (n=5). As a way to measure rhythmic gene expression in vivo in these plants, they transplanted firefly luciferase gene (which is responsible for the insect's glow) into the plants. The plants exhibited rhythms in bioluminescense when their CAB genes (CAB is a light harvesting protein) were being "turned on" rhythmically. In other words, the luciferase gene was used as an intra-cellular marker to detect the plants' own gene activity rhythms[27]. In other experiments, they isolated and cloned a photoreceptor gene from rice. Multiple copies of this phytochrome gene were transplanted to tobacco with the result that the tobacco plants became hypersenstive to light because of the higher than normal number of photoreceptors[28]. Scientists are attempting to do a similar experiment with rice in the hope that they will be able to grow rice under low-light conditions and produce more crops per year.

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