The daily opening and closing of flowers and the rhythmic leaf movement of some plants suggests, even to the casual observer, that plants have an internal clock.
For example, in 1751 Linnaeus published Philosophia Botanica in which he noted what time of day flowers of various species open and close.
And also in this book, Linnaeus conceived the idea of a floral clock (“horologium florae“) garden by which one could estimate the time of day by observing which flowers were open and which had closed. (Click on photo of floral clock below for more information.)
Darwin, assisted by his son Francis, studied the diurnal movement of leaves (sometimes called “sleep movements”, a.k.a., nyctinasty). In their book The Power of Movement in Plants the Darwins argued that the plants had an internal clock that generated the observed rhythms, rather than them being solely imprinted by the diurnal cycle.
Of course, we now know that these “sleep” movements in plants are manifestations of the circadian rhythm, which is evident in most organisms.
What Sets the Clock?
Think about it…what happens during the course of a typical 24-hr period on Earth? In simplest terms, it cycles between light/warm and dark/cool.
So, what sets (entrains) the biological clock of plants are mainly light/dark transitions, augmented or reinforced by diurnal cycles in temperature. In other words, light (dawn/dusk) acts to reset the clock, but temperature also has an effect, albeit not very well defined.
It turns out that, in most plants, the leaves play a central role in sensing the light that entrains the biological clock. But it’s not chlorophyll that is the light-sensing pigment, but two other non-photosynthetic pigments called phytochrome and cryptochrome. (Much more about these two photoreceptors another time.)
Basically, How Does the Clock Work?
Research on the cellular mechanisms of circadian (“about a day”) rhythms in plants has greatly advanced our understanding of how the clock works at the molecular level. (For an excellent review from an historical perspective see Ref. 1 below.)
Briefly, the clock works at the individual cell level and consists of three basic components as shown in the diagram below.
It turns out that plants likely have at least three such mechanisms, all interlocked in a complex system, working inside leaf cells. As mentioned above, phytochrome and cryptochrome are the photoreceptors. These modify other proteins involved in a transcription/translation feedback loop that serves as the central oscillator.
The collective output chiefly consists of proteins, and maybe even RNA, that serve to modify the plant’s metabolism and development. These output signals may even travel from the leaves through the phloem to other parts of the plant.
Some Additional News About Plant Circadian Rhythms
Leaves may have three interlocking clocks, but there may be only one root clock, and it’s apparently a slave of the leaf clocks.
The circadian rhythm also apparently results in the rhythmic growth of plants.
Researchers at the University of Texas at Austin have shown that modifying the internal clock may result in bigger plants.
Much has been learned about clock genes in plants and how they relate to clock genes in animals.
Some More Recent News
Prof. Michael Thomashow and colleagues at Michigan State University have shown that the circadian clock may contribute to plant cold tolerance through regulation of a cold-response pathway.
According to a recent report by Dr. Xing-Wang Deng and coworkers at Yale University, manipulating plants’ circadian clock may allow farmers and horticulturists to grow plants in different seasons and places that are not currently possible.
Bottom line: For hundreds of years people have recognized that plants have an internal clock, but only recently have plant molecular biologists discovered the complex inner workings of this timepiece.
1. McClung, C. R. (2006) “Plant circadian rhythms.” The Plant Cell, Vol. 18, pp. 792-803. (Full Text)
2. Pruneda-Paz, J. L. and S. A. Kay (2010) “An expanding universe of circadian networks in higher plants.” Trends in Plant Science, Vol. 15, pp. 259-265. (Full Text)
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Next Time: Why Plants Tell Time
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