Light and Plants : Circadian Clock of Plants – Part 1
In the Light Science Vol 2 – Light & Body Clock, we talked about the biological clock of humans. As its sequels, I’d like to discuss Circadian Clock of Plants in the two following newsletters of the Light and Plants series.
The plant was the first living organism on the earth where the circadian clock was recorded in the history. Androsthenes of Thasos, one of the general of Alexander the Great, found around BC 400 that the tamarind plant changes the angle of leaves regularly on the daily base. 
It is for tamarind to maximize light absorption (i.e. photosynthesis) by orienting the leaf surface perpendicular to Sun’s ray, and it happens for a while even when the sun is clouded. It also happens to sunflowers that when sunflowers are staked to limit their sun-facing movement, plant growth and visitation by pollinators to flowers that is important for successful pollination and reproduction significantly decrease (Figure 1).

(Figure 1) Sunflowers move their flowers and leaves to face the sun to maximize photosynthesis and visitation by pollinators such as honeybees. (, Plant circadian clock mystery solved)
Moreover, circadian clock plays a role in modulating plant defense system that the level of plant defense against insects is maximized during the day time when insect attack is most frequent. As the results, plants whose day/night cycle is artificially inverted are very susceptible to insects that cabbage loopers (Trichoplusia ni) gain much more weight when grown on plants whose day/night cycle is artificially inverted than on plants whose day/night cycle is synchronized to the insect (Figure 2).

(Figure 2) The role of circadian clock in modulating plant defense against insects. (Goodspeed, et al., PNAS 2012). A. A cabbage looper (Trichoplusia ni) feeding on Arabidopsis thaliana plant. B. The size of loopers fed on plants whose day/night cycle is synchronized to that of insects. C. The size of loopers fed on plants whose day/night cycle is inverted.
Increasingly more physiological processes in plants are revealed to be under control of the circadian clock. It is obvious that plants and other living organisms have been forced to adapt to the rhythmic environment formed by the rotation of the earth.
Jinwon Kim, Ph.D. 

* Next week’s topic : How Sunlight Burns Fat

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