Circadian rhythms are the daily repeating patterns of many organisms, such as the closing of flowers at night or opening of stomata during the day. An interesting property of circadian rhythms is that many are endogenous, arising from an internal biological clock. For example, in 1729, a French scientist discovered that circadian movements were not dependent on the daily cycling of light and dark, even though they were synchronized with it.
Photosynthesis is an important circadian rhythm in plants.
The two pathways of photosynthesis, photophosphorylation and the Calvin-Benson Cycle, proceed within the chloroplast. Because photophosphorylation requires light energy to proceed, both pathways of photosynthesis stop in the dark. It is therefore beneficial for plants to coordinate their photosynthetic activity and stomata activity with daily changes in light availability. The internal clock coordinating photosynthesis produces a circadian rhythm. Photosynthesis dominates the metabolism of green plant cells.