Almost all living organisms adapt their behavior to the circadian rhythm tied to day and night, and plants are no exception. However, to this day, studies have only focused on small plants grown in pots. Now, using laser scanning to examine the day-night cycle of trees, a team of scientists from Austria, Finland and Hungary has successfully examined the sleep movement of fully-grown trees and revealed that trees go to sleep, too.
"Our results show that the whole tree droops during night which can be seen as position change in leaves and branches," said Eetu Puttonen of the Finnish Geospatial Research Institute and lead author of the study. "The changes are not too large, only up to 10 cm for trees with a height of about five meters, but they were systematic and well within the accuracy of our instruments."
In order to ensure that the effects of weather and location didn't skew their results, the team conducted their experiment two times with two different trees; the first was examined in Finland and the second in Austria. Both tests were conducted under the same conditions: close to solar equinox and in a calm environment with no wind or condensation.
Overnight, the leaves and branches of the trees seemed to droop gradually, reaching their lowest point a few hours before sunrise. When morning came, they returned to their original position within a few hours, although the team is still unsure if this "awakening" stems from the sunlight or their own internal clock.
"Plant movement is always closely connected with the water balance of individual cells, which is affected by the availability of light through photosynthesis," said András Zlinszky of the Hungarian Academy of Sciences and co-author of the study. "But changes in the shape of the plant are difficult to document even for small herbs as classical photography uses visible light that interferes with the sleep movement."
Using a laser scanner, the team was able to overcome this difficulty through minimal plant disturbance due to its utilization of infrared light. This infrared light is reflected by the leaves, which allowed the team to map full-sized trees within minutes at a high resolution.
"The next step will be collecting tree point clouds repeatedly and comparing the results to water use measurements during day and night," Puttonen said. "This will give us a better understanding of the trees' daily tree water use and their influence on the local or regional climate."
The findings were published in the Feb. 29 issue of the journal Frontiers in Plant Science.
