Most living organisms adapt their behavior to the rhythm of day and night and plants are no exception. Flowers open in the morning and close at night and recently scientists have discovered that trees also rest their branches when ‘asleep’. Branches of birch trees have been seen drooping by as much as 10 centimeters at the tips towards the end of the night.
Scientists from Austria, Finland and Hungary used laser scanners between sunset and sunrise. From the time it takes beams to bounce back from branches and leaves, they could measure the movements of each tree, in three dimensions and at resolutions of centimeters.
The team scanned two birch trees in two geographically separate experiments; one in Finland and one in Austria, each over the course of a single night under similar conditions. They made 11 scans of the Finnish tree, approximately one per hour, and 77 of the Austrian tree, around one every 10 minutes. Scientists decided to use laser scanning instead of observing their movement photographically where they would have to illuminate the trees with light that could affect the outcome.
Both tests were done close to solar equinox, under calm conditions with no wind or condensation
“We’re certain it’s not caused by another effect,” says Norbert Pfeifer of the Technical University of Vienna in Austria. “The experiment is the first of its kind,” says Eetu Puttonen of the Finnish Geospatial Research Institute in Masala. “These studies have only been done before in small plants, but here, it was possible to do it outside in fully grown trees.”
The leaves and branches were shown to droop gradually, with the lowest position reached a couple of hours before sunrise. In the morning, the trees returned to their original position and it is still not clear whether they were ‘woken up’ by the sun or by their own internal rhythm.
The drooping effect is probably caused by loss of internal water pressure within plant cells which is called turgor pressure. “It means branches and leaf stems are less rigid, and more prone to drooping under their own weight,” says Zlinszky.
Zlinszky explains that Turgor pressure is actually influenced by photosynthesis, the process by which plants use sunlight to create sugar from carbon dioxide and water. Photosynthesis stops in the dark, so this in itself may explain why the branches droop.
The trees may also be “resting” their branches. During the day, branches and leaves are angled higher, allowing leaves to catch more sunlight, because there’s less self-shading from leaves above. But this is energy-intensive and serves no purpose at night, when there’s no light.
It is important to see whether other tree species also ‘sleep’
“I’m confident it will apply to other trees,” says Zlinszky. The ideal targets would be poplars and chestnut trees, because researchers have decoded the genomes of both and identified genes linked with circadian rhythms, which could help them see which genes are involved in the behavior.
“There have been some studies on circadian rhythms in trees, mostly studying gene expression, but this latest research is a beautiful way to watch it happen in individual trees,” says C. Robertson McClung of Dartmouth College in Hanover, New Hampshire. “It shows things are happening in the real world.”
McClung says that studies in sunflowers have tied circadian rhythm to the ability of water to travel in the plant’s stem. “The water supply might underlie the effects seen,” he says. “It’s reasonable to suspect that it’s not just the water supply, but that the ability of the plant to transport it might be controlled by the plant itself.”
Future studies will certainly give us a better understanding of the trees’ daily tree water use and their influence on the local or regional climate.
Published in Frontiers in Plant Science, DOI: 10.3389/fpls.2016.00222