After a long day standing up straight, trees call it a night and let their branches droop.
Charles Darwin first described his observation of the overnight movement of plant leaves and stalks as “sleep.” Flowers are known to open in the morning, and some tree leaves close at night. A range of plants appear to adapt their behavior to day and night cycles, but until now, scientists never knew whether trees sleep as well.
Researchers from Austria, Finland and Hungary recently measured the sleep movement of fully grown trees using a time series of three-dimensional laser scans that detected changes in tree shape over time. Their research was published in the journal Frontiers in Plant Science.
"Our results show that the whole tree droops during night which can be seen as position change in leaves and branches," said study lead author Eetu Puttonen from the Finnish Geospatial Research Institute, in a press release. He notes that the changes were subtle — a 5-meter (16.4-feet) tree would only droop down 10 cm (3.9 inches) in a given night — but they were consistent.
The experiments were repeated in Finland and Austria to rule out effects of location. In both cases, the weather was calm, with no wind.
The leaves and branches fell gradually during the night, reaching their lowest position a couple of hours before sunrise. By morning, the trees returned to their original position.
"Plant movement is always closely connected with the water balance of individual cells, which is affected by the availability of light through photosynthesis,” explained study co-author András Zlinszky from the Hungarian Academy of Sciences.
However, it is unclear whether the trees were "woken up" by the sun or by their own internal rhythm. The fact that some of the branches started moving to their original position before sunrise supports the latter idea, but further experiments are needed to confirm.
Next, the researchers plan to compare tree movements to their water usage during the day and night, which will help elucidate what is really driving their night and day rhythms.
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