Trees witnessed sleeping for the very first time

Most living organisms including plants adapt to the regular pattern of days and nights. Hence most flowers open their petals in the morning, while the leaves of several tree species close shut for the night. The circadian rhythm of plants has been in the scope of research for centuries. It was Carl von Linné who first observed that flowers stored in dark cellars for days on end still kept opening and closing their petals, while Charles Darwin described the night movements of shoots, which he identified as the “sleep” of plants.

Abandoning the lab

To this day, experiments concerning the sleep of plants are conducted on small potted plants in laboratories. Thus, it was unknown until now how much the regularities observed in labs may be applied to trees. A new method for measuring the sleeping movements of trees has been devised by an international research group of Finnish, Austrian and Hungarian scientists. With the help of a laser scanner they made a very precise and detailed model of the examined tree, repeating the process on an hourly basis during the entire night. It turned out that the branches and leaves of the trees drooped by as much as 10 centimetres during the night.

“We measured a small but systematic change,” said Eetu Puttonen of the Finnish Geodetic Institute and leader of the experiment. “The leafy branches of 5 meter birch trees gradually drooped by 8 to 10 centimetres by night. Their lowest position was right before sunrise and after dawn it took a couple of hours for them to return to their original position. Whether the branches were “woken up” by the rays of the Sun or by their own inner rhythm independent of the Sun still remains to be seen.”

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In order to exclude external effects, the same experiment was conducted on two trees separated by several thousand kilometres, during the autumnal equinox: one in Austria, the other in Finland. The circumstances were favourable, as there was neither wind nor dew.

Branches witnessed moving for the first time

“Chronobiology, i.e. the scientific study of periodic phenomena in living organisms operates primarily on a cellular and on a molecular level. Earlier, independent studies have mainly concentrated on a cellular level on the genetic background of the daily rhythm of plants. Our method, however, could mean a breakthrough in the investigation of the spatial alterations affecting the entire plant. The sleeping movements of certain trees, such as the leaf movements of acacia species, have long been known. We did not suspect, however, that this is also the case in many other species. What is more, the fact that branches also move was especially unexpected on our part, however logical it may seem in hindsight,” stated András Zlinszky, researcher at the Balaton Limnological Institute of MTA’s Centre for Ecological Research. “This phenomenon was not noticed earlier, because the changes in the shape of the plants are harder to measure on small pot-plants in a laboratory environment, as the movements are on a much smaller scale. Additionally, the light needed to take photographs during the experiment may influence the sleeping movements of the plant. The laser scanner used in our experiment operates in the infrared range and focuses on a given point of a few millimetres for a fraction of a second in order to scan the shape of the tree. Eight million points were scanned in a couple of minutes with millimetre precision and we have thus successfully proven that plants change their shape during the night.”

Plant movement is strongly connected to the water content of the cells and thus to the water balance of the whole tree. The next step in the research project is to examine further trees and to intently follow the water-flow in the trunks and the branches. These experiments may reveal how much water trees extract from the soil and how much they evaporate through their leaves. We may also gain insight into how trees influence the temperature and humidity of their immediate environment during the day and night.

The results have been published in the open access article, Frontiers in Plants: Puttonen, E., Briese, C., Mandlburger, G., Wieser, M., Pfennigbauer, M., Zlinszky, A., Pfeifer N. (2016). Quantification of Overnight Movement of Birch (Betula pendula) Branches and Foliage with Short Interval Terrestrial Laser Scanning. Frontiers in Plant Science, 7: 222. doi: 10.3389/fpls.2016.00222. New Scientist Online also reported on the experiment.