We humans sleep, animals sleep too, but do plants sleep? For the first time, researchers have caught trees in the act of ‘sleeping’ with their branches drooping down by as much as 10 centimeters during the night. Researchers hypothesize that, this change in stance during night, could be an indicator of ‘the resting’ state in trees.
Ever since the origin of life on earth, most living creatures have aligned their lifestyle, behaviour around the 24 hour day-night rhythm. Most animals are active during the day and go into sleep mode as the sun goes down. Interestingly, the number of hours animals spend in this ‘resting state’ is quite varying ranging from 2 hours in horses to 20+ hours among bats. Below is a list of some known animals with their sleep periods.
Horses – 2 hours
Elephants – 3+ hours
Cows – 4.0 hours
Humans – 8.0 hours
Rabbits – 8.4 hours
Chimpanzees – 9.7 hours
Dogs – 10.1 hours
Cats – 12.5 hours
House mice – 12.5 hours
Lions – 13.5 hours
Little brown bats – 19.9 hours
But, have you ever wondered with the question: do plants sleep? We see them standing erect all the time – day in and day out, and would have naturally dismissed any thoughts of associating plants with ‘sleep’. On the contrary, plants have their entire physiological functioning aligned to the earth’s 24-hour light-dark cycle. Flowers bloom in the morning and but fold and shrink at night. Even certain plants leaves close at night.
This day and night cycle scientifically termed as the ‘circadian rhythm’ have been a subject of interest among botanists for centuries. Closing of petals at night time, now technically called as nyctinasty, was first observed and recorded by Androsthenes of Thasos, a naval admiral under Alexander the Great. Scientists are still not sure as to why flowers have evolved to behave in this manner. One of the widely held theories is that it is to conserve energy at night.
Charles Darwin recorded the movement of plant leaves and stalks throughout the night and labeled it as ‘sleep’. Carolus Linnaeus, an 18th century taxonomist, observed that the flowers in dark cellars continued to open and close. Fascinated by this plant behavior, he came up with a floral clock called ‘sleep’ in plants, depicting at what particular times in the day certain flowers opened and closed their buds.
Surprisingly, even to this day, such studies into plants circadian rhythm is been carried out on small plants grown in pots and not on fully grown outdoor trees. This is quite understandable as monitoring and measuring large scale outdoor trees can be difficult and cumbersome.
For the first time, researchers from Austria, Hungary and Finland have recorded the ‘sleep’ movements in large trees using laser scanners. These movements were observed to happen systematically over several hours lasting from sunset to sunrise, ruling out the occasional wind effect. It was observed that, close to sunrise the branches were hanging lower than that around sunset.
“Our results show that the whole tree droops during night which can be seen as position change in leaves and branches”, says Eetu Puttonen from Finnish Geospatial Research Institute. “The changes are not too large, only up to 10 cm for trees with a height of about 5 meters, but they were systematic and well within the accuracy of our instruments.”
In order to be doubly sure that their recordings are correct and devoid of any major errors, researchers made their laser scanning on two different trees, one in Austria and the other in Finland. To rule out any weather conditions corrupting the data they scheduled their recordings on a day near to solar equinox, with no wind or condensation.
What the researchers observed is that, the leaves and branches droop gradually and systematically, attaining the lowest position, a couple of hours prior to sunrise. At the break of dawn, the branches spring back to their original position within a few hours. Here, the researchers are not yet clear about whether the tree was ‘woken up’ by the sun or was it because of their own internal rhythm.
Why branches droop at night
But seriously, can drooping of branches be considered as ‘sleep mode’ in plants? Well, that depends on how you define sleep. In animals, the circadian rhythm determines when an animal goes to sleep and when it is up. Every morning the sunlight entering through the eyes activates a bunch of cells in the brain which regulate the levels of melatonin– a hormone which controls drowsiness. More the melatonin, sleepier is the feeling. Sunlight brings down the melatonin level during day, while darkness keeps the level high, driving the animal to sleep.
Unlike animals, plants do not have a central nervous system – a key factor in what we define as ‘sleep’. However, just like animals, plants do have a circadian rhythm tuned to the earth’s 24 hour light-dark cycle, which they maintain even when they are exposed to light for extended periods.
Plant cells have the ability to change their shape by varying the internal pressure of the fluid. Though small in magnitude, this internal pressure called as turgor pressure, can bring in considerable changes in movements at the macro level of tree branches. Using this mechanism, trees angle their leaves towards the sun throughout the day, thereby maximizing their exposure to sunlight.
At night, without sunlight, the extra energy needed to maintain pressure is no longer generated and as a consequence, the internal cell pressure drops, there by drooping the branches and leaves to a ‘resting’ position. Researchers believe this branch drooping could be a ploy of plants to conserve energy at a time when photosynthesis is not possible. This doesn’t mean that the entire plant is shutdown. It is just that the plant shifts its attention from photosynthesis to other equally important physiological processes like metabolism and growth.
Mapping the tree movements
Recording these branch movements is a tricky business. As András Zlinszky from Centre for Ecological Research, Hungarian Academy of Sciences explains, “Plant movement is always closely connected with the water balance of individual cells, which is affected by the availability of light through photosynthesis. 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.”
However, with laser scanning, disturbance to the plant is minimal, as only specific points on a plant are illuminated for a fraction of a second using infrared light. In each scan, a cloud of points consisting of millions of points are used to make the measurements. This technique can automatically map an entire tree with sub-centimeter resolution within minutes.
Norbert Pfeifer from TU Wien, Vienna says, “We believe that laser scanning point clouds will allow us to develop a deeper understanding of plant sleep patterns and to extend our measurement scope from individual plants to larger areas, like orchards or forest plots.”
“The next step will be collecting tree point clouds repeatedly and comparing the results to water use measurements during day and night”, says Eetu Puttonen. “This will give us a better understanding of the trees’ daily tree water use and their influence on the local or regional climate.”
“Quantification of Overnight Movement of Birch (Betula pendula) Branches and Foliage with Short Interval Terrestrial Laser Scanning”, Eetu Puttonen et al, Frontiers in Plant Science, 29 Feb, 2016.
Feature Image: via commons.wikimedia