Do plants sleep, or just become nycti-nasty?
The Greeks knew about it. Charles Darwin wrote about it. You have almost certainly noticed that some plants fold their leaves or close their flowers at night. Nyctinasty is just one of the things plants do when it's dark.
Other plants show nyctitropism, where a part of the plant is re-positioned. If that re-positioning happens to bring the plant to the surface of a waterbody we might call the plant nyctipelagic. The common thread here of course is 'nycti', the night.
It has been discovered recently that many more plants than we thought have nocturnal activities, if I can call them that. The Hungarian and Danish authors of a publication I was reading - thanks to a steer from colleague Peter Symes - calls them 'sleep movements'.
Their experimental site was a 'gardening shop' in Western Hungary, restricting, as they put it 'the choice of plants...somewhat'. But they managed to test a nice mix of familiar plants using a laser scanner to detect any movements in their canopy.
The scientists retested a birch tree (Betula), which had previously (in Finland and Austria) been shown to droop by up to 8 cm overnight, then return back to their original position by dawn. In that early study, the same movement was measured in trees 2000 km apart so attributed to the species rather than some kind of odd downdraft. Yet in our Hungarian gardening shop no pattern could be discerned above the 'noise'.
The Honey Locust (Gleditschia triacanthos), was the star of the Hungarian study, with its crown moving up to 9 cm overnight. Most other plants only moved up to 1 cm, in cycles of only a few hours. The Oleander (Nerium oleander) had a pattern not unlike the previously measured birch, although the movements were far less.
A maple (Acer palmatum) had, according to the researchers, a strong circadian rhythm (i.e. part of a 24 hour cycle that would occur, at least for some time, in the absence of light), but in an opposite direction to other plants. The one palm they tested, the Chusan Palm (Trachycarpus fortunei), was difficult to interpret. It doesn't have limbs to lower so all the movement was in its leaves, or fronds. They noticed a short downward movement, then up, then down, then up... Conifers, such as Cedar of Lebanon (Cedrus libani), seem to have no sleep movements.
From 21 species a few general patterns were identified. The first was what they term 'sleep', when a canopy moved significantly during the night but returned to its original position towards dawn (8-12 hours later). This sleep was considered to be a potential circadian rhythm.
Another was called 'drift', where the plant moves but in only one direction. That is, it doesn't return to its original state at the end of the night. A third is an 'oscillation', with cycles of only a few hours, and finally there is movement best described as 'noise' - where the movements are small and difficult to detect and/or explain.
Which is all fine and interesting as far as it goes. The study is very descriptive and doesn't really answer questions about whether these cycles are circadian or controlled by the day/night cycle (all measurements were taken over a single autumn night). There is also no repeating of the tests or good controls to determine what might cause these movements. The authors realise this and suggest are range of additional experiments at the end of their paper.
As to why plants fold their leaves, close their flowers and drop (or raise) their canopy? Folding leaves is thought to reduce the loss of water from leaves during the night when the plant doesn't need to have its leaf pores open to take in carbon dioxide. Closing flowers will protect those sometimes fragile petals from wind and grazing damage while their pollinators are asleep in their beds (which obviously doesn't apply to moth and bat pollinated plants...).
Moving the canopy around, as measured in this study, might be the result of changes in turgidity of plant cells, making it truly a kind of resting state for the plant. But that's just conjecture and not entirely satisfying. What's more exciting is discovering a new phenomenon that might end up telling us a little more about how plants work, or sleep.
Image: at top, my picture of the moon on the night (31 January 2018) the Earth got in the way. The next picture is of a snowfall, at night, when we were living in Kew Gardens, London. The third is nearby, at dawn on the River Thames, from my kayak on one frosty morning. The final one is a Christmas Trees, constructed from a living conifer at Wakehurst Place, near London, photographed of course at night...
Postscript: The weekend before last there was a nice interview with one of the authors of this paper by Hilary Harper on ABC RN's Blueprint for Living.