Stephen Armstrong {journalist on the Guardian newspaper}, reports that trees undergo many adjustments from sunset to sunrise – even falling asleep during the hours of darkness.
Charles Darwin showed that plants actually grow in spurts late at night, with plant stems elongating fastest in the hours just before dawn. They also drink heavily. Plants absorb water in their roots {a process called transpiration} and then, as water evaporates from the leaves, “it pulls up” other water molecules behind it to fill the space {a phenomenon called cohesion}. In trees, the long slender capillaries that suck up water can fail across the day if too much water evaporates too rapidly – internal columns of water break and fail. At night, trees refill, swelling every leaf and twig.
In 2016, researchers at the Centre for Ecological Research in Thiny, Hungry, pointed laser scanners at two birch trees – one in Austria and one in Finland – to measure any changes to their shape. Between two hours after sunset and just before sunrise, the trees branches drooped – by as much as 10cm {4 inches}.
To rule out effects of weather and location, the experiment was done twice with two different trees. Both tests were done in calm conditions with no wind or condensation.
The drooping effect is caused by water pressure dropping within plant cells. Some plants even wrap their leaves away at night when they’re not needed and pump water back into cells at the base of the leaf to open them in the morning.
So how can branches droop as more water is being pumped in? Zlinszky speculates that the trees may be resting – literally going to sleep, just like humans. Plants don’t have the central nervous system that control what we think of as “sleep” – but they do have circadian rhythms, tuned to Earth’s 24-hour light-dark cycle, even if they’re kept in light full-time.
During the day, branches and leaves are angled higher, straining up to catch as much sunlight as possible. Pumping water into every cell is an energy-intensive business and serves no purpose after the sun goes down. Trees like all plants rely on a mixture of photosynthesis and respiration to survive. Photosynthesis uses light from the sun to power a chemical reaction – between water, carbon dioxide and a handful of minerals – that produce complex organic molecules. Having torn the carbon dioxide apart to grab the raw ingredients to make these molecules, the plant then throws away the rest – the oxygen, which is pumped out of the plants during the day.
As animals, we use oxygen as fuel in burning up our fuel – specifically glucose, which we also get from plants, just one of the reasons they’re so important to us. But glucose is an important source of energy for plants too, and to burn it they also need oxygen. Hence plants respire – or breathe – in a comparable way to us. Respiration doesn’t require light, so when a tree stops photosynthesising it carries on “breathing” throughout the night, taking in oxygen and releasing carbon dioxide.
Of course, for us to be here, trees can’t use as much oxygen as they produce – plants generally produce four times the oxygen they need for respiration. In effect, trees are producing food during the day and consuming it at night. Work by a team from the John Innes Centre in Norwich has discovered that they use a chemical calculator to divide the amount of stored energy by the length of the night, so they don’t starve. This calculator is so adaptable that plants used to 12 hour days can adjust to an 8 hour day, recalculating their metabolic rate to their normal nightly rhythm.
But for some plants, this inefficient combination of photosynthesis and respiration is deadly. “Trees in hot climates need to preserve water, so they don’t want to open their stomata in the midday sun,” explains Sandra Knapp, a botanist at the Natural History Museum. To avoid this, they only open these microscopic pores at night – when the plant has to take in enough oxygen and CO2 to last the day; they store this CO2 in a chemical called malic acid, then release it at night.
So next time you are passing through the woods at night and all seems quiet and still, listen a little harder. All around you, trees are relaxing, letting branches hang, eating, stretching and sucking up water.
Reference:
theguardian.com
Tree Weaver 