All living beings, including us and green plants, obey a circadian rhythm. It corresponds to a biological rhythm that lasts about 24 hours. For example, the fact that we alternate a waking phase and a sleeping phase marks our daily life.
These rhythms are governed by our biological clocks, which are themselves sensitive to various factors. One of them is the presence of light. In nature, it is the perception of light that tells us when to get up and when to go to sleep.
Plants therefore do not use water at night, which is precisely when they fill up for the day. So we talk about plant transpiration. This transpiration is therefore higher during the day and decreases strongly during the night.
How does plant transpiration work?
This sweating determines a much higher water consumption than animals that generally consume relatively little water, in the order of one twentieth of their weight per day. For a pedunculated oak from an alluvial forest, the average consumption is 260 litres/day.
According to various studies, the water needs resulting from 10 to 50% of the plant transpiration of the trees are satisfied by the water stored in the woody plants, the rest being supplied by the water stored in the soil. Plants, especially trees, lose stored water by transpiration during the day and recharge their stock by root extraction mainly at night.
Role of plant transpiration
Raw sap is a very diluted solution of low osmotic pressure. The sap circulates in the wood vessels at a speed of 1 to 6 m/h, up to 100 m/h for maximum transpiration.
The water is transpired by the leaf, all the more so as the climatic demand is high.
It “flows” from the soil where it is little retained (high water potential) to the leaves where it is more retained (low water potential). This movement can be described by a transfer equation where the greater the difference in water potential and the lower the resistance to transfer, the greater the flow.
In this way, the more the plant transpires, the more effective the suction will be, and the more water the plant will absorb into the soil. The plant thus uses phenomena of variations in the opening of the stomata in order to vary the force of absorption when the soil or the air is too dry. But this is true only up to a certain threshold beyond which the plant will be at a stage of too much water stress, forcing it to close the stomata in order to preserve itself.
Is it dangerous to sleep next to a green plant?
Indeed, during the night, due to the absence of light, the process of photosynthesis is interrupted in plants. Instead of absorbing CO2 and rejecting oxygen, the opposite happens.
A phenomenon that could therefore, in theory, cause asphyxiation of people nearby. But this hypothesis is exaggerated, because plants actually emit only a minute amount of carbon dioxide.
The latter is indeed lower than that of a cat or a dog sleeping next to its master, but also lower than that of a spouse, or even the sleeper’s own exhalations.
In 2011, a British scientist even took the demonstration to the point of locking himself up for two days in a 30 m2 room with 160 plants. And he came out of it in perfect health.
The fact that the plants emit carbon dioxide should not worry you. Indeed, you do not risk asphyxiating yourself if you put a plant in your room (unless you install a baobab tree, and again!):
- If plants release carbon dioxide (CO2) at night, it is in very small quantities.
- Moreover, over a 24-hour cycle, the plant actually absorbs much more CO2 than it releases, and produces much more oxygen than it consumes.
To sum up, plants use practically no water at night, they emit carbon dioxide. Plants lose the water they have stored at night through a process called plant transpiration. We explain how this works in this article.