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Below-ground hydraulics control stomatal closure

Botanists find that during soil drying, stomatal regulation is controlled by below-ground hydraulics in a predictable way.

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Stomata are tiny pores in leaves that allow the plant to respire. We typically think of plants breathing in carbon dioxide, but they also let gases out. Along with gases, plants can lose water that they have drawn up from the soil. In order to prevent excessive water loss, plants must be able to control their stomata, shutting them when necessary to avoid losing too much moisture. But how is this done? Mohanned Abdalla and colleagues used a novel root pressure chamber to measure the relation between transpiration and leaf xylem water pressure. In their article in Annals of Botany, the scientists show how below-ground hydraulics control stomatal closure during soil drying.

A mess of cells bunched together with what appear like lips lying between some of them.
Stomata. Image: Canva.

To examine the relationship between stomata and below-ground hydraulics, Abdalla and colleagues took tomato plants and grafted them onto two kinds of rootstock. One had short roots, while the other had long roots and so could reach deeper into the soil when looking for moisture. They then used a root pressure chamber system that could simultaneously measure transpiration and leaf xylem water pressure within an intact plant.

The team found the relationship was linear in wet conditions but became non-linear as the soil dried. Plants with shorter root systems required larger gradients in soil water pressure to sustain the same transpiration rate and exhibited an earlier non-linearity and stomatal closure.

Abdalla and colleagues conclude that the relation between stomatal conductance and leaf water potential is affected by root length and below-ground hydraulics, particularly soil hydraulic conductance. They state: “Our results show a tied link between soil hydraulic conductivity, active root length and stomatal conductance, and the coordination between these variables is central in predicting the ability of plants to cope with water shortage.”


Abdalla, M., Ahmed, M.A., Cai, G., Wankmüller, F., Schwartz, N., Litig, O., Javaux, M. and Carminati, A. (2022) “Stomatal closure during water deficit is controlled by below-ground hydraulics,” Annals of Botany, https://doi.org/10.1093/aob/mcab141

Alex Assiry

Alex Assiry is an editorial assistant in the Annals of Botany Office. When not working, Alex listens for the opportunity to help.


  • “We typically think of plants breathing in carbon dioxide, …” – do “we”? If so then we are wrong. Plants do not “breathe” in the sense of forced gas exchange as in mammals. The process is a passive diffusion process (perhaps with some involvement of pressure differences under limited circumstances). So to use the word breathes is quite wrong. Please check before expressing erroneous ideas.

    • Looking through search logs, I see common queries “Do trees breathe in when we breathe out?”, “Do trees breathe in oxygen at night?”, and “How do trees breathe when they shed leaves?”. While it is literally wrong to say plants breathe, it is also a common thought. Does ‘breathe’ work as a metaphor?

      Here I’m less sure. In your case it clearly does, as you understand fully how the metaphor breaks down. In the case of other people, particularly newcomers to botany, it might not and there could be some confusion between breathing and respiration. However, to many of the people who don’t appreciate there is a difference, the word respiration doesn’t mean a lot, while ‘breathe’ at least suggests an exchange of gases.

      That indicates there is a useful blog post to be written on how plants breathe that makes clear they don’t and why that’s interesting. That is on my to-do list, and when this post went live my thought was “I really need to get it written”, because I was expecting at least one complaint.. I’m hoping next month I can explain why that to-do list is just getting longer and longer at the moment.

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