Drought: what if the soil held the answers?

Whether they measure a few centimeters or more than 100 meters, plants seem to close their pores at the same tension threshold. This discovery by Mathieu Javaux, professor and researcher at UCLouvain, and his colleagues, reveals how the soil imposes its laws on vegetation. 

Thanks to knowledge sharing and a mathematical model, the discovery of this potential universal limit of -1.3 MPa enriches our knowledge of plants and could well open up new perspectives on how to adapt agriculture to future droughts. 

Plants and soil: a relationship under strain 

Like us, plants sweat. They lose water through tiny holes on the surface of their leaves called stomata. These pores are used to capture CO₂, which plants need to grow. In order to collect this gas, the stomata must open, causing them to lose water.

“This loss may explain why 500 liters of water are needed to produce 1 kg of wheat”, explains Mathieu Javaux, professor at the Faculty of bioscience engineering and researcher at the Earth and Life Institute.

However, this transpiration is not a waste: it circulates sap, allows nutrients to be taken up from the soil, and cools the plant.  

Transpiration also feeds the continental water cycle. In fact, “on a continental scale, transpiration accounts for 60% of total precipitation”, reveals Mathieu Javaux. 

However, the problem is that if the plant loses too much water, it risks dehydration. Hence the initial question: when do stomata close?

In plants, water flows from the roots to the leaves through a network of vessels called xylem. This continuous column is subject to significant tension due to transpiration, as if the plant were sucking water upward through a giant straw.

Regulating this tension remains complex. If it becomes too strong, there is a risk of embolism. Air bubbles can form in the xylem, interrupting the flow of water.

Without water, the plant dries out and eventually dies. This is why regulating this tension is vital and raises a second question: what determines the closing tension of stomata? 

The story of a potential universal limit 

To answer these questions, Mathieu Javaux, Tim Brodribb (University of Tasmania), and Andrea Carminati's team (ETH Zurich) compiled all existing data on tension during stomatal closure. These measurements had been shared by various laboratories on 19 different varieties.

At the same time, a mathematical model was also used to simulate the behavior of water in the soil and the plant.

The result: the threshold of -1.3 megapascals (MPa) emerged as a constant at which stomata begin to close, regardless of plant type.

Why this universal limit? It is no coincidence; it stems from the laws of soil hydraulics: above this tension, extracting water from the soil becomes much less efficient for the plant. 

But then, why are some plants more resistant to drought? 

The threshold of -1.3 MPa marks the beginning of stomatal closure, not their total closure. Indeed, some plants can close their stomata quickly, while others keep them open longer.

Both strategies have their advantages and disadvantages.

On the one hand, closing stomata quickly improves survival in arid conditions, but at the expense of growth.

On the other hand, keeping stomata open longer promotes growth but increases vulnerability to drought.

This explains why some plants lose less water than others and are therefore more resistant to drought.

Other factors also come into play, such as root size, leaf surface area, and resistance to embolism.

These findings were published in January in the international journal Science and are the result of a collaboration between numerous laboratories.

This proves, once again, the importance of knowledge sharing, particularly in the context of climate change. 

Reference

Andrea Carminati et al., Soils drive convergence in the regulation of vascular tension in land plants. Science 391, 476-479 (2026). DOI: 10.1126/science.adx8114

This article was originally written in French by the AREC team of UCLouvain. It is available to be read here.