When heat shuts down plants' climate regulation

Heat is an enormous stressor, also for plants. An Innsbruck-based research team looked into the correlation of dose and effect, finding that the damage to leaf health caused by very high temperatures is exacerbated by prolonged heat spells. If that coincides with a lack of water, matters become even worse.

Biology and Medicine

by Verena Ahne

August 31, 2026

The climate crisis became painfully evident during the scorching summer of 2026—with consequences not only for humans and animals. The extreme heat led to aridity and drought. This was evident in trees, shrubs, ornamental and crop plants, which displayed brown, crispy edges on leaves, withered branches, and premature fall foliage. These changes cannot be explained by the lack of water alone. The “Plant water use under heat” project, which was funded by the FWF, impressively demonstrated that it is the heat itself that does this visible damage to our plants, on which we rely for oxygen.

A team of researchers at the University of Innsbruck explored how high temperatures affect leaves. They were particularly interested in the following questions: Do different plants, from ground cover alpines found in high altitudes to deciduous trees populating rain forests, react differently to heat? To which extent can plants adapt to rising temperatures? At which temperatures do plants start to suffer from irreparable damage? And what’s the role of water supply in all of this?

About the project

In the course of evolution, plants have developed numerous mechanisms to deal with external stressors. The research project attempted to pinpoint the threshold at which various plants, from trees to alpine herbs, lose their ability to deal with extreme temperatures.

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Nature’s own AC system

Botanist Gilbert Neuer, the project’s lead, and his team were especially curious about the links between dose and effect. “In the course of evolution, plants have developed a whole host of strategies to withstand extreme temperatures,” Neuner explains. In regions habitually exposed to extreme cold, they grow close to the ground and remain small and compact to increase their body temperature.

Where heat is the issue, plants rely on a sophisticated cooling mechanism. Not unlike human skin, leaves regulate their temperature via small pores called stomata, through which water evaporates. This kind of transpiration functions as the leaf’s built-in AC system. And it also cools down the plant’s surroundings by several degrees: it is thanks to “sweating” plants that temperatures are noticeably lower in forests or below trees compared to meadows without any trees or bushes, crop fields or the concrete desert found in cities.

How hot is too hot?

Leaf temperatures above 50° Celsius, which are not uncommon in south-facing alpine slopes protected from winds or in treetops in tropical areas, can be dealt with through this evaporation mechanism. But are there ambient temperatures at which this will no longer be the case? “In the past, researchers used a standardized test to check heat tolerance,” Neuner explains. It consisted of exposing plant samples to a certain temperature for 30 minutes and checking it for damage afterwards.

Such tests showed that almost all plants on Earth can handle air temperatures between 44 and 55° Celsius. “The results are astonishingly consistent and also coincide with the maximum temperatures found in these biomes,” the plant expert says, adding that only aquatic plants are more sensitive to outside temperatures while some desert plants can stomach record highs. However, these tests “at best investigated midday heat” of no more than 30 minutes.

“We wanted to explore what happens when we change these parameters, more specifically, the duration a plant is exposed to the heat or the temperature itself.” To this end, they used differential calorimetry, among other things, which allowed them to heat plants collected outside in the lab in a controlled manner, after which the researchers could analyze various molecular processes occurring in the leaves. “For example, we checked how leaves open or close their stomata or how heat changes plant cuticles.” Further analyses of molecular processes in leaves’ membranes included testing at which temperature protein denaturation kicks in.

Damage increases with heat

The experiments showed a clear and exponential connection between dose and effect. “A leaf won’t be bothered by an air temperature of 45° Celsius if it only lasts for half an hour,” Neuner specifies. “But when it stays hot for four hours, we see the same kind of injuries already at 40°. And at extreme temperatures above 50° Celsius, the same damaging effect can be seen after several minutes.” Very fittingly, the researcher published a study titled “The dose makes the poison”. After all, one thing is obvious in cities at the moment: the longer leaves are exposed to heat, the lower their heat-tolerance gets. And the higher temperatures rise, the faster real damage is done.

Waxes, lipids, proteins

After all, heat affects not only transpiration but the leaf’s entire physiology. It is the reason for a whole set of changes, some of which a plant is able to repair when temperatures drop again. Once a certain threshold is reached, however, the injuries become irreversible. But what exactly causes the damage?

The researchers started out by closely examining plants’ cuticles, a thin wax layer found on leaves. “The cuticle of cabbage is very pronounced, but every leaf has a cuticle,” Neuner explains by way of an example. It protects plants from pathogens, serves as a mechanical barrier, reduces damage caused by feeding insects – and it also keeps the leaf from losing an excessive amount of water.

Plants can adjust the composition of this protective layer to respond to a high level of solar radiation. “We were able to observe that alpine azalea, a ground cover with evergreen leaves, for example, reduces the amount of flavonoids in its cuticular membrane when it grows on south-facing slopes. This makes it much better equipped to withstand heat than alpine azalea found on north-facing slopes,” Neuner explains. The same properties found in bees’ wax, which melts at high temperatures, are also found in cuticles: “The hotter it gets, the more porous it becomes. This causes the plant to lose a lot more water.”

Initially, the researchers suspected this was what caused leaf injuries. “And yet: the cuticle only becomes porous once the leaf has already suffered severe damage,” project lead Neuner reports. Proteins and lipids in a leaf’s membrane also change due to scorching heat. Proteins undergo a denaturation process, “similar to the way protein turns solid and white when it is fried.” Yet this is another change that only happens when the leaf is already damaged. “So it seems that the process causing the injury starts at a much earlier stage,” says Neuner, wistfully adding: “We really wanted to solve this puzzle. But it’s still unknown what actually causes the damage.”

Adaptability at the brink?

What does all of this mean in a rapidly warming world? “It is difficult to make forecasts because so much depends on water supply – here at home as well as anywhere else in the world,” says Neuner, admitting that he never ceases to be amazed by plants’ ability to adapt. “There is something called a safety margin, but in some regions, it has become dangerously small.”

Research conducted in tropical regions that is yet to be published suggests that there are concrete physiological limits. “Due to the unambiguous dose-effect connection, we thought that tropical plants would have adapted to the high temperatures in their region,” he says about a research trip to Costa Rica. “But that was not the case. This really surprised us. The trees growing in rainforests are not that much better at handling heat than the azalea found in the Alps. They merely offset the higher ambient temperatures through particularly strong transpiration.” If the typical rain to which rainforests owe their name stops falling, the heat might start to turn into a critical danger to trees.

In fact, dry seasons have already become considerably longer. In recent years, some rainforests even experienced droughts for the first time. Whether there will be enough water in the future will depend on how forests develop, in rainforests as much as here in Austria.

All of this doesn’t give us much reason to be optimistic. The rapidly rising temperatures we witness today mean that plants would need much more water than in the past – not only to grow but to undergo the crucial process of transpiration. But in many places around the world, people more and more often wait for snow or for light but long rainfall in vain. As leaves close their stomata to save water, their natural AC function stops, which makes them overheat, hurts their leaves and the plant’s overall resistance. This also means that plants evaporate less, which in turn becomes a factor in global warming. What’s more, heat also dehydrates both land and air, which only exacerbates dry spells. It is a parching, desiccating vicious circle. The real-world effects can already be seen on a global scale.

About the researcher

Gilbert Neuner studied botany at the University of Innsbruck, where he was appointed full professor in 2001. He leads the stress physiology research group at the Department of Botany, which mainly explores the mechanisms plants – particularly those found in alpine regions – develop to withstand stress factors such as heat, drought, cold, ice and strong solar radiation. Neuner also serves as the deputy head of the Alpine Biology and Global Change doctoral college. The research project “Plant water use under heat” (2021–2025) was funded by the Austria Science Fund (FWF).