Though we cannot hear it, plants are constantly emitting and absorbing gases through millions of little mouths on the surface of their leaves and stems.
Just by opening and closing, these pores (“stomata” in Greek and botanist-speak) fulfil crucial functions in the plant — allowing the entry of carbon dioxide (CO2) for the solar-powered process of photosynthesis, through which plants produce their own food (as well as the oxygen we breathe).
Whenever they’re open, stomata also allow water vapour to escape, cooling down sun-exposed leaves and creating the physical traction that pulls water up through the plant from the roots.
But in a warmer world with more frequent droughts, stomata can also be liability, presenting plants with a balancing act: Keep stomata open and make sugars, or close them to save water. Which is why an increasing number of researchers are keen to study stomata and figure out ways to manipulate them and develop hardier crops.
“Water is the No. 1 limitation to crop production globally,” said plant physiologist Andrew Leakey of the University of Illinois Urbana-Champaign at a 2025 meeting of the Society for Experimental Biology that featured a surprising number of talks on stomata. “It’s a very relevant issue today that will only become more significant as climate change progresses.”
In a typical stomate structure, two bean-shaped or dumbbell-shaped guard cells surround a pore. Depending on environmental conditions such as the amount of water in the plant, the guard cells shrink to close the pore, or swell to open it, allowing the movement in and out of water vapour, carbon dioxide and oxygen.
Though photosynthesis requires those pores to open, there’s a great amount of variability in how much water is lost in the process. One of the most efficient crops is sorghum, a drought-adapted cereal that originated in Africa and is popular across the tropics and subtropics. A 2024 study found that in many sorghum plants, stomata move fast — opening just long enough to let in the CO2 needed for photosynthesis and closing right after. “I’d never seen such a tight relationship in any of the species I’ve studied,” said study coauthor Tracy Lawson, now also at Illinois Urbana-Champaign. She hopes that disentangling the mechanism behind this precise coordination might inspire efforts to improve thirstier crops.
Some scientists had been initially optimistic that higher CO2 levels in the air — amounts have increased from 280 parts per million to 422 parts per million since the Industrial Revolution — might mean that plants would lose less water, because they don’t have to open their stomata as much to get enough CO2. Starting in 2004, Leakey and colleagues set out to test this. They surrounded plots in a soybean field with a network of computer-controlled pipes puffing out extra CO2, effectively mimicking an atmosphere with 550 to 585 parts per million of the gas.
After eight years of growing soy under these conditions — sometimes using retractable awnings to create artificial droughts — the researchers found that soy plants did, indeed, keep their stomata open for less time in response to higher CO2. They also increased water efficiency and rates of photosynthesis. In the wettest years, this led to a 20 to 25 percent increase in soybean yield.
But there was a catch: When the plants were exposed to drought conditions, “the benefits of high CO2 can diminish to nothing,” Leakey said. That’s because at high CO2 levels, the plants grew much bigger early in the season — and then required more water later. This suggests that we cannot count on rising CO2 to improve water efficiency, since droughts are becoming more frequent.
A deceptively simple way of helping plants preserve water might be to reduce the number of stomata in plant leaves. This has been achieved in various crops by boosting the activity of a gene called EPF1 that regulates how many stomata develop. The resulting stronger genetic signal leads to crops with fewer stomata — and improved water-use efficiency.
Leakey’s lab, for its part, reported last year that modifying an EPF gene reduced the number of stomata in sorghum by about 30 to 60 percent. That was enough to lower water loss by 30 to 34 percent without limiting the plants’ rate of photosynthesis or growth.
In sugarcane, however, the team found they couldn’t reduce the number of stomata as drastically. Like in sorghum, the remaining stomata opened wider — and in sugarcane’s case, since there were still enough stomata around, the extra dilation eliminated any water savings. In a follow-up analysis of published research across 10 different plant species, including wheat and corn, Leakey and colleagues observed this to be a widespread phenomenon. While reducing the number of stomata almost always saved some water, the savings were less than expected.
Tinkering with the EPF genes can have undesired effects, too. The sorghum plants that were modified to have fewer stomata, for example, also had much smaller, underdeveloped flowers and produced significantly fewer seeds, presumably because the genes are involved in these processes too. Leakey and his team are now working to make more precise modifications that affect the genes’ activity only during early leaf development.
The team is also increasingly using artificial intelligence to speed up the discovery of promising genes to modify, and to evaluate new, genetically changed crop variants in the lab and field.
This can make a big difference. For an experiment on sorghum published in 2021, Leakey’s team grew 869 genetic variants of the crop in field trials across two years. “There were 4,000 leaf samples with almost 3 million stomata on them, which we counted,” he recalled. In the past, such counting was done manually, by painting nail varnish on leaves, peeling it off and counting the marks left by stomata under the microscope. “People in my lab don’t particularly enjoy doing that thousands of times,” he said.
Instead, his team uses a method called optical tomography in which “you just take a piece of frozen leaf, use double-sided sticky tape to attach it to a microscope slide, and then in about a minute, you can scan the surface.” A machine-learning tool then counts the stomata.
In other progress, Leakey’s team has developed a system that allows them to watch stomata under the microscope while measuring the gases passing through them. And researchers are working on ways to use drones to monitor the growth and leaf temperatures of plants in fields at a previously unimaginable scale. This makes it possible to far more quickly evaluate new genetic variants in many different environments and under a variety of conditions.
This knowledge can then be used in models that allow researchers and potentially breeders to predict how tinkering with genes will affect the opening and closing of stomata, says ecophysiologist François Tardieu of the French National Research Institute for Agriculture, who specialises in maize. He has shown that more stomatal opening predicts better yield in favourable conditions — but, significantly, less yield in stressful conditions such as heat or drought.
This may have important implications, Tardieu said. Breeding companies tend to focus on measuring yield — and though this has so far been very effective, yield-based selection may be too slow and reactive to keep up with increasing hot and dry spells. “If we want to prepare for climate change, perhaps we’ll have to change our ways,” he said.
In addition to conducting experiments at much larger scales, that might mean zooming in on what plants are doing when we haven’t been paying attention — at night, for example. That is when plant physiologist Lorna McAusland of the University of Nottingham, England, can often be found roaming around her test fields. “Our nights are warming up at about 1.4 times the rate of daytime temperatures,” she said, “because of increasing cloud cover. And for crops, warmer nights may mean lower yields.” In rice, for example, scientists have found that with every 1 degree Celsius increase at night, yields declined by 10 percent. Such trends have been documented for crops like wheat, too.To study the reaction of stomata to these trends, McAusland and her team heated their fields using infrared light. (This was not without risk: “I actually burned my hair in the field,” she recalled.) They then measured the gases passing through pores at night in 12 wheat varieties. Those that open their stomata more at night tended to maintain their yields compared with varieties that close them, the scientists found.
McAusland’s research underscores the importance of breeders knowing what stomata are up to in the crops that they select — at night as well as in the daytime. Plants don’t just use opened stomata at night to cool down; she has evidence they use them to absorb condensed water from their own leaves, which may be increasingly important in the face of worsening droughts. “Before this trait gets bred out,” she said, “we really need to work out why they do it.”
This article was originally published on Knowable Magazine.
