Southern Spain’s landscape in high summer is perhaps best described as “crunchy.” Under the unrelenting sun, grass turns to brown straw, and almost everything green shrivels and dies—except for the clustered carline thistle, a plant with humble yellow flowers and a surprising superpower.
Every August this thistle is one of the only plants to flower in most of Spain’s arid Mediterranean habitats, giving it a virtual monopoly on the local bees and other pollinators. But how can the thistle survive, much less bloom, when its neighbors are reduced to twigs and dust?
Spanish National Research Council evolutionary ecologist Carlos M. Herrera was conducting a census of pollinators in the Sierra de Cazorla mountain range when he peered into a thistle blossom, lightly touching the flower, to see how much nectar was inside. To his astonishment, it felt unmistakably cool—even after hours in direct sun. Herrera says he immediately sensed he was “about to make a discovery.”
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During scorching Spanish heat waves, Herrera measured the temperature inside thistle flower heads and the ambient air temperature less than an inch away. He found the flower heads were routinely nine degrees Fahrenheit cooler than their surroundings, with the difference approaching 18 degrees F for some flowers on the hottest days. His observations are detailed in the Scientific Naturalist.
Sanna Sevanto, a physicist and plant physiologist at Los Alamos National Laboratory who studies how plants respond to environmental stress, says Herrera’s finding is exciting and could confirm a risky plant survival strategy that has, until now, only been theorized.
Sevanto and other scientists have documented apparent self-cooling in tree leaves, but that effect is probably coincidental, she says. To perform photosynthesis, leaves need access to carbon dioxide, which enters through tiny pores called stomata on a leaf’s surface. When stomata open to let carbon dioxide in, some water escapes, thereby causing evaporative cooling that lowers the leaf’s temperature slightly.
But for the Spanish thistles, evaporative cooling could be the goal rather than just a side effect of photosynthesis. Herrera suggests the plant could essentially be sweating: sacrificing precious water, extremely scarce in Spain’s arid summers, to prevent its delicate reproductive organs from overheating.
Herrera says that such cooling could occur anywhere that the flower head contains water, including in its liquid nectar. Some flower species have stomata on their petals, which Sevanto says would be an easy route for releasing water. Opening stomata in a drought is a big gamble, though, and she notes that so far “we have not observed a plant that would do it to cool themselves.”
As well as checking the thistle’s petals for stomata under a microscope, Herrera has more experiments planned for the next sweltering summer. For one, he will manipulate a plant’s water supply to try to prove its cooling is really a “sweating” action, and in a second, he will look for novelties in the thistle’s root structure that could explain how it is able to take in enough water to invest in cooling.
As heat waves become more frequent and intense with climate change, it’s increasingly important to study unusual adaptations that help plants survive heat stress. But ultimately, “whether these plants will have water enough to keep the system working,” Herrera says, is “another story.”