In the sweltering summer days, the landscape of southern Spain appears monotonous due to the parched yellow grass and withering greenery. However, a thistle named Carlina corymbosa unexpectedly blossoms with small yellow flowers under this merciless sunshine, causing wonder. This plant becomes a precious nectar source for bees and pollinators scarce in water, especially in August when they are the only flowering plants in Spain’s Mediterranean arid regions.
While researching Carlina corymbosa in Spain’s Cazorla Mountains, biologist Carlos M. Herrera discovered that even after hours of exposure to the sun, these flowers still feel cool to the touch. Further investigation showed that the temperature inside the capitula of the thistle is typically lower than the surrounding environment, sometimes with a difference of up to 10 degrees Celsius.
Physicists from the Los Alamos National Laboratory in the United States, along with plant physiologist Sanna Sevanto, believe this finding demonstrates the survival strategies plants adopt when facing extreme environmental pressures. Sevanto and other researchers had previously observed leaves cooling themselves, but the case of Carlina corymbosa suggests that transpirational cooling is not just a side effect of photosynthesis but may also be a deliberate mechanism to lower the temperature of reproductive organs.
Herrera points out that as long as the capitula contain moisture (such as nectar), the thistle can perform this cooling mechanism. Sevanto adds that at the stomata of these petals, the evaporative cooling from water evaporation can help maintain the safety of the reproductive organs, thus aiding the plant’s survival in harsh conditions.
In arid environments, plants open their stomata for transpiration to lower body temperature, which is a strategy comparable to a gamble on water management. Currently, the scientific community has not observed plants employing this mechanism for cooling.
Researcher Herrera is planning a series of experiments on the Carlina corymbosa. First, he will observe the stomata of the thistle’s petals under a microscope. Then, during the hot summer, he plans to verify whether this cooling method is akin to sweating by adjusting the plant’s water supply. In addition, he intends to study the root structures of the thistle to discover their unique design, which allows them to absorb enough water to support cooling.
As climate change progresses, plants face increasing heat stress, making the exploration of adaptive strategies to extreme temperature conditions particularly crucial. However, Herrera points out that whether these plants can obtain enough water to ensure the process of transpirational cooling is yet another entirely different question.
