Hike through Death Valley, a desert basin in California and parts of Nevada, and you're guaranteed to see a landscape covered in colorful geological formations.
But the place is also well known for being the lowest, driest, and hottest place in North America.
By "hottest" is meant not only that the average daytime temperature in summer reaches 49°C (120°F), but that it has also been reported to rise as high as 56.7°C (134.1°F).
It is therefore not surprising that vast areas of the park are completely devoid of any vegetation.
However, one plant species has adapted particularly well to these harsh conditions: Tidestromia oblongifolia ( also known as Arizona honey grass ) , a shrubby native flowering plant.
When you imagine organisms that thrive in extreme heat, you probably picture microbes thriving in hot springs or deep-sea hydrothermal vents. However, complex plants and animals have many more moving parts that need to be coordinated, making survival in extreme heat much more challenging.
A new preprint study has found that the key to this plant's ability to survive in very high temperatures is that the honey plant can cool its leaves significantly, well below the ambient air temperature.
It cools so efficiently that it can withstand temperatures approaching the upper limit of the tolerable load for complex life forms, which is 60 °C (140 °F).
To understand how this plant copes with its conditions, the researchers collected seeds from 223 wild plants across the species' range, including populations from Death Valley itself.
They then subjected more than 1,200 seedlings to a grueling heat regime, gradually acclimating them before exposing them to temperatures of 60°C (140°F) for six to eight hours daily for more than a week.
Not all plants survived. Depending on the genetic line, survival rates ranged from just under 2 percent to over 34 percent, suggesting that some individuals are better adapted to extreme heat than others.
Plant T. oblongifolia , growing in Death Valley, California, photographed as part of a previous study of the plant. (Karin Prado)
Using infrared cameras, the researchers recorded that on the hottest days, the temperature of some leaves was more than 10°C lower than the ambient air temperature, and in the most extreme cases, the difference reached a staggering 13°C.
Even after eight days of continuous exposure to 60°C, the surviving plants maintained leaf temperatures between approximately 54°C and 59°C—still incredibly high, but just below the presumed upper temperature limit for complex eukaryotic organisms. How well the plants cooled themselves could mean the difference between life and death.
The key point of this study was to show that this plant is not only able to tolerate higher temperatures than other plants, but also to avoid them through active cooling.
Other studies suggest that heat adaptation is mediated by interactions between mitochondria and chloroplasts, causing the chloroplasts to change shape.
By conducting a more thorough study, the team tried to elucidate the biological mechanism underlying this powerful cooling system.
Whole-genome analysis identified three DNA regions associated with survival in high temperatures, suggesting a genetic basis for this ability. Other tests revealed clear differences between leaves exposed to cooler temperatures and those exposed to hotter temperatures.
Plants with cooler leaves activated mechanisms associated with keeping porous leaf surfaces, called stomata, open. These tiny openings allow water to evaporate from the leaf surface, creating the same cooling effect as sweat evaporating from human skin.
To test whether this cooling mechanism was indeed responsible for survival, the researchers artificially closed the stomata.
«"We found that forcibly closing the stomata (pores on the plant leaf) prevents chilling," Joanna Feehan, first author of the study and a research scientist at the Plant Sustainability Institute in Michigan, told ScienceAlert.
«"Transpiration is partly accomplished by evaporation through the stomata, so we know this is an important part of their cooling mechanism.".
While researchers are beginning to understand the mechanism, it remains a mystery where the water needed for this significant evaporative cooling comes from.
Most desert plants conserve water rather than evaporate it. Unlike many desert species, honey grass is not particularly drought-tolerant and lacks the ability to store water, relying instead on groundwater available beneath the soils of Death Valley.
Feehan has a hypothesis.
«"We believe that Tidestromia, "Probably a very good root system capable of absorbing water, specialized hydraulics that allow for efficient water movement throughout the plant, and a salt/sugar/protein composition within the cells that keeps them hydrated during transpiration for cooling," she said.
The study, which has not yet been peer-reviewed, also has important caveats. The experiments were conducted on young seedlings grown under controlled conditions with ample water, not on mature plants adapted to the many challenges of the desert.
The researchers also identified only three broad genetic regions associated with heat tolerance, so further research will be needed to pinpoint the specific genes responsible.
See also: A glacial lake has reappeared in Death Valley after record rains.
However, lest you think this is just a funny story, it should be noted that this work can have a direct impact on anyone who eats food.
«"We are breaking global temperature records, and crop yields are very vulnerable to rising temperatures," Feehan said.
« Tidestromia "It has learned to survive at temperatures that are lethal to plants, animals, and fungi. If we can study these survival mechanisms, we will have more, and likely very unique, tools for increasing crop resilience to extreme heat.".
The full text of the study can be found on BioRxiv.
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