This Desert Plant in Death Valley Can Air-Condition Its Own Leaves : ScienceAlert

Hike through Death Valley, a desert basin in California and parts of Nevada, and you’re guaranteed to see wall-to-wall colorful geology.

But it’s also well-known for being the lowest, driest, and hottest place in North America.

By hottest, that means that not only do normal daily temps hit 49 °C (120 °F) in the summer, but it’s reported to have reached as high as 56.7 °C (134.1 °F).

So, unsurprisingly, there are vast areas of the park where no plants grow at all.

But one species of flora has adapted especially well to these punishing conditions: Tidestromia oblongifolia (AKA Arizona honeysweet), a scrubby native flowering plant.

Think of extreme heat-loving organisms, and you’d probably picture microbes flourishing in hot springs or deep-sea hydrothermal vents. Complex plants and animals have far more moving parts to coordinate, making extreme heat much harder to survive.

Key to its ability to survive very high temps, a new preprint study has found, is the fact that the honeysweet can significantly cool its own leaves well below the temperature of the surrounding air.

It cools itself so effectively that it can survive temperatures brushing up against the upper limit of what complex life can tolerate, which is 60 °C (140 °F).

To understand how it copes, 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 regimen, gradually acclimating them before exposing them to 60 °C (140 °F) for six to eight hours every day over more than a week.

Not every plant made it. Depending on the genetic line, survival ranged from just under 2 percent to more than 34 percent, suggesting that some individuals are better adapted to extreme heat than others.

This Desert Plant in Death Valley Can Air-Condition Its Own Leaves — *T. oblongifolia* growing in Death Valley, California, photographed as part of a previous study on the plant. (Karine Prado)

Using infrared cameras, the researchers recorded that on the hottest days, some leaves measured more than 10 °C cooler than the air around them, with the most extreme cases reaching an astonishing 13 °C difference.

Even after spending eight consecutive days in 60 °C heat, surviving plants kept their leaf temperatures between roughly 54 °C and 59 °C – still incredibly hot, but just below the estimated upper thermal limit for complex eukaryotic life. How well the plants cooled determined life or death.

Key to this research was showing that this plant wasn’t just able to tolerate higher temperatures than other plants could, but that it could avoid them by active cooling.

Other research suggests the heat adaptation is enabled by mitochondria and chloroplasts interacting, resulting in the choroplasts changing shape.

Digging deeper, the team searched for the biological machinery behind this powerful cooling system.

Genome-wide analyses identified three regions of DNA associated with heat survival, indicating that this ability has a genetic basis. Other tests showed clear differences between cooler and hotter leaves.

Plants with cooler leaves activated pathways linked to keeping leaf pores, called stomata, open. These tiny openings allow water to evaporate from the leaf surface, producing the same cooling effect as sweat evaporating from human skin.

To test whether this cooling mechanism was truly responsible for survival, the researchers artificially shut the stomata.

“We found that forcing the stomata (the pores on a plant leaf) to close interfered with cooling,” Joanna Feehan, the first author of the study and postdoctoral fellow at the Plant Resilience Institute in Michigan, told ScienceAlert.

“Part of transpiration is evaporation through stomata, so we do know this is a major component of their cooling mechanism.”

While researchers are starting to understand the mechanism, it’s still a mystery where the water needed for this significant evaporative cooling comes from.

Most desert plants conserve water, rather than evaporating it. Unlike many desert species, honeysweet isn’t especially drought-tolerant or water-storing, instead relying on groundwater available beneath Death Valley’s soils.

Feehan has a hypothesis.

“We think Tidestromia probably has a root system that is very good at taking up water, specialized hydraulics that move water well through the plant, and compositions of salts/sugar/proteins inside their cells that keep them hydrated as they transpire to cool,” 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 plentiful water, rather than mature plants coping with the myriad complexities of the desert.

The researchers also identified only three broad genetic regions linked to heat tolerance, so pinpointing the specific genes responsible will require further work.

Lest you think this is just some good trivia fodder though, this work might have a direct impact on anyone who eats food.

“We’re breaking heat records globally and crop yields are very vulnerable to increasing temperatures,” said Feehan.

“Tidestromia has figured out how to survive at temperatures that are otherwise lethal to plants, animals, and fungi. If we can learn those survival mechanisms, we will have more, and likely very unique, tools to make crops more resilient against heat waves.”

Read the full study at BioRxiv.

This article was fact-checked by Rachel Garner and edited by Peter Dockrill. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.