For plants, breathing is a balancing act between gathering what they need from the atmosphere and not losing too much water. A new study shows how some plants are able to regulate this mechanism and stay hydrated, even at very high temperatures.

The wax can make the surface of some leaves waterproof. Image credits: Rei.

In the 1950s, the German botanist Otto Ludwig Lange was studying plants in Mauritania — a country in western Africa that’s mostly covered by a desert. Lange noticed that leaves can heat up to temperatures as high as 56 degrees Celsius (133 Fahrenheit). He couldn’t figure out how they do it — how do plants get so hot without losing all their water?

In an attempt to solve that, botanists Markus Riederer and Amauri Bueno from Julius-Maximilians-Universität Würzburg in Germany, succeeded in revealing the secret studied the complex structure of a plant leaf.

Leaves are covered with a “skin” called a cuticle. The cuticle consists of lipids and polymers impregnated with tax, and it acts coherent outer covering of the plant. If you’ve ever seen droplets of water sitting on top of a leaf, it’s the wax-rich cuticle holding it in place.

This protective layer contains numerous pores (called stomata), which open and close according to the plant’s needs. The problem is that when these pores open up to allow the plant to breathe, they also allow water to evaporate. For desert plants, this is particularly troubling, but as Riederer and Bueno found, they have different ways of dealing with this.

Date palms have an innovative strategy to survive extreme heat.

For instance, a plant called a colocynth (Citrullus colocynthis), also called a bitter apple or a bitter cucumber opens up its pores when the heat gets going. This allows some of its water to evaporate as sweat, cooling down the leaves. This process is water-intensive, but the colocynth can afford it because it has a deep root which allows it to gather sufficient water. Decades ago, Otto Ludwig Lange noted that the plant can keep its leaves up to 15 degrees cooler than the surrounding air, and now the mechanism is better understood. However, the date palms have a radically different approach.

The date trees can’t afford to lose water, so they don’t “sweat”. As a result, their leaves get much hotter than the surrounding desert — up to 15 degrees hotter. The secret to its survival lies in the cuticle, specifically in the wax in the cuticle.

Unlike that of the colocynth and most other plants, the wax in the date palm’s skin is much more water-proof, due to its different water composition. While it’s not clear exactly what causes this different composition, the results are important because they could be used in agriculture, encouraging crop growers to select plants with certain cuticle waxes because they have better chance of survival in hot locations.

The study was published in the Journal of Experimental Botany.