Scientists Discover How Plant Sunscreen Works

esther wieringa, 'groen' via Flickr. CC BY-NC-ND 2.0

Plants are sun worshippers; they bask in it all day long, soaking up its rays so that they can generate energy from the process of photosynthesis. But they’re not immune to the damage that UV radiation can cause, and just like humans they need protection from it. Obviously, plants can’t slather themselves in sunscreen like we do, so how do they do it? According to new research, published in the Journal of the American Chemical Society, plants synthesize their own highly efficient biological “sunscreen.”

This sunscreen comes in the form of a group of molecules called sinapate esters which are found near the leaf surface. These chemicals have a dual role: they absorb photons of light for photosynthesis while also protecting against the damaging effects of UV. The most important of these compounds is sinapoyl malate, which provides the majority of the sun shield.

While researchers were aware of sinapoyl malate in plants, they didn’t know how it carried out its UV filtering role. To find out more, a team of scientists led by Purdue University’s Timothy Zwier began intensely probing the compound in the lab

In order to record the spectrum of UV light absorbed by sinapoyl malate, they began by using a laser to transition the molecule from a solid into a gas. Next, they brought the substance to close to absolute zero (-273.15oC or -459.67oF), which makes the spectrum easier to interpret. Absolute zero, or zero Kelvin, is the point where no more energy can be removed to a system. Finally, they used three different lasers to analyze the shapes the molecule adopted as it absorbed different wavelengths of UV-B light, which is known to damage both plant and human DNA.

They found that sinapoyl malate was remarkably efficient at absorbing UV light. In fact, its efficiency was one of the highest that can be achieved, absorbing all wavelengths of UV-B radiation with “no gaps in coverage,” according to Zwier. This broad spectrum, he says, is key to a good sunscreen, because it blocks all the harmful radiation.

“This is about the biggest absorption efficiency you can find in a molecule,” added Zwier. “So it does an excellent job capturing the UV-B light while letting other wavelengths of light necessary to plant life slip right through.”

While this molecule may be incredibly efficient at blocking harmful UV wavelengths, the researchers have no intentions to use it to develop human sunscreens. That’s because other natural substances, called cinnamates, that are already used in sun lotions are just as good. However, they could use this information to produce plants that are even more resistant to UV radiation. 

[Via Purdue University, JACS, New Scientist and Science Alert]

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