Plants need sunlight to live, but that doesn’t mean that more light always leads to healthier sprouts. They need to block out the Sun’s most harmful rays just like we do, or else their cells could get seriously damaged. Plants need sunlight for photosynthesis, where the incoming light is absorbed by a pigment called chlorophyll and is involved in chemical reactions that make glucose for energy.
But not all light is created equal. Different colors of light carry different amounts of energy: red light is less energetic than green light, which is less energetic than blue or violet light. Plants can use pretty much any color of visible light for photosynthesis, but chlorophyll barely absorbs green light. That’s why plant stems and leaves look green. And that’s just the light we can see.
Beyond violet is ultraviolet light, or UV. And UV has enough energy to break bonds in molecules like DNA. That’s why it causes sunburns in humans: Ultraviolet light can damage the DNA in your skin cells, and your immune system then kills those cells to try and prevent bigger problems like cancer. Your body makes a dark pigmented molecule called melanin that absorbs UV light to protect your skin from damage.
More sun exposure generally causes special skin cells to produce more melanin for more protection, which is what makes you tan. But UV light isn’t just harmful to humans; it can damage DNA or other organic molecules in lots of organisms, including plant cells. I mean, plants are just sitting there in the sun all the time! They are the shade… they can’t move into the shade.
Too much UV can even mess up photosynthesis, because it can damage structures that help capture energy from sunlight, like important proteins, chlorophyll, or photosynthetic genes. Just like humans make melanin to protect against the Sun’s rays, plants have ways of defending themselves. They make a variety of compounds called flavonoids that absorb UV.
Just like in humans, plants that haven’t built up enough of those chemicals before being exposed to lots of bright sunlight can end up with cell death and whitish burns on their leaves. It’s sometimes known as leaf scorch.
Some people even say that watering plants during the day makes them more susceptible to leaf scorch. The reasoning goes: the water drops act like little lenses that focus sunlight down to hot points all over the plant’s leaves. Like frying ants with a magnifying glass. But when scientists tested this out, they found that it just doesn’t work like that for most plants.
Water droplets vary in shape and size too much, and none of the drops bend light enough to focus it on the surface of the leaves they’re already sitting on. So even though plants might need sunlight more than us, they can still get a little toasted if they don’t have enough protection.