Around 700 million years ago, plants began to appear on land. One of the key adaptations needed for this transition was some way to protect against the Sun’s ultraviolet (UV) rays; plants in the sea had hitherto been shielded by the seawater.
Scientists have known since 2011 that plant cells have a protein called UVR8 that can detect shorter wavelength UVB rays, which is the type of UV radiation most responsible for sunburn. This protein signals the cells to begin producing compounds that block further UV damage and repair DNA damage.
In 2014, researchers at Purdue University in the US identified one of these protective compounds as ‘sinapoyl malate’ and found that this molecule harnesses quantum mechanical effects in order to absorb UVB rays. The ability to produce this natural sunscreen seems to be common to all land plants and algae, which suggests it is an ancient adaptation.
Plants aren’t invulnerable to the Sun, however. Prolonged exposure to the UVB in strong sunlight causes cell damage to the leaves and bark of many plants. This is worse when plants are dehydrated, because this limits their ability to move sunscreen chemicals to the worst-affected sites.
Ironically, there is a widely-held belief among gardeners that watering plants in the midday sunshine can cause sunburn, because droplets of water supposedly act as tiny lenses to focus the sunlight onto the leaf surface. However, this myth was debunked in 2011 by researchers at Eötvös University in Budapest, Hungary. They used computer modelling and direct experiment to show that the refractive index of water isn’t strong enough to focus sunlight from a water droplet onto the surface of a leaf.