Plants have more leaf surface area which is exposed to Sun and plants do need sunlight for Photosynthesis to produce food and oxygen. But how plants avoid getting sunburned ? (Sunburn for plants might mean like leaf getting dry or premature falling off etc). To explain this first we need to know the leaf structure and its mechanism to absorb light and heat.
(Image courtesy Complete Biology)
The major functionality of Waxy Cuticle on upper surface is(Leaf Terminology):
- Insulation - Acts as an insulator preventing more heat absorption by the leaf surface. The lower surface of the leaf has 'Stomata' with 'Guard cell' which keeps the leaf temperature moderate by preventing maximum water evaporation. The plants have well adapted to various environment conditions by having various compositions in its waxy cuticle. This has prevented plants from losing water and getting leaf dry.
- Light reflection - Surface of plant leaves also has hairy structures which will reflect the light and thus reducing the leaf surface from maximum light fall.
- As we know the diffusion and osmosis process in plants to balance the water content in the cell across its body, we can say that depending on environment and temperature plants have mechanism to deliver water to leaves.
To summarize plants have developeb mechanism to prevent heating up of leaf surface which will dry it out. Hence they have evolved and adapted themselves to various environment to avoid getting sunburned. Similar question was answered here Why do plants do not burn
On the other side if there is any change in the environmental condition due to global warming or Ozone hole effect then dangerous rays from the Sun could damage the leaves.