Garden Mythbusters: Does Sunlight and Water Mixing Really Burn Leaves?
Two years ago, four Hungarian scientists published a paper called “Optics of sunlit water drops on leaves: conditions under which sunburn is possible” in the journal New Phytologist. Given the near-universal belief that water drops can scorch plant leaves on a sunny day (e.g. the RHS book How To Garden: “Under a hot midday sun, water droplets on leaves will act as miniature magnifying glasses and may scorch them”), you may be surprised — or you may not — that no one had previously checked to see if this actually happens.
First of all, the short answer is no.
Are there any circumstances under which water drops on leaves can cause sunburn? Yes, but only if the leaf has a dense covering of water-repellent hairs, in which case drops can be held above the leaf surface, allowing them to focus light on the surface itself.