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Watering plants midday triggers sunburn, research shows

Gardeners have long contended that the watering of plants and flowers at high noon only leads to sunburn. Now new research from a joint Hungarian-German study validates their position. Published in the journal New Phytologist, the results of the study into sunlit water droplet...

Gardeners have long contended that the watering of plants and flowers at high noon only leads to sunburn. Now new research from a joint Hungarian-German study validates their position. Published in the journal New Phytologist, the results of the study into sunlit water droplets sheds light on a question that has perplexed many scientists. Research investigating whether watering plants in the midday sun is detrimental to their health has been insufficient at best. Thanks to the work conducted by the researchers at Eötvös Loránd University in Budapest and the Max Planck Institute for Meteorology in Hamburg, we can perhaps get a better understanding of how forest fires are triggered, and could even raise awareness of the impact sunburns have on human health. 'The problem of light focusing by water droplets adhered to plants has never been thoroughly investigated, [either] theoretically [or] experimentally,' explained research leader Dr Gabor Horvath from the Department of Biological Physics at Eötvös Loránd University. 'However, this is far from a trivial question. The prevailing opinion is that forest fires can be sparked by intense sunlight focused on water drops on dried-out vegetation.' The researchers performed computational and experimental studies to figure out how the contact angle between the water droplet and a leaf affects the light environment on a leaf blade. What they discovered was that water droplets on a smooth surface (e.g. Ginkgo biloba leaves) do not trigger leaf burn. On the contrary, floating fern leaves that are rich in wax hairs are at much higher risk for leaf burn. The hairs hold the water droplets 'in focus' above the leaf's surface and act like a magnifying glass. 'In sunshine, water drops residing on smooth hairless plant leaves are unlikely to damage the leaf tissue,' Dr Horvath said. 'However, water drops held by plant hairs can indeed cause sunburn and the same phenomenon can occur when water droplets are held above human skin by body hair.' Despite the potential implications for forest fires, the researchers are cautious about whether the same process could effectively trigger forest fires. 'If the focal region of drops falls exactly on the dry plant surface, intensely focused sunlight could theoretically start a fire,' Dr Horvath pointed out. 'However, the likelihood is reduced as the water drops should evaporate before this, so these claims should be treated with a grain of salt.'

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Germany, Hungary