Optics of sunlit water drops on leaves: conditions under which sunburn is possible
Article first published online: 8 JAN 2010
© The Authors (2010). Journal compilation © New Phytologist (2010)
Volume 185, Issue 4, pages 979–987, March 2010
How to Cite
Egri, Á., Horváth, Á., Kriska, G. and Horváth, G. (2010), Optics of sunlit water drops on leaves: conditions under which sunburn is possible. New Phytologist, 185: 979–987. doi: 10.1111/j.1469-8137.2009.03150.x
- Issue published online: 9 FEB 2010
- Article first published online: 8 JAN 2010
- Received: 19 September 2009, Accepted: 5 November 2009
- environmental optics;
- leaf burn;
- plant leaf;
- ray tracing;
- solar radiation;
- water drop
- •It is a widespread belief that plants must not be watered in the midday sunshine, because water drops adhering to leaves can cause leaf burn as a result of the intense focused sunlight. The problem of light focusing by water drops on plants has never been thoroughly investigated.
- •Here, we conducted both computational and experimental studies of this phyto-optical phenomenon in order to clarify the specific environmental conditions under which sunlit water drops can cause leaf burn.
- •We found that a spheroid drop at solar elevation angle θ ≈ 23°, corresponding to early morning or late afternoon, produces a maximum intensity of focused sunlight on the leaf outside the drop’s imprint. Our experiments demonstrated that sunlit glass spheres placed on horizontal smooth Acer platanoides (maple) leaves can cause serious leaf burn on sunny summer days.
- •By contrast, sunlit water drops, ranging from spheroid to flat lens-shaped, on horizontal hairless leaves of Ginkgo biloba and Acer platanoides did not cause burn damage. However, we showed that highly refractive spheroid water drops held ‘in focus’ by hydrophobic wax hairs on leaves of Salvinia natans (floating fern) can indeed cause sunburn because of the extremely high light intensity in the focal regions, and the loss of water cooling as a result of the lack of intimate contact between drops and the leaf tissue.