Department of Biological Physics, Physical Institute, Eötvös University, Budapest, Hungary.
New Phytol. 2010 Mar;185(4):979-87. doi: 10.1111/j.1469-8137.2009.03150.x. Epub 2010 Jan 8.
*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 theta approximately 23 degrees, 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.
人们普遍认为，中午阳光强烈时不能给植物浇水，因为附着在叶子上的水滴会因阳光聚焦而导致叶子灼伤。然而，水滴对植物的聚光问题从未得到过彻底的研究。在这里，我们对这种植物光学现象进行了计算和实验研究，以澄清在特定的环境条件下，阳光照射下的水滴会导致叶子灼伤。我们发现，在太阳仰角约为 23 度的情况下（对应于清晨或傍晚），一个球形水滴会在叶子外部产生最大的阳光聚焦强度，超出了水滴印迹的范围。我们的实验表明，在阳光明媚的夏日，将阳光照射下的玻璃球放在水平光滑的槭树（枫树）叶片上会导致严重的叶片灼伤。相比之下，在水平无毛的银杏和槭树叶片上，阳光照射下的从球形到扁平透镜形的水滴不会造成灼伤损伤。然而，我们表明，由于在聚焦区域内的光强度极高，以及由于缺乏水滴与叶片组织之间的紧密接触而导致的水分冷却损失，在 Salvinia natans（满江红）叶片上由疏水性蜡毛“保持聚焦”的高折射球形水滴确实会导致晒伤。
