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Variability and Extremes in Leaf Wettability and Run-Off Properties in Plants from Various Habitats

Abstract

The natural variability and extremes of surface wettability properties of leaves was investigated in 396 plant species out of 85 families growing in three different continents at various elevations (186-5268 m) and in all major ecological zones. This should yield the range of leaf surface wettablility properties but also allow detecting extreme hydrophobic and hydrophilic leaf surfaces. Surface wettability properties were measured by contact angle θ of defined water droplets applied to the leaves and their run off angle. Wettability of leaf surfaces differed significantly between species (16.7° <θ<169.7°). 72.6% of the species investigated showed extreme surface properties, being either highly wettable or extreme-hydrophilic (37.8 %) or highly non-wettable or extreme-hydrophobic (34.8%). Leaf surface wettability was in some families specific. Oxalidaceae and Fabaceae showed hydrophobic properties, Ericaceae and Moraceae were rather hydrophilic. Many families comprised all wettability classes. Sixteen leaf surfaces were extreme-hydrophilic (θ=27-40°) and with one exception found in the tropical to subtropical ecological zones. Extreme-hydrophilic properties were detetcted more frequently on glabrous and adaxial leaf surfaces. Twenty-four leaf surfaces were superhydrophobic (θ>150°), were from virtually all ecological zones and observed mostly abaxial and on pubescent leaves. On extreme-hydrophilic leaf surfaces water droplet run off occurred at a significantly lower mean angle of inclination (8°) as compared to super-hydrophobic (19°) leaf surfaces. Within each leaf wettability category, significant differences between species with respect to run off angle were found. Leaf surface wettability properties in the 396 species tested were highly divers. Extreme surface properties were only partly related to the ecological zone or a peculiar family and run off and leaf surface wettability turned out to be independent functional traits.

Biva Aryal and Gilbert Neuner

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