Leaf surface structures enable the endemic Namib desert grass Stipagrostis sabulicola to irrigate itself with fog water

Journal of The Royal Society Interface, Aug 2012

The Namib grass Stipagrostis sabulicola relies, to a large degree, upon fog for its water supply and is able to guide collected water towards the plant base. This directed irrigation of the plant base allows an efficient and rapid uptake of the fog water by the shallow roots. In this contribution, the mechanisms for this directed water flow are analysed. Stipagrostis sabulicola has a highly irregular surface. Advancing contact angle is 98° ± 5° and the receding angle is 56° ± 9°, with a mean of both values of approximately 77°. The surface is thus not hydrophobic, shows a substantial contact angle hysteresis and therefore, allows the development of pinned drops of a substantial size. The key factor for the water conduction is the presence of grooves within the leaf surface that run parallel to the long axis of the plant. These grooves provide a guided downslide of drops that have exceeded the maximum size for attachment. It also leads to a minimum of inefficient drop scattering around the plant. The combination of these surface traits together with the tall and upright stature of S. sabulicola contributes to a highly efficient natural fog-collecting system that enables this species to thrive in a hyperarid environment.

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Leaf surface structures enable the endemic Namib desert grass Stipagrostis sabulicola to irrigate itself with fog water

A. Roth-Nebelsick 2 M. Ebner 1 T. Miranda 1 V. Gottschalk 3 D. Voigt 0 S. Gorb 0 T. Stegmaier 3 J. Sarsour 3 M. Linke 3 W. Konrad 1 0 Zoological Institute, University of Kiel , Am Botanischen Garten 1-9, 24098 Kiel , Germany 1 Department of Geosciences, University of Tu bingen , Sigwartstrasse 10, 72076 Tu bingen , Germany 2 State Museum of Natural History , Rosenstein 1, 70191 Stuttgart , Germany 3 Institut fu r Textil- und Verfahrenstechnik Denkendorf , Ko rschtalstrae 26, 73770 Denkendorf , Germany The Namib grass Stipagrostis sabulicola relies, to a large degree, upon fog for its water supply and is able to guide collected water towards the plant base. This directed irrigation of the plant base allows an efficient and rapid uptake of the fog water by the shallow roots. In this contribution, the mechanisms for this directed water flow are analysed. Stipagrostis sabulicola has a highly irregular surface. Advancing contact angle is 988 + 58 and the receding angle is 568 + 98, with a mean of both values of approximately 778. The surface is thus not hydrophobic, shows a substantial contact angle hysteresis and therefore, allows the development of pinned drops of a substantial size. The key factor for the water conduction is the presence of grooves within the leaf surface that run parallel to the long axis of the plant. These grooves provide a guided downslide of drops that have exceeded the maximum size for attachment. It also leads to a minimum of inefficient drop scattering around the plant. The combination of these surface traits together with the tall and upright stature of S. sabulicola contributes to a highly efficient natural fog-collecting system that enables this species to thrive in a hyperarid environment. 1. INTRODUCTION Wetting properties are important for many biological processes as well as for technical applications. A wide variation of wetting effects can be obtained by combining surface chemistry and surface structures [1]. Plant surfaces represent a group of biological surfaces that recently have generated much interest the Lotus effect leading to highly water repellent leaves as an example [2,3]. The underlying benefits of wetting effects on plant surfaces are manifold ranging from preventing pathogens from settling (as proposed for many waterrepellent leaves), to ensuring the floating ability of aquatic plants [4] or to facilitating the catching of prey by carnivorous plants [5]. Wetting properties are also involved in the interactions between plants and dew or fog [6]. In many regions of the world, fog and dew represent regularly occurring phenomena, and the impact of these events on hydrology and ecology of the local vegetation is often substantial [7 9]. For example, fog drip (the shedding of droplets from leaves to the ground) and stem flow (the running-off of water from plant stems) may alter the local hydrological conditions [10 12]. In various arid regions, fog represents a valuable water source for the plants. This applies particularly for deserts with regular fog events, such as the hyperarid Namib Desert of South Western Africa [13]. Here, many organisms are adapted to use fog [14]. As fog collection has attained increasing interest during the last years as a sustainable water source in arid environments, a better understanding of the strategies that are employed by fog-harvesting organisms is expected to contribute to further improve the already existing technical fog collectors and fog-collecting strategies. There is evidence that Stipagrostis sabulicola, a grass species endemic to the sand dunes of the Namib, depends to a large degree on fog collection ([15,16] and citations therein). This species is able to extract substantial quantities of water from fog with collection rates of 4 5 mm3 mm22 of leaf surface and fog event as was shown by measurements in the field [15,16]. During these fog events, intense droplet formation can be observed on the plant which consists of stiff culms with heights of up to 2 m. The scattering of droplets is quite rare despite the sagging of the upper parts of the culms. It can be observed in the field that droplets cling to the plant, coalesce with other droplets and then, after reaching a maximum size, glide downwards towards the roots. The collected water is mostly conducted towards the culm base. It becomes available for the roots and a fog event can be optimally exploited [16]. This study investigates how S. sabulicola achieves directed water conduction and prevents dropletscattering during fog events. The results are compared with droplet behaviour on industrial fibres containing longitudinal ridges. 2. MATERIAL AND METHODS 2.1. Plant material All studies were conducted with fresh or dry plant material from its original habitat. Plant material was collected close to the Gobabeb Research Station, located within the Naukluft National Park, Namibia (http://www.gobabebtrc.org/). The surface exposed to the fog is preferentially that (...truncated)


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A. Roth-Nebelsick, M. Ebner, T. Miranda, V. Gottschalk, D. Voigt, S. Gorb, T. Stegmaier, J. Sarsour, M. Linke, W. Konrad. Leaf surface structures enable the endemic Namib desert grass Stipagrostis sabulicola to irrigate itself with fog water, Journal of The Royal Society Interface, 2012, pp. 1965-1974, 9/73, DOI: 10.1098/rsif.2011.0847