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)