Frictional properties of flower stems in the plant Hippeastrum reginae (Amaryllidaceae)
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T.C. BIOLOGICAL AND BIOMIMETIC MATERIALS
Frictional properties of flower stems in the plant Hippeastrum reginae (Amaryllidaceae) Elena V. Gorb1 · Stanislav N. Gorb1 Received: 19 May 2020 / Accepted: 12 June 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Plant flower stems covered with three-dimensional (3D) wax prevent access of ants that rob flower nectar and do not contribute to the plant cross pollination. This phenomenon, called greasy pole syndrome, is caused by the ant’s failure to gain a foothold on slippery waxy stems due to dislodged wax particles. The aim of this study was to analyze frictional properties of 3D wax projections specific to flower stems in Hippeastrum reginae by means of a microtribometer. Using the cryo scanning electron microscopy approach, we obtained images of shock-frozen plant surfaces with intact and smeared wax layers. Compared to wax-free stem samples, intact waxy stems showed significantly reduced frictional coefficient. The obtained results allowed us to provide some plausible explanations about the mechanism of the slipperiness of the 3D wax coverage on plant surfaces involved in the greasy pole syndrome. It is most probably based on easy delamination of the layered structure of single wax projections and on finally building of smearing surfaces with presumably lower intrinsic friction between individual layers. This knowledge can potentially contribute to the development of future physical control barriers based on dry lubricants with application in pest control. Keywords Attachment · Delamination · Frictional coefficient · Greasy pole syndrome · Slippery surface · Wax projections
1 Introduction Plant surface micro- and nanostructures play an important role in interactions between plants and a variety of environmental factors [1–3]. It is well known that plant surfaces can mediate insect attachment depending on the presence of trichomes, cuticular folds, three-dimensional (3D) epicuticular waxes or fluid coverage (see review by Gorb and Gorb [4]). Especially 3D epicuticular waxes have been repeatedly shown to reduce the attachment ability and impede locomotion of insects (reviewed in Ref. [5]) by making plant cuticles slippery. The protective function of 3D epicuticular waxes coverage against insects has been experimentally supported mainly for plant leaves in representatives of various plant families. However, waxy stems (especially * Elena V. Gorb [email protected]‑kiel.de Stanislav N. Gorb [email protected]‑kiel.de 1
Department of Functional Morphology and Biomechanics, Zoological Institute, Kiel University, Am Botanischen Garten 9, 24118 Kiel, Germany
flower stems) have their protective function mainly against ants robbing flower nectar and not contributing to the plant cross pollination. This phenomenon is called greasy pole syndrome (GPS) [6–10]. Based on field observations and experiments, it has been previously demonstrated that ants failed to gain a foothold on waxy stems due to dislodged wax crystals and, therefore, fell to th
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