Oil adsorption ability of three-dimensional epicuticular wax coverages in plants

Elena V Gorb,Philipp Hofmann,Alexander E Filippov,Stanislav N Gorb

doi:10.1038/srep45483

Elena V Gorb, Philipp Hofmann + Show 2 more

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https://doi.org/10.1038/srep45483

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Abstract

Primary aerial surfaces of terrestrial plants are very often covered with three-dimensional epicuticular waxes. Such wax coverages play an important role in insect-plant interactions. Wax blooms have been experimentally shown in numerous previous studies to be impeding locomotion and reducing attachment of insects. Among the mechanisms responsible for these effects, a possible adsorption of insect adhesive fluid by highly porous wax coverage has been proposed (adsorption hypothesis). Recently, a great decrease in insect attachment force on artificial adsorbing materials was revealed in a few studies. However, adsorption ability of plant wax blooms was still not tested. Using a cryo scanning electron microscopy approach and high-speed video recordings of fluid drops behavior, followed by numerical analysis of experimental data, we show here that the three-dimensional epicuticular wax coverage in the waxy zone of Nepenthes alata pitcher adsorbs oil: we detected changes in the base, height, and volume of the oil drops. The wax layer thickness, differing in samples with untreated two-layered wax coverage and treated one-layered wax, did not significantly affect the drop behavior. These results provide strong evidence that three-dimensional plant wax coverages due to their adsorption capability are in general anti-adhesive for insects, which rely on wet adhesion.

Highlights

We have previously proposed four hypotheses explaining the mechanisms of insect adhesion reduction on plant surfaces covered with three-dimensional epicuticular waxes[17]
It has been found that plants differ in their contaminating effects on insect pads: the contaminating ability depended on the micro-morphology of the wax projections and was related to both the largest dimension and the aspect ratio of the projections[27]
Our experiments with the water drops on the artificial smooth solid sample (SS) and two N. alata pitcher samples with either one (LL) or two wax layers (UL) demonstrated very similar behavior of the fluid, despite of some differences in the surface wettability between the samples

Summary

Introduction

We have previously proposed four hypotheses explaining the mechanisms of insect adhesion reduction on plant surfaces covered with three-dimensional epicuticular waxes17. (1) Wax projections create micro-roughness, which greatly decreases the real contact area between the plant surface and insect attachment organs called adhesive pads (roughness hypothesis). (2) Wax projections are detachable structures contaminating insect pads (contamination hypothesis). (3) Highly porous wax coverage may absorb the fluid secretion from the insect pad surface (fluid absorption hypothesis). (4) Insect pad secretion may dissolve epicuticular plant waxes (wax dissolving hypothesis). We have previously proposed four hypotheses explaining the mechanisms of insect adhesion reduction on plant surfaces covered with three-dimensional epicuticular waxes. As for the fluid adsorption hypothesis, only a very few relevant studies demonstrating the effect of the fluid reduction on the insect attachment to artificial surfaces have been performed. Since the comparison of the changes in contact angles of water and oil between solid and nano-porous surfaces indicated a strong adsorption ability of the nano-porous samples for both polar and non-polar fluids, it was concluded that due to their high porosity, the nano-porous samples can adsorb the fluid from insect pads, thereby reducing insect attachment. The oil was employed as a rough approximation of the oily pad secretion found previously in beetles e.g. refs 46–50

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