The effect of orchid leaf ornamentation on snail adhesion

Protective
structures in the epidermis are essential for land plants to defend themselves against
herbivores. Intriguingly, field studies show that some orchid species are less
prone to herbivory than other species. In this study, we investigated the
effect of leaf ornamentation of the terrestrial orchids Orchis mascula and Calanthe
triplicata and epiphytic orchids Dendrochilum
pallidiflavens and Trichotosia ferox on
consumption and attachment of herbivorous land snails, using histochemistry,
feeding and centrifuge experiments. Size and ornamentation of wax layers and
density and histochemistry of epicuticular hairs and exudates on the orchid
leaves were assessed with light microscopy, scanning electron microscopy and
transmission electron microscopy. The possible contribution of epicuticular
hairs to palatability was investigated by feeding snails untrimmed and trimmed
orchid leaves. Total forces needed to detach two differently shaped snail
species, Subulina octona and Pleurodonte isabella, were measured
using a turntable equipped with a synchronized strobe. Snails were placed in
two positions, either perpendicular or parallel to the main veins on the orchid
leaves and on the adaxial (=upper) or abaxial (=lower) side. The results
obtained provided three new insights. First of all, trimming of epicuticular
hairs increased the palatability of young leaves of Calanthe triplicate significantly. Secondly, a perpendicular or
parallel position of the snails to the main veins did significantly affect the
attachment performance of the smaller species tested, but only on the leaves of
Orchis mascula that are protected by
a wax layer. Thirdly, snails came off significantly faster on the leaf sides
covered high density of lignin filled epicuticular hairs. Our study highlights
the importance of histology in combination with attachment force and feeding
experiments for obtaining a better understanding of the defence mechanisms
employed by different species of epiphytic and terrestrial orchids to deter
herbivorous snails.