Holding on or falling off: The attachment mechanism of epiphytic Anthurium obtusum changes with substrate roughness

Abstract

Premise: For vascular epiphytes, secure attachment to their hosts is vital for survival. Yet studies detailing the adhesion mechanism of epiphytes to their substrate are scarce. Examination of the root hair-substrate interface is essential to understand the attachment mechanism of epiphytes to their substrate. This study also investigated how substrate microroughness relates to the root-substrate attachment strength and the underlying mechanism(s). Methods: Seeds of Anthurium obtusum were germinated, and seedlings were transferred onto substrates made of epoxy resin with different defined roughness. After 2 months of growth, roots that adhered to the resin tiles were subjected to anchorage tests, and root hair morphology at different roughness levels was analyzed using light and cryo scanning electron microscopy. Results: The highest maximum peeling force was recorded on the smooth surface (glass replica, 0 µm). Maximum peeling force was significantly higher on fine roughness (0, 0.3, 12 µm) than on coarse (162 µm). Root hair morphology varied according to the roughness of the substrate. On smoother surfaces, root hairs were flattened to achieve large surface contact with the substrate. Attachment was mainly by adhesion with the presence of a glue-like substance. On coarser surfaces, root hairs were tubular and conformed to spaces between the asperities on the surface. Attachment was mainly via mechanical interlocking of root hairs and substrate. Conclusions: This study demonstrates for the first time that the attachment mechanism of epiphytes varies depending on substrate microtopography, which is important for understanding epiphyte attachment on natural substrates varying in roughness.

Smoothing out the misconceptions of the role of bark roughness in vascular epiphyte attachment

“…It is assumed that these plants secrete the adhesive substance to establish a first bond upon initial contact between root and substrate. The specific attachment mechanism of the aroid seedling varied with roughness, shifting from an adhesive mechanism on a smooth surface to an interlocking mechanism as substrate topography became increasingly coarser (Tay et al ., 2022). Therefore, on natural bark surfaces, which are unlikely to have a homogenous topography, attachment of a seedling to the bark surfaces is probably achieved by a combination of strategies.…”

“…Even without additional mechanical disturbances, root attachment has to be strong enough to support the plant's own weight, withstand gravitational pull and, in the case of tank bromeliads, the added weight of impounded water and organic matter. While there are some quantitative measurements of root‐substrate attachment strength from Anthurium obtusum seedlings (Tay et al ., 2022), it is unknown how attachment strength and gravitational pull scale with plant size. Nevertheless, one can assume that mature plants also produce more and thicker roots, which might have different biomechanical properties compared with the young fresh roots assessed in that study.…”

“…0.5 μm (Tay et al ., 2021, 2022), allowing them to readily flatten against the substrate, thus replicating the microcontours of the surface (see fig. 4 in Tay et al ., 2022), and to grow between the layers of bark substrate and crevices, producing an additional interlocking mechanism that promotes attachment. Unlike ground‐rooted terrestrial plants, in which root hairs are ephemeral and are typically confined to the area directly behind the zone of root elongation (Galway, 2000; Grierson & Schiefelbein, 2002), root hairs of epiphytes persist beyond the distal part of the root apex.…”

“…Although a defining characteristic of an epiphyte is its structural dependence on a host, the literature detailing the fundamental mechanism of how the plant attaches to its host remains scarce. To date, research on attachment strategies in plants is mainly limited to climbers (Isnard & Silk, 2009), with only two studies that explicitly examined and provided a mechanistic understanding of the attachment mechanism of vascular epiphytes – a study with an epiphytic orchid (Tay et al ., 2021) and another with an aroid (Tay et al ., 2022). Nevertheless, before these recent studies, numerous publications touched on the topic of attachment of epiphytes to particular host trees, and often in conjunction with bark traits, that is, bark stability and bark rugosity (Callaway et al ., 2002; Vergara‐Torres et al ., 2010; Chomba, 2014; Wagner et al ., 2015).…”

Phorophyte size and soil profiles differentially correlate with community structure among hemiepiphytes and nomadic vines

“…Different life history strategies should indeed result in differential phorophyte and microsite preferences; however, such a prediction has rarely been tested, especially in tropical forests (Zotz, 2016). In summary, hemiepiphyte germination success is putatively influenced by phorophyte plant characteristics such as phorophyte bark quality, micro‐architectural features (e.g., knotholes and bifurcations) (Laman, 1995; Tay et al, 2022; Zotz, Almeda, et al, 2021), and that of nomadic vine is limited by various factors associated with the understory such as solar radiation, leaf litter, and soil quality (Cockle, 2001). In light of these demographic contrasts, we predict that nomadic vines' occupancy is weakly related to phorophyte size (Orihuela & Waechter, 2010), and hemiepiphytes, due to their above‐ground germination, should be favored on larger phorophytes which support conditions for the establishment of above‐ground plants (Wagner et al, 2015).…”