Getting a Grip on the Adhesion Mechanism of Epiphytic Orchids – Evidence From Histology and Cryo-Scanning Electron Microscopy

Abstract

Plants and animals evolve different attachment structures and strategies for reversible or permanent adhesion to different substrate types. For vascular epiphytes, having the ability to permanently attach to their host plants is essential for establishment and survival. Unlike mistletoe roots, roots of vascular epiphytes do not penetrate the host tissues but instead achieve attachment by growing in close contact to the surface of the substrate. However, the fundamental understanding of the attachment functions of epiphytic roots remains scarce, where majority of studies focused on the general root morphology, their functional properties and the descriptions of associated microbial endophytes. To date, research on attachment strategies in plants is almost entirely limited to climbers. Therefore, this study aims to fill the knowledge gap and elucidate the attachment functions of roots of epiphytic orchids. With the use of histology and high-resolution cryo-scanning electron microscopy (cryo-SEM) technique with freeze fracturing, the intimate root-bark substrate interface of epiphytic orchid Epidendrum nocturnum Jacq was investigated. Results showed a flattened underside of the root upon contact with the substrate surface, and the velamen layer appeared to behave like a soft foam, closely following the contours of the substrate. Root hairs emerged from the outermost velamen layer and entered into the crevices in the substrate, whenever possible. A layer of amorphous substance (glue-like substance) was observed on the surface of the root hairs. Combining the observations from this study and knowledge from previous studies, we hypothesised that epiphytic orchid roots produced a layer of glue-like substance to adhere the root to the substrate. Then root hairs are produced and enter into the voids and crevices of the substrate. This further generates a mechanical interlocking mechanism between root and substrate, thus reinforcing the attachment of the root (and hence the whole plant) to its substrate.

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

“…Our observation of a glue‐like substance is not the first for epiphytes. It has been described for epiphytic orchids (Tay et al, 2021) and ferns (Testo and Sundue, 2014). Such a substance has also been observed at the root–substrate interface in numerous attachment systems of climbers and seagrasses (e.g., Darwin, 1867; Groot et al, 2003; Melzer et al, 2011; Steinbrecher et al, 2011; Bohn et al, 2015; Zenone et al, 2020b).…”

“…The recent study by Tay et al (2021) found that on a relatively smooth bark substrate, the mechanics of attachment for epiphytic orchids occurs at the scale of the root hair–substrate interface where the actual contact area is defined (see fig. 6 of Tay et al, 2021). Therefore, the concept of rugosity in terms of how it is measured and how it relates to epiphyte abundance and richness should be discussed based on how attachment is defined in the first place.…”

“…For these so‐called bark epiphytes (Zotz, 2016), roots grow fully exposed on the surface of the substrate. These roots do not penetrate the host's tissues (unlike parasitic mistletoe roots), but instead achieve attachment to their host by maximizing the area of contact with the substrate (Tay et al, 2021), or by mechanical interlocking with the dead bark tissues. To resist dislodgement of the epiphyte from the host, these roots have to withstand a considerable amount of mechanical loading, including (1) the weight of the epiphyte (self‐loading), (2) in the case of tank bromeliads—additional impounded water and organic material in the tank (Brandt et al, 2017; Zotz et al, 2020), and (3) external forces such as wind and rain (Telewski, 1995; Tay et al, 2022).…”

“…When the root is jammed tightly into grooves of the bark, a large area of the root is in direct with the substrate, which could in turn strengthen the attachment of the plant to the tree. The recent study by Tay et al (2021) found that on a relatively smooth bark substrate, the mechanics of attachment for epiphytic orchids occurs at the scale of the root hair–substrate interface where the actual contact area is defined (see fig. 6 of Tay et al, 2021).…”

“…Orchids, aroids, and bromeliads have noticeable root hairs that emerge only from the side of the root that is in direct contact with the substrate (Dycus and Knudson, 1957; Benzing, 1970; Brighigna et al, 1990; Mathews et al, 1997; Stern and Judd, 1999; Stern, 2014; Ponert et al, 2016; Muthukumar and Kowsalya, 2017; Deseo et al, 2020). Root hairs of an epiphytic orchid were observed to enter microcrevices on the substrate surface, providing some degree of interlocking mechanism between the root and substrate (Tay et al, 2021), contributing to the overall anchoring strength of the plant to its host. However, the mechanical attachment of epiphytes to their hosts has received little scientific attention and usually discussed in relation to bark properties such as rugosity and substrate stability.…”