Mind the Gap: Reach and Mechanical Diversity of Searcher Shoots in Climbing Plants

Abstract

Climbing plants need to reach supports and position their leaves for light capture. Vines and lianas develop a large diversity of self-supporting shoots among diverse species and different kinds of attachment. A searcher’s reach is a crucial trait for colonising supports in complex three-dimensional spaces. We explore the reach capacity and diversity of searcher shoots among representative temperate and tropical climbing plants. We investigate the overall range of variation between short- and long-reach searchers; the mechanical and anatomical organisations underlying reach capacities; how searcher architectures are linked to different climbing strategies such as stem twining, tendril climbing, root climbing, and branch-angle-hook climbing. We investigated reach and mechanical and anatomical organisations (stem rigidity and stiffness, stem and tissue geometry) in 29 climbing plant species from temperate and tropical habitats. Searchers show a wide range of maximal reach per species from 0.1 to 2.5 m. Flexural rigidity (EI) at the base of searchers increased with reach length; overall this increase was proportional although some longest-reaching shoots develop proportionally thinner searcher bases with higher stiffness [structural Young’s modulus (Estr)] than shorter-reach shoots. Bases of short-reach searchers rely more on primary tissues compared to long-reach shoots, which rely more on wood production. We identified different mechanical architectures for a given reach capacity across all species. These are linked to different kinds of attachment mechanisms, support foraging, and possibly leaf display. Plants attaching by twining of the main stem showed a wide range of reach capacity. They also developed lighter, more slender, less rigid, but generally relatively stiff (higher Estr) shoots compared with tendril climbers and branch-angle-hook climbers. Differences in the mechanical architecture of searcher shoots in climbing plants are informative for understanding how diverse climbing plant species explore and colonise different kinds of three-dimensional spaces. This is a key feature that distinguishes different habitat preferences. We discuss how such knowledge is not only important for understanding functional biology and ecology of climbing plants but is also of interest for developing new technologies in soft robotics that mimic climbing plants that can navigate through unstructured environments.

Trellis-forming stems of a tropical liana Condylocarpon guianense (Apocynaceae): A plant-made safety net constructed by simple “start-stop” development

“…This suggests a hard-wired, cutoff point where stiff juvenile wood development is stopped. It appears that the trellis-forming capacity across gaps can only be formed at a primary distance of c. 1.5 m represented by searcher maximal length—the approximate length of self-supporting searchers in this species ( Hattermann et al., 2022 ). There is no indeterminate or extended phase of support exploration as self-supporting stems.…”

“…Instead, there appears to be limit of three-dimensional (3D) space for which this species is adapted to exploit. In comparison with other searcher lengths ( Putz, 1984 ; Hattermann et al., 2022 ), different climbing plants have arguably undergone selection for specific habitats that include different distances between supports. In other words, at least for C. guianense , the plant does not invest materially in a self-supporting mode of growth for searching aerially further than that it was under selection for.…”

“…Trellises represent areas to sample where young to older shoots are abundant and where the growth and light conditions are similar. Previous studies have determined the overall changes in anatomy and stem mechanics during its life history ( Rowe and Speck, 1996 ); made comparisons of this species with other phylogenetically and morphologically diverse climbers ( Rowe et al., 2004 ); analyzed the chemical composition and ultrastructural organization of the early stiff and later compliant types of wood ( Hoffman et al., 2003 ) and most recently compared the length and reach of searcher stems with other diverse climbing plants ( Hattermann et al., 2022 ). An early study ( Rowe and Speck, 1996 ) demonstrated that anatomical and mechanical properties of young trellis shoots could vary.…”

Microspines in tropical climbing plants: a small-scale fix for life in an obstacle course

“…S7 ). Sled experiments were designed to apply a normal downwards force that approximated the ‘resting force’ for searcher shoots of each species at their maximum length or reach ( Hattermann et al , 2022 ). The results reported in this paper essentially represent the part of the stem that initiates initial contact to host supports and then twines around the host stem.…”

A growing soft robot with climbing plant–inspired adaptive behaviors for navigation in unstructured environments

“…Photoreceptors and gravity-sensing cells are distributed throughout the apical shoot and are involved in phototropic (33,34) and gravitropic responses (35,36), respectively. To locate, reach, and interact with obstacles and potential supports, climbing plants produce an initial self-supporting stem whose stiffness and density decrease adaptively after location and attachment to an external support (37,38). It has been suggested that support localization by plants is driven by attraction to shade (39)(40)(41).…”