Cladode junction regions and their biomechanics for arborescent platyopuntias

The three terminal cladode junction regions along branches of four arborescent platyopuntias-Nopalea cochenillifera, Opuntia ficus-indica, Opuntia robusta, and Opuntia undulata-were investigated for their reaction wood traits, flexural stiffness, and elastic moduli. The junctions had suberized tissue, phloem, wood, and pith and were surrounded by a collar composed of epidermis, hypodermis, and chlorenchyma. Gelatinous fibers, which were often associated with lignified axial parenchyma and vessels, occurred only in the wood of the two older junction positions for the three species with the most massive cladodes (the Opuntia spp.). Gelatinous fibers were more frequent in lateral regions, indicating that they are formed in response to large static loads and that they limit lateral movement of the branches. Axial parenchyma cells in regions under tension often stained lightly and had irregularly thickened walls. Pith eccentricity, which resulted from a greater wood extent in compressive than in tensile regions, occurred for all three positions for all four species, except for the youngest junctions of O. robusta and O. undulata. Greater wood extent in compressive regions may be because the wood is primarily made up of parenchyma, which resists compressive forces better than tensile forces. Flexural stiffness of junctions did not differ between bending parallel to the cladode face toward compressive regions versus bending perpendicular to the cladode face toward lateral regions but increased with increases in junction age, reflecting increases in junction cross-sectional area. Collars significantly resisted compressive stresses, accounting for 34% of junction flexural stiffness. Junction region elastic moduli varied among species but did not increase with junction age. Thus, the biomechanics for the cladode junctions of arborescent platyopuntias are affected mostly by the wood cross-sectional area and the resistance provided by the collar.