Biomechanics and anatomy of cladode junctions for two Opuntia (Cactaceae) species and their hybrid

Hybridization between the introduced arborescent Opuntia ficus-indica and t he native shrubby O. lirroralis has led to populations, referred to as O. " occidentalis," which form thickets that can dominate hillsides of chaparral and that can survive fires. Because the thickets apparently develop via ve getative reproduction, O. "occidentalis" was hypothesized to have a greater ability than its parent species to reproduce vegetatively due to weaker cl adode junctions. Of the three taxa, the junctions for O. "occidentalis" had the least amount of wood, despite having cladode masses and junction cross -sectional areas similar to those of O. littoralis. The cladodes of O. "occ identalis" resisted deflection about their junctions the least and their ju nctions required the least amount of applied mass and the smallest bending moment to fail mechanically. The junction wood for all three taxa consisted mostly of parenchyma, with lesser amounts of cells with thickened secondar y cell walls, indicating that some junction strength depended on hydrostati c pressure, especially for terminal junctions. Libriform fibers, which cont ribute to support and resist bending moments, were about 80% less frequent in the sub-subterminal junctions of O. "occidentalis" than in O. ficus-indi ca and O. littoralis. Vascular tracheids, which probably reduced shear amon g cells in the wood, were 90% less frequent in the terminal and sub-subterm inal junction wood of O. "occidentalis" compared to O. littoralis. Thus woo d characteristics can account for the weaker junctions of O. "occidentalis" compared to those of O. ficus-indica and O. littoralis, which apparently i ncreases the ability of the hybrid to reproduce vegetatively.