Compressive/Tensile Stresses and Lignified Cells as Resistance Components in Joints between Cladodes of Opuntia laevis (Cactaceae).

Abstract

The Cactaceae are a diverse group of plants with a wide variety of morphologies. Many species of Opuntia have segmented stems in which terminal cladodes may be separated from main-stem cladodes with varying amounts of resistance. From a geometric approach, derivations were used to calculate normal (axial and bending) and shear (transverse force and torque) stresses at joints due to the weight of the cladodes. Normal and shear stresses act perpendicular and parallel to (along) the cross sections of joints, respectively. Normal stress caused by bending was >10 times that of the mean value of any other stress. Analyses were performed to determine the relationship between maximum normal stress and the amount of lignified xylem cells. Such cells had thicker cell walls compared with the various other cells of stem joints that had thin cell walls and that thus would provide the most resistance to normal stresses. An analogy was made between cactus joints and a composite beam with reinforcing rods. In such joints, thin-walled parenchyma cells might be analogous to concrete that has little resistance to tensile stress, while the thick-walled, lignified xylem cells would be analogous to reinforcing rods. There were statistically significant relationships between normal stresses (from bending and axial loads) and mean percentage of lignified xylem cells (r=0.73) and between normal stresses and total areas of lignified xylem cells (r=0.65) (more stress, more reinforcing xylem cells). Tensile portions of cactus joints had 23% lignified xylem cells, while compressive portions had only 10% lignified xylem cells in joint areas (more tension, more reinforcing xylem cells). In addition, tensile joint tissues had two to three times more thick-walled, lignified xylem cells in the outer 30% of the radius compared with other joint tissues types (more reinforcing near the surface). To our knowledge, this is the first publication to present mechanical stresses at stem joints of cacti and the first to relate these stresses to characteristics of resisting tissues in the joints of a cactus.