Compressive/tensile stresses and lignified cells as resistance components in joints of Opuntia ficus-indica and Opuntia parryi var. parryi (Cactaceae)

to being separated from the plant. Bending stress (due solely to the weight of the branches) was the predominant stress (mean bending stresses were 167 and 383 kPa for 0. ficus-indica and 0. parryi var. parryi, respectively) between stem segments. Axial, shear, and torsion stresses were relatively low compared with bending stresses for both species. Data of 0. ficus-indica and 0. parryi var. parryi show that 23.6 and 25.3% of tensile portions of joints were composed of lignified xylem cells, respectively, while compressive portions of the same joints had only 10.8 and 14.7% lignified xylem cells, respectively. The relative radial positions of lignified xylem cells for compressive, tensile and lateral portions of joints for 0. ficus-indica and 0. parryi var. parryi were analyzed. In general, lignified xylem cells were closer to the external surface in tensile tissues than in compressive and lateral tissues of joints. Thus, lignified xylem cells are located in a position to provide a high level of resistance to bending. Maximum bending stresses were positively related with amounts of lignified xylem cells in joints for both species. For 0. ficus-indica the best-fit line was y = 1.48 x + 24.2 (P = 0.012) with an r2 = 0.76. For 0. parryi var. parryi, the best-fit line was y = 54.0 x - 11.7 (P = 0.01) with an r2 = 0.92. The 1.48 slope value for 0. ficus-indica was low compared with the 54.0 slope value for 0. parryi var. parryi. Large slope values for a species may reflect a greater ability of lignified xylem cells to resist stress. In five Opuntia species for which data are available, there was a strong negative relationship between slope values and joint diameter. However, high slope values occurred in species with more horizontal stem segments. Overall, these results show bending stress is the main stress between stem segments, lignified xylem cells in stem joints provide the main resistance to joint stresses, and slope values of stress versus amounts of lignified xylem cells may be related to plant morphology.