Compressive/Tensile Stresses and Lignified Cells as Resistance Components in Joints between Stem Segments of Opuntia fulgida and Opuntia versicolor (Cactaceae)

Abstract

Many species of Cactaceae, a diverse group of plants with a wide variety of morphologies and reproductive strategies, have segmented stems in which terminal stem segments may be separated from main stem segments with varying amounts of resistance. Terminal stem segments that are removed with little resistance may easily form adventitious roots to produce new plants asexually, whereas other species with high resistances to stem segment removal may predominately reproduce sexually. The purposes of this study were (1) to quantitatively determine the stresses at joints between stem segments of two species of Opuntia (Opuntia fulgida [jumping cholla] and Opuntia versicolor), (2) to determine whether lignified cells of the xylem cells in joints provide the major source of resistance to these stresses, and (3) to determine whether the resistances are related to whether species reproduction is correlated with resistance stresses. The overall concept of this study is that lignified xylem tissues are the main tissue of resistance to mechanical stresses (especially tensile stress) at stem segment joints since all other tissues have relatively thin‐walled cells. This condition is analogous to reinforcing rods in concrete. The computer‐aided design package, coupled with a geometric evaluation of relative positions of stem segments, provides adequate estimates of joint‐stress parameters among stem segments. Bending stresses at joints were more than 10 times greater than any other stresses. Expressing joint stress as a function of area of lignified xylem cells is a valid way of expressing the mechanics of joint integrity. Areas of lignified xylem cells were about two times greater for tensile tissues than they were for compressive tissues, and tensile joint portions exhibited a greater percentage of lignified xylem tissues closer to the external surface. Stresses at joints as a function of area of lignified xylem cells were about four times greater for O. fulgida than for O. versicolor, and the relatively high bending stress values ($$\mathrm{mean}\,=2955$$ kPa) at joints of O. fulgida coincide with its ability to have terminal stem segments removed easily and its ability to reproduce asexually by rooted stem segments. In contrast, lower bending stress values ($$\mathrm{mean}\,=1021$$ kPa) at joints of O. versicolor coincide with its greater ability to retain stem segments and its characteristic of reproducing predominately by seeds.