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Wood biomechanics and anatomy of PACHYCEREUS PRINGLEI¹

² Department of Plant Biology, Cornell University, Ithaca, New York 14853-5908 USA; and ³ Instituto de Ecologia UNAM, Apartado Postal 1354, Hermosillo, Sonora CP83000, Mexico

ABSTRACT

We report the longitudinal, biomechanical, and anatomical trends observed for tissue samples drawn from the parallel aligned, prismatic woody vascular bundles running the length of a Pachycereus pringlei plant measuring 5.22 m in height. The main vertical stem of this plant was cut into five segments (labeled A through E in the acropetal direction) measuring ~1.02 m in length. Four of the 14 vascular bundles in each segment were surgically removed to obtain 20 vascular bundle segments that were tested in bending to determine their stiffness measured in the radial E_R and tangential E_T direction. We also determined the lignin content of representative samples of wood.

A nonlinear trend in stiffness was observed: E_R and E_T were highest in segments B or C (1.67 GN/m and 1.09 GN/m, respectively), lower in segment A (E_R = 1.18 GN/m and E_T = 0.35 GN/m), and lowest in segment E (E_R = 0.03 GN/m and E_T = 0.20 GN/m). Similar longitudinal trends were seen for axial tissue volume fraction and fiber wall thickness, which achieved their highest values in segment B (69.8% and 6.59 µm, respectively). Wood stiffness also correlated significantly with cell wall lignin content: with respect to segment B (which had the highest lignin content, and was thus used as the standard reference for percent lignin content), lignin content, was 15, 60, 85, and 43% in segments E, D, C, and A, respectively. Fiber cell length, which increased toward the base of the stem and toward the vascular cambium in the most proximal vascular bundle segment, did not correlate with E_R or E_T.

Basic engineering principles were used to calculate stem stresses resulting from self-loading and any wind-induced bending moment (produced by drag forces). Calculations indicated that the less stiff wood produced in segment A eliminates a rapid and potentially dangerous increase in stresses that would otherwise occur in segments B or C. The less stiff wood in segment A also reduces the probability of shear failure at the cellular interface between the wood and surrounding tissues in this portion of the stem.

We conclude that P. pringlei wood stiffness is dependent on the volume fraction and lignification of axial tissues, less so on fiber wall thickness, and that wood development in this species is adaptively responsive to self-loading and differentially applied external mechanical forces

Key Words: biomechanics • Cactaceae • lignin • plant stems • Young's modulus • vascular tissues • wind drag • wood

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