STEM BIOMECHANICS OF 3 COLUMNAR CACTI FROM THE SONORAN DESERT

Citation
F. Molinafreaner et al., STEM BIOMECHANICS OF 3 COLUMNAR CACTI FROM THE SONORAN DESERT, American journal of botany, 85(8), 1998, pp. 1082-1090
Citations number
34
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Plant Sciences
Journal title
ISSN journal
0002-9122
Volume
85
Issue
8
Year of publication
1998
Pages
1082 - 1090
Database
ISI
SICI code
0002-9122(1998)85:8<1082:SBO3CC>2.0.ZU;2-Z
Abstract
The allometric relationship of stem length L with respect to mean stem diameter D was determined for 80 shoots of each of three columnar cac tus species (Stenocereus thurberi. Lophocereus schottii, and S. gummos us) to determine whether this relationship accords with that predicted by each of three contending models purporting to describe the mechani cal architecture of vertical shoots (i.e., geometric, stress, and elas tic similitude, which predict L proportional to D-alpha with alpha = 1 /1, 1/2, and 2/3, respectively). In addition, anatomical, physical, an d biomechanical stem properties were measured to determine how the ste ms of these three species maintain their elastic stability as they inc rease in size. Reduced major axis regression of L with respect to D sh owed that alpha = 2.82 +/- 0.14 for S. thurberi, 2.32 +/- 0.19 for L. schottii, and 4.21 +/- 0.31 for S. gummosus. Thus, the scaling exponen ts for the allometry of L differed significantly from that predicted b y each of the three biomechanical models. In contrast, these exponents were similar to that for the allometry previously reported for saguar o. Analyses of biomechanical data derived from bending tests performed on 30 stems selected from each of the three species indicated that th e bulk stem tissue stiffness was roughly proportional to L-2, while st em flexural rigidity (i.e., the ability to resist a bending force) sca led roughly as L-3. Stem length was significantly and positively corre lated with the volume fraction of wood, while regression analysis of t he pooled data from the three species (i.e., 90 stems) indicated that bulk tissue stiffness scaled roughly as the 5/3-power of the volume fr action of wood in stems. These data were interpreted to indicate that wood served as the major stiffening agent in stems and that this tissu e accumulates at a sufficient rate to afford unusually high scaling ex ponents for stem length with respect to stem diameter (i.e., dispropor tionately large increments of stem length with respect to increments i n stem diameter). Nevertheless, the safety factor against the elastic failure of stems (computed on the basis of the critical buckling heigh t divided by actual stem length) decreased with increasing stem size f or each species, even though each species maintained an average safety factor equal to two. We speculate that the apparent upper limit to pl ant height calculated for each species may serve as a biomechanical me chanism for vegetative propagation and the establishment of dense plan t colonies by means of extreme stem flexure and ultimate breakage, esp ecially for S. gummosus.