PREFERENTIAL STATES OF LONGITUDINAL TENSION IN THE OUTER TISSUES OF TARAXACUM-OFFICINALE (ASTERACEAE) PEDUNCLES

Citation
Kj. Niklas et Dj. Paolillo, PREFERENTIAL STATES OF LONGITUDINAL TENSION IN THE OUTER TISSUES OF TARAXACUM-OFFICINALE (ASTERACEAE) PEDUNCLES, American journal of botany, 85(8), 1998, pp. 1068-1081
Citations number
33
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Plant Sciences
Journal title
ISSN journal
0002-9122
Volume
85
Issue
8
Year of publication
1998
Pages
1068 - 1081
Database
ISI
SICI code
0002-9122(1998)85:8<1068:PSOLTI>2.0.ZU;2-9
Abstract
We tested Wilhelm Hofmeister's hypothesis that the outer layers of her baceous stem tissues are held in a preferential state of longitudinal tension by more internal stem tissues that are held in a reciprocal sl ate of compression. We measured (1) the biaxial stiffness of dandelion peduncles that were barometrically inflated with a Scholander pressur e bomb. and (2) the stiffness and mechanical behavior of different lay ers of tissues that were surgically manipulated as longitudinal strips placed in uniaxial tension. Hofmeister's hypothesis predicts that ste ms will shorten and expand in girth as their volume transiently increa ses (due to barometric or hydrostatic inflation), that they will longi tudinally rupture when excessively inflated, and that the principal st iffening agents in their outer tissues will be aligned in the longitud inal direction with respect to stem length. Our experiments confirmed these predictions: (1) the longitudinal strains observed for inflated peduncles were negative and smaller than the circumferential strains s uch that stems contracted in length and expanded in girth, (2) peduncl es longitudinally ruptured when excessively inflated, (3) surgical exp eriments indicated that the epidermis was stiffer in longitudinal tens ion than any other immature peduncle tissue and was as stiff as any ot her tissue region in mature stems, and (4) microscopic analyses showed that the net orientation of cellulose microfibrils in the cell walls of the outer region of stem tissues was parallel to stem length. A str ong positive correlation existed between the tensile stiffness of tiss ues and the net orientation of cell wall microfibrils. A biomechanical model for the dandelion peduncle wall was constructed based on the th ermal behavior of a cross-ply unidirectional composite material whose adjacent plies contain orthogonally oriented stiffening agents. This m odel accurately predicted the magnitudes of the tensile stiffness of d ifferent tissue regions, the proportional thickness of these legions, and the formation of a bulge in the epidermis and a crease on the inne r surface of the stem wall when the innermost region of stem tissues w as transversely cut. For each of its variants possessing an outer Fly with longitudinally aligned stiffening agents (i.e., a 0 degrees ply) and containing at least one internal ply with transversely aligned sti ffening agents (i.e., a 90 degrees ply), the model predicted a state o f preferential longitudinal tension in the 0 degrees ply (or plies) an d of preferential longitudinal compression in the 90 degrees ply as po sited by Hofmeister's hypothesis.