The role of the epidermis as a stiffening agent in Tulipa (Liliaceae) stems.

We investigated the hypothesis that the epidermis is a tension-stressed "skin'' whose contribution to stem stiffness depends on the turgor pressure exerted on it by an hydrostatically inflated inner "core'' of tissues. This hypothesis was tested by relying on the intensities of bending stresses due to stem flexure, which must reach their maximum levels at the outer surface of epidermis such that damage to the surface of the stem should produce the most significant decrease in overall flexural stiffness. We discerned whether the principal tension supporting members at the stem surface (cellulosic microfibrils) were oriented parallel or normal to stem length by comparing the bending stiffness of stems before and after their surface cells first received three parallel longitudinal incisions followed by one helical incision, and by comparing the bending stiffness of stems for which the sequence of cuts was reversed. The same protocol was also applied to stems with various water potentials to determine the effect of hydrostatic pressure on stem stiffness contributed by the surface. Based on the behavior of 82 turgid Tulipa stems, parallel cuts reduced, on average, stem stiffness by 8%, whereas a subsequent helical incision further reduced stiffness by 42%. In contrast, an initial helical incision reduced stem stiffness by 50%, while three subsequent parallel cuts through the same stems did not significantly further reduce stiffness. These results suggested that the net orientation of cellulose microfibrils in the outer epidermal walls was parallel to stem length. This was confirmed by microscopic observations of cells with dichroic staining and polarized light. The responses to surgical damage were directly proportional to stem water potential. We thus conclude that the epidermis, probably in conjunction with a single layer of subepidermal collenchyma cells, acts as a tension-stiffening agent that can contribute as much as 50% to overall stem stiffness We present a simple mechanical model that can account for all our observations.

[1]  J. E. Gordon,et al.  Structures: Or Why Things Don't Fall Down , 1978 .

[2]  E. Stadelmann.  The Derivation of the Cell Wall Elasticity Function from the Cell Turgor Potential , 1984 .

[3]  K. Mühlethaler.  Plant Cell Walls , 1961 .

[4]  GROWTH PATTERNS OF PLANTS THAT MAXIMIZE VERTICAL GROWTH AND MINIMIZE INTERNAL STRESSES , 1982 .

[5]  R. E. Mark.  Cell Wall Mechanics of Tracheids , 1967 .

[6]  K. Thimann,et al.  DIFFERENTIAL GROWTH IN PLANT TISSUES , 1938 .

[7]  S. Timoshenko,et al.  Theory of elasticity , 1975 .

[8]  K. Niklas.  DEPENDENCY OF THE TENSILE MODULUS ON TRANSVERSE DIMENSIONS, WATER POTENTIAL, AND CELL NUMBER OF PITH PARENCHYMA , 1988 .

[9]  U. Kutschera,et al.  Differential effect of auxin on in vivo extensibility of cortical cylinder and epidermis in pea internodes. , 1987, Plant physiology.

[10]  J. Vincent.  The influence of water content on the stiffness and fracture properties of grass leaves , 1983 .

[11]  Ulrich Kutschera,et al.  Tissue stresses in growing plant organs , 1989 .

[12]  D. Penny,et al.  Tissue interactions in indoleacetic acid-induced rapid elongation of lupin hypocotyls , 1983 .

[13]  C. H. Hertz,et al.  On the Relation between Turgor Pressure and Tissue Rigidity. I. Experiments on Resonance Frequency and Tissue Rigidity , 1958 .

[14]  Stephen A. Wainwright,et al.  Mechanical Design in Organisms , 2020 .

[15]  D. B. Sellen,et al.  The Response of Mechanically Anisotropic Cylindrical Cells to Multiaxial Stress , 1983 .

[16]  S. Timoshenko.  Theory of Elastic Stability , 1936 .

[17]  Mechanical properties of the rice panicle. , 1982, Plant physiology.

[18]  P. A. Roelofsen.  The plant cell-wall , 1959 .

[19]  M. Probine.  Cell Growth and the Structure and Mechanical Properties of the Wall in Internodal Cells of Nitella opaca: III. SPIRAL GROWTH AND CELL WALL STRUCTURE , 1963 .

[20]  H. Burström,et al.  Growth, Turgor, Water Potential, and Young's Modulus in Pea Internodes , 1967 .

[21]  R. D. Preston,et al.  The physical biology of plant cell walls , 1975 .

[22]  Y. Masuda,et al.  Role of the epidermis in auxin-induced elongation of light-grown pea stem segments , 1971 .