Mechanical properties within the growth zone of corn roots investigated by bending experiments. II: Distributions of modulus and compliance in bending

A bending technique was used to infer the spatial distributions of rheological properties within the growth zone of the root of corn, Zea mays. "Bending modulus" (ratio of stress to strain, calculated from engineering theory of bending) falls from 20 MPa near the root tip (3 mm from the tip) to 6 MPa at the location 6 mm from the tip and then remains uniform through the basal region of the growth zone. Where growth stops, at 11-12 mm, there is a sharp rise in bending modulus. The profile of bending moduli is not changed by root incubation temperature during the growth period prior to bending, but it is shifted to the left in roots growing more slowly than the average at either of two temperatures (19 and 29 C). The spatial distribution of "compliance" (reciprocal of bending modulus and a measure of tissue extensibility) resembles the distribution of swelling in response to osmotic perturbation. The distribution of compliance does not parallel that of growth rate. Attempts to explain the discrepancy between compliance and growth rate lead us to examine the theoretical basis for the calculations and to suggest that the dependence of compliance on rate of stretching is physiologically important. PLANT ORGANS grow as their cell walls extend under an internal hydrostatic pressure. The mechanical properties of cell walls have been the subject of much study because of their important role in growth (reviewed in Cleland, 1971; Taiz, 1984; Cosgrove, 1986). Over the past 60 years several techniques have been used to analyze wall properties. In early investigations, deflections of plant tissue segments hung with weights characterized tissue extensibility (Heyn, 1931). In later work, an instron stressstrain analyzer permitted examination of forcedisplacement-time relationships and resulted in a characterization of the extensibility of plant walls under various environmental conditions (Cleland, 1967; Haughton and Sellen, 1969; Masuda et al., 1974). These studies clarified the effects of hormones on plant growth mechanics. In recent review articles, Silk (1984) and Cosgrove (1986) emphasized the importance of refining the earlier studies to include a consideration of the distribution of extensibility within the growth zone. In most organs, growth is distributed over less than a centimeter oftissue; and an understanding ofgrowth I Received for publication 13 May 1987; revision accepted 17 September 1987. Supported in part by NSF grants DCB-841704 and RII8503652. 2 Current address: Department of Information and Computer Science, University of California, Irvine, California 92717. 3Send reprint requests to this address. dynamics requires a knowledge of tissue extensibility on a spatial scale of millimeters. A newer approach, the pressure probe, shows promise for resolving the spatial distribution of wall extensibility within the plant organ (Cosgrove, 1985; Van Volkenburgh and Cleland, 1986). Here we report the results of a complementary technique, based on the physics of beam bending, to estimate the intraorgan distribution of mechanical properties. We evaluate the spatial distribution of local curvature which develops in response to an imposed bending moment and use the engineering theory of bending to infer the distribution of mechanical properties which might be related to growth. MATERIALS AND METHODSExperimental conditions for growing seedlings of corn (Zea mays crow single cross hybrid WF9 x HO 17) and the apparatus for bending roots were described in a companion paper (Silk and Beusmans, 1988). "Swell and shrink" experiments were conducted to verify the distribution of compliance computed from the bending experiments. Root tips were excised and marked with a pen at approximately millimeter increments and were placed in distilled water for two minutes, photographed, placed in -10 bar mannitol solution for two minutes and rephotographed. Root images were enlarged 20-fold, and percent length change was calculated from digitized photographs of the marked roots.