Session: 17-01-01 Research Posters

Paper Number: 72101

Start Time: Thursday, 02:25 PM

72101 - The Influence of Water Content on the Apparent Young's Modulus of Maize Stalk Tissues 

Plant stems often utilize a common structural architecture: a tough outer rind which is filled by a foam-like interior tissue known as pith. A similar architecture is found in human bone and the shaft of bird feathers. One unique aspect of plant stems is that the foam-like pith region is filled with living cells that are capable of regulating their internal pressure. When these cells are pressurized (turgid), the tissue is stiff and when the cells are flaccid, the tissue is limp. A common example is found in crispy carrots (turgid cells) vs. limp carrots (flaccid cells). In plants such as maize, the pressure of these cells may regulate the material properties of the stem. Indeed, tests performed in the field reveal that stalks exhibit noticably different behavior when tested in the early hours of morning (turgid cells) vs. in the mid-afternoon heat (flaccid cells). The purpose of this study was to quantify the contribution of water content on the material properties of maize tissues. To our knowledge, such a study has never before been undertaken.

Maize specimens were grown in the controlled environment of a greenhouse to full maturity. At maturity, stalk segments of approximately 6-8 cm in length were prepared for testing. Segments were chosen for their uniformity and lack of curvature. Segments were initially cut with sharp pruning shears several centimeters from the segment to be tested. These specimens were then cut to length using a diamond abrasive cutting blade. Specimens were then immersed in cool tap water. The osmotic pressure between the solute inside each cell and the tap water encouraged maximum turgor pressure. After equilibrating in tap water, segments were tested with a universal testing machine outfitted with two spherical platens to account for the angles of each end face. Compression testing progressed at a rate of 2mm/minute to a total displacement of approximately 0.5 mm, according to an established testing protocol. After testing, each end face was scanned using an optical scanner and the stalk segments were set on a rack when they slowly lost water to the atmosphere. Stalks were retested at intervals of 4, 8, and 24 hours. Specimens were weighed immediately prior to each test. At the end of the testing period, specimens were oven dried and the final weight was obtained. In addition to these tests, other specimens were dissected into rind-only and pith-only specimens. These specimens were tested in the same manner.

Results showed that the cellular pith tissue was composed primarily of water, and that water significantly stiffened by the presence of water, with an average reduction in tissue stiffness of approximately 25% attributable to water content. In contrast, the rind tissue contained much less water (by weight) and actually became more stiff as water content decreased. However, this increase was relativley minor (~5%). This study also provided the first known estimates of the ratio of pith to rind stiffness (~30:1).

This study was the first known instance in which water content was related to the mechanical tissue properties of maize stems, and the first to report the ratio of rind to pith stiffness. The results are in line with informal observations of overall stalk response, though the influence of water was found to be less significant than anticipated. Further analysis and testing will be needed to determine whether or not water content influences other mechanical properties such as tensile yield stress, ultimate compressional strength, etc. This study provides valuable initial data in support of multi-scale modeling, structural optimization, and other research activities.

Presenting Author: Brandon Sutherland Brigham Young University

Authors:

Brandon Sutherland Brigham Young University
Douglas Cook Brigham Young University