A Biomechanical Approach to Xylem Structure and Function

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0112213 SperryA major constraint on plant productivity is the need to transport water to the foliage. This vital function is performed by the xylem tissue (wood) of the plant. The proposed research will quantify the design constraints of xylem in relation to water transport using state-of-the-art finite element methods developed for applications in fluid and solid mechanics. Advances in high performance computing make it feasible to apply these methods to the complex geometries of biological materials, including the conducting tubes (conduits) of the xylem. The result of the research will be quantitative links between the microscopic and macroscopic structure of xylem in relation to water transport and mechanical properties. Knowledge of the functional significance of wood's structural properties will have a number of broader implications ranging from the bioengineering of timber, to the interpretation of the climatic record of tree rings, and the analysis of evolutionary and ecological trends in xylem structure. The work will be interdisciplinary, involving graduate students and collaboration across three departments of the host institution (biology, math, and mechanical engineering). Hypotheses of structure-function linkages generated by computational analysis will be tested by direct experiment and with a comparative data set including xylem from different organs (root-stem-leaf), species from distantly related vascular plant groups (e.g., conifers vs. flowering plants), and plants of different growth types (vines vs. trees). The research has four specific objectives. The first objective is to quantify the relationship between conduit wall structure and reinforcement against the extremely negative water pressures required for xylem transport. The hypothesis is that conduit reinforcement will scale with operating pressure so as to minimize investment in wall material. This hypothesis is supported by preliminary work showing a strong proportionality between the ratio of conduit wall thickness to lumen diameter and a proxy of operating pressure across 12 species of conifers and 37 species of flowering plants. When scaled to the tissue level, this predicts that wood density must increase with more negative operating pressures, as was observed. The implication is that wood strength may be more related to conduit reinforcement than to support of the plant against gravity or wind loading. The present research will extend these results by using sophisticated finite element packages to analyze conduit and tissue stresses, allowing us to factor in the structure of the interconduit pits and surrounding tissue. The second objective is to quantify the mechanics of the valve function of interconduit pits and their role in causing transport failure by the air-seeding of cavitation (vaporizing of water under negative pressure). Preliminary work indicates that air seeding occurs at a constant wall stress across species. The hypothesis is that air seeding will also occur at an approximately constant pit membrane stress, meaning that cavitation resistance will be determined by pit membrane material properties, and especially by pit geometry. The third objective is to quantify the link between xylem structure and its conducting capacity. Here we will use advanced methods in fluid mechanics to solve the complex problem of flow through the irregular shapes of xylem conduits and their interconnecting pits. The fourth objective will bring together the individual structure-function relationships quantified in the first three objectives to evaluate the extent to which xylem design of diverse species maximizes conducting capacity while meeting constraints related to conduit support and prevention of cavitation.