A Biomechanical Approach to Xylem Structure and Function

木质部结构和功能的生物力学方法

• 批准号: 0112213
• 负责人: John Sperry
• 金额: $ 23.5万
• 依托单位: University of Utah
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2001
• 资助国家: 美国
• 起止时间: 2001-08-01 至 2004-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0112213&HistoricalAwards=false
• 关键词: Biomechanical; Approach; Xylem; Structure; Function

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.

植物生产力的一个主要限制是需要将水输送到叶子。这种重要的功能是由植物的木质部组织（木材）执行的。拟议中的研究将量化木质部的设计限制，在水的运输使用国家的最先进的有限元方法开发的应用程序在流体和固体力学。高性能计算的进步使得将这些方法应用于生物材料的复杂几何形状，包括木质部的导管（导管）成为可能。研究的结果将是木质部的微观和宏观结构之间的定量联系，与水分运输和力学性能。木材结构特性的功能意义的知识将有一些更广泛的影响，从木材的生物工程，树木年轮的气候记录的解释，并在木质部结构的进化和生态趋势的分析。这项工作将是跨学科的，涉及研究生和主办机构的三个部门（生物，数学和机械工程）的合作。通过计算分析产生的结构-功能联系的假设将通过直接实验和比较数据集进行测试，所述比较数据集包括来自不同器官（根-茎-叶）的木质部、来自远缘维管植物组的物种（例如，针叶树与开花植物），以及不同生长类型的植物（藤蔓植物与树木）。 研究有四个具体目标。第一个目标是量化导管壁结构和增强对木质部运输所需的极负水压之间的关系。假设是，管道加固将与操作压力成比例，以最大限度地减少对壁材料的投资。这一假设是支持的初步工作显示导管壁厚管腔直径的比例和代理的操作压力在12种针叶树和37种开花植物之间的强比例。当缩放到组织水平时，这预测木材密度必须随着更负的操作压力而增加，正如所观察到的那样。这意味着木材强度可能更多地与管道加固有关，而不是与植物抵抗重力或风荷载的支撑有关。本研究将通过使用复杂的有限元软件包来分析导管和组织应力，从而扩展这些结果，使我们能够考虑导管间凹坑和周围组织的结构。第二个目标是量化的力学的阀门功能的管道间坑和它们的作用，造成运输故障的空泡（水在负压下蒸发）的空气播种。初步工作表明，空气播种发生在一个恒定的壁应力跨物种。该假设是，空气播种也将发生在一个近似恒定的坑膜应力，这意味着抗气蚀性将由坑膜材料的性能，特别是坑的几何形状。第三个目标是量化木质部结构与其导电能力之间的联系。在这里，我们将使用先进的流体力学方法来解决复杂的问题，通过不规则形状的木质部导管和它们的相互连接的坑的流动。第四个目标将把前三个目标中量化的个体结构-功能关系结合在一起，以评估不同物种的木质部设计在多大程度上最大化传导能力，同时满足与管道支持和防止空化有关的限制。