Structure-Function Trade-Offs in Xylem

植物从大气中吸收的每公斤干物质中，必须通过木质部移动200-300公斤的水。Xylem的运输必须是经济的，否则工厂无法在如此低的碳-水交换率下盈利。驱动力是免费的：在细胞壁孔毛细吸力耦合到木质部导管的负压梯度。但是，负木质部压力会对传导效率产生影响，因为导管网络必须安全，不会因空化而功能障碍，也不会因内爆而崩溃。效率的发展受到安全问题的限制。拟议中的研究将量化主要木质部类型的效率与安全权衡，测试主要木质部性状的功能意义的假设。第一个目标是量化通过导管间凹坑的流动阻力及其对限制总木质部阻力的重要性。凹坑是关键的结构，它通过空气播种防止空化，但会限制水流并削弱壁。假设是，高的空气播种压力是以牺牲高的坑阻力为代价的。 第二个目标是确定空泡的空气播种如何发生在不同的坑类型，以及膜的强度如何约束坑的尺寸。假设是，坑尺寸最大化的点，膜强度是有限的，空气播种发生在膜破裂的边缘。第三个目的是确定木材细胞壁中的机械应力的程度，从负木质部压力。预测是，液压应力是显着的，需要补偿钢筋，降低传导效率。这些目标将通过实验和生物力学建模的结合来实现，这些实验和生物力学建模跨越了主要的木质部类型和系统发育组，包括蕨类植物、裸子植物和被子植物。这项研究测试了许多长期存在的木材结构和功能的假设。这些结果对于解释木质部进化、评估水运输的成本和揭示增强竞争能力的机制以及实现基因组学将基因与木材功能联系起来的承诺是必要的。 这项研究将为各级学生创造研究机会。

Plants must move on the order of 200-300 kilograms of water through their xylem for every kilogram of dry mass they assimilate from atmospheric CO2. Xylem transport has to be economic or plants could not turn a profit with such a poor carbon-for-water exchange rate. The driving force is free: capillary suction at cell wall pores coupled to a negative pressure gradient in xylem conduits. But negative xylem pressure exacts a toll on conducting efficiency, because the conduit network must be safe from dysfunction from cavitation and from collapse by implosion. The evolution of efficiency is limited by safety concerns. The proposed research will quantify the efficiency vs. safety trade-off across major xylem types, testing hypotheses for the functional significance of major xylem traits. The first objective is to quantify the flow resistance through inter-conduit pits and its importance for limiting total xylem resistance. Pits are crucial structures that protect against cavitation by air-seeding but constrict water flow and weaken walls. The hypothesis is that a high air-seeding pressure comes at the expense of a high pit resistance. The second objective is to determine how air-seeding of cavitation occurs in different pit types, and how the strength of the membrane constrains pit dimensions. The hypothesis is that pit dimensions are maximized to the point where membrane strength is limiting and air-seeding occurs at the verge of membrane rupture. The third objective is to determine the extent of mechanical stress in wood cell walls from negative xylem pressure. The prediction is that hydraulic stresses are significant and require compensating reinforcement that reduces conducting efficiency. The objectives will be accomplished by a combination of experimentation and biomechanical modeling across major xylem types and phylogenetic groups including ferns, gymnosperms, and angiosperms. The research tests many long-standing hypotheses of wood structure and function. The results are necessary for interpreting xylem evolution, evaluating the cost of water transport and exposing mechanisms that enhance competitive ability, and fulfilling the promise of genomics for linking genes to wood function. The research will create research opportunities for students at all levels.