Structure-Function Trade-Offs in Xylem

植物从大气中吸收二氧化碳，每吸收一公斤干质量，就必须通过木质部输送200-300公斤的水。木质部运输必须是经济的，否则在如此低的碳水交换率下，工厂无法盈利。驱动力是自由的：细胞壁孔的毛细管吸力与木质部导管的负压梯度耦合。但负木质部压力会对传导效率造成损害，因为导管网络必须避免空化和内爆造成的功能障碍。效率的发展受到安全问题的限制。该研究将量化主要木质部类型之间的效率与安全性权衡，测试主要木质部性状功能意义的假设。第一个目标是量化通过导管间坑的流动阻力及其对限制总木质部阻力的重要性。坑是防止空气播撒产生空化的关键结构，但会限制水流并削弱墙壁。假设是高空气播种压力是以高坑阻力为代价的。第二个目标是确定空化的空气播种如何在不同的坑类型中发生，以及膜的强度如何限制坑的尺寸。假设坑的尺寸被最大化到膜的极限强度和空气播种发生在膜破裂的边缘。第三个目标是确定木质部负压力对木材细胞壁的机械应力程度。预测是水力应力很大，需要补偿加固，从而降低传导效率。该目标将通过实验和生物力学模型的结合来完成，这些模型将跨越主要的木质部类型和系统发育类群，包括蕨类植物、裸子植物和被子植物。该研究验证了许多长期以来关于木材结构和功能的假设。这些结果对于解释木质部的进化，评估水分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.