Structure-Function Trade-Offs in Xylem

植物从大气中吸收的每一公斤干物质，必须通过木质部大约200-300公斤的水分。木质部的运输必须是经济的，否则植物就不能在如此低的碳水交换率下盈利。驱动力是免费的：细胞壁孔的毛细吸力与木质部导管中的负压力梯度相耦合。但负木质部压力会影响传导效率，因为导管网络必须是安全的，不会因空化而出现功能障碍，也不会因内爆而崩溃。效率的演变受到安全问题的限制。这项拟议的研究将量化主要木质部类型的效率和安全性之间的权衡，测试主要木质部性状的功能意义的假说。第一个目标是量化流经导管间纹孔的流动阻力及其对限制木质部总阻力的重要性。坑是关键的结构，可以防止空气播种造成的气蚀，但会收缩水流并削弱壁面。假设是，高播种压力是以牺牲高坑阻力为代价的。第二个目标是确定空化的空气播撒如何在不同类型的坑中发生，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强度有限的程度，并且在膜破裂的边缘进行空气播种。第三个目标是确定木质部负压力对木材细胞壁机械应力的影响程度。据预测，水力应力很大，需要补偿加固，从而降低了传导效率。这些目标将通过包括蕨类、裸子植物和被子植物在内的主要木质部类型和系统发育类群的实验和生物力学建模的组合来实现。这项研究检验了许多关于木材结构和功能的长期假设。这些结果对于解释木质部进化，评估水分运输的成本和揭示提高竞争能力的机制，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。这项研究将为各个层次的学生创造研究机会。

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.