The dynamics of embolism formation and repair in xylem conduits: from bubble scale to loss in plant hydraulic transport capacity

对所有树种的水分利用和抗旱后死亡能力有一个清晰的认识是必要的。然而，尽管进行了数十年的研究，描述植物水分运动仍然是一项艰巨的科学挑战，在缺乏这一基本知识的情况下，森林的可持续管理将是不可能的。所有类型的植物，包括硬木和针叶树，在它们的茎中都有维管组织的核心束。这些循环维管束由称为木质部或木材的中空导管组成，它们专门用于有效地输送水分，同时最大限度地抵抗干旱（水力安全）。在炎热和干燥的日子里，当水需求增加时，植物中不能保证有足够的水输送。在这种情况下，木材中的水分变得非常不稳定，一旦气泡进入并扩散到这些中空细胞内，树木就会死亡。通过将建模活动与木材中空气进入和气泡形成相关的视觉和生理过程结合起来，将建立植物的水分运输和抗旱性。拟议项目的主要影响将是通过预测干旱后树木的死亡率，促进在水关系和维管植物的水力结构领域的发现，同时促进对下一代科学家和教育工作者的培训。这项研究的结果可以直接用于未来植被对气候响应的预测。这里的发现也将对决策者、植物育种者和土地管理者感兴趣，他们关注干旱对某些物种的生产力和分布的潜在影响。该项目的总体目标是从气泡尺度开始，将木本植物木质部在张力下栓塞形成和去除的动力学联系起来，并整合沿着单个管道产生的动力学，从而得出整个植物对空化曲线的脆弱性。具体目标是：1)测量和模拟空化诱导气相完全栓塞整个木质部导管所需的时间尺度；2)研究气泡形成从一个管道扩散到另一个管道的条件；3)考察张力作用下气泡吸收发生的可能性；4)确定栓塞形成与水力传导能力丧失之间的关系。为了实现项目目标，将解决一系列科学问题：研究问题1（气泡规模）：在单个木质部导管中栓塞形成和去除的机制和动力学是什么？在管道结构和木质部张力不同的情况下，气泡完全充满和排出管道所需的时间是多少？研究问题2（导管连通性）：栓塞填充导管所需的时间是否取决于导管的大小和端壁坑膜的刚性？研究问题3（扩展到整个植物）：单个导管中气泡形成的动力学能否解释整个器官对栓塞曲线的脆弱性？蒸腾植物中气泡形成和扩散的公式将代表着理论计算方法的重大进步，实验技术现在已经成熟，可以实现这一进展。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

A clear understanding of water usage and resistance to mortality following drought is necessary across tree species. However, despite decades of research, describing plant water movement remains a formidable scientific challenge, and in the absence of this fundamental knowledge, sustainable management of forests will be impossible. All type of plants, including hardwoods and coniferous trees, have core bundles of vascular tissue inside their stems. These circulatory vascular bundles are composed of hollow conduits known as xylem, or wood, that are specialized to transport water efficiently while allowing maximum resistance to drought (hydraulic safety). Sufficient water transport in plants is not guaranteed when water demand increases during hot and dry days. Under those conditions, water in wood becomes highly unstable and trees can die once an air bubble enters and spreads inside those hollow cells. By integrating modelling activities with visual and physiological processes related to air entry and air bubble formation in wood, plant water transport and resistance to drought will be established. The primary impacts of the proposed project will be to advance discovery in the field of water relations and hydraulic architecture of vascular plants by predicting tree mortality following drought, while promoting training for the next generation of scientists and educators. Results from this research can be directly used in future predictions of vegetation responses to climate. The findings here will also be of interest to policy makers, plant breeders and land managers concerned with potential drought impacts on productivity and distributions of certain species.The overall objective of this project is to link the dynamics of embolism formation and removal under tension in xylem of woody plants starting from the bubble scale and integrating the resulting dynamics along individual conduits to arrive at whole-plant vulnerability to cavitation curves. The specific objectives are to 1) measure and model the timescale required for the gas phase induced by cavitation to fully embolize an entire xylem conduit; 2) investigate the conditions under which bubble formation can spread from one conduit to another; 3) investigate the possibility of bubble resorption under tension to occur; and 4) determine the relation between embolism formation and loss of hydraulic conductivity. To achieve the project objectives, a series of science questions will be addressed: Research question 1 (bubble scale): what are the mechanisms and dynamics of embolism formation and removal in a single xylem conduit? What is the time required for a gas bubble to completely fill and to drain a conduit for varying conduit structures and xylem tensions? Research question 2 (conduit connectivity): Is the time needed for the embolism to fill a conduit dependent on conduit size, and the rigidity of the end-wall pit membrane? Research question 3 (Up-scaling to whole-plant): Do the dynamics of bubble formation in a single conduit explain the whole organ vulnerability to embolism curves. The formulation of bubble formation and spread in transpiring plants will represent a major step forward as the theoretical computational methods, and experimental techniques are now ripe for making this progress.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.