EAGER: A novel mechanism regulating leaf water transport: Reversible collapse of xylem conduits

本项目研究植物如何保护其维管系统免受过度蒸发负荷的损害。这项工作的意义源于这样一个事实，即光合作用依赖于植物从土壤中输送水分的能力。因此，了解植物如何保护其水分运输系统将有助于阐明农业和自然生态系统生产力的制约因素。一项新的发现表明，末端的输水容器位于树叶中，由非活细胞组成，其功能就像一个机械阀门：随着系统中压力的增加而关闭，随着系统放松而重新打开。这项研究的主要目标是确定终端管道的阀状行为是否可以防止系统中的应力达到可能对上游管道造成持久损害的水平。第二个目标是探索胁迫诱导的末端导管快速关闭和叶表皮气孔缓慢关闭之间的联系。这项研究的发现将有助于开发对干旱具有弹性的作物品种。该项目将加强对学生的研究、指导和培训，并提供植物生物学的基本见解，这些见解将被纳入教学并向公众传播。植物生态生理学的一个基本问题是植物如何保护自己免受气蚀的影响。与土壤干燥相关的对植物维管系统的挑战发生得很慢，因此气孔关闭、根萎缩和叶片脱落是调节木质部潜力的有效手段。相反，相对于气孔关闭的能力，蒸腾速率的漂移可能更快，从而使叶片暴露在潜在的破坏性水势之下。尤其是被子植物，气孔不会被动地跟踪叶水势，蒸腾作用的增加会导致最初的开放，而不是关闭。最近的一项发现表明，在最小的叶脉中，最小的叶脉与蒸发部位接触最密切，因此经历了最负的压力，管道变形(坍塌)而不是空化，形状的变化完全(且迅速)可逆。该项目的目标是研究(1)末端导管坍塌导致的导水率降低是否保护上游木质部免受引起空化的水势的影响，以及(2)坍塌如何促进蒸腾作用的气孔调节。这项研究为液体和气相运输的协调提供了一个新的视角，对植物生产力和干旱反应都有影响。更多地了解木质部机械特性的作用可能会为作物品种的表型鉴定和了解植物多样性提供新的目标。

This project investigates how plants protect their vascular system from damage due to excessive evaporative loads. The significance of the work arises from the fact that photosynthesis is dependent upon a plant's capacity to transport water from the soil. Thus, understanding how plants protect their water transport system will illuminate constraints on the productivity of agricultural and natural ecosystems. A new finding shows that the terminal water transporting vessels, which are located in leaves and consist of non-living cells, function like a mechanical valve: closing off as the stresses in the system increase and re-opening as the system relaxes. The principle goal of this research is to determine if the valve-like behavior of terminal conduits prevents the stresses in the system from reaching levels that could cause lasting damage to upstream conduits. A second objective is to explore linkages between the rapid stress-induced closure of terminal conduits and the slower closure of stomatal pores in the leaf epidermis. The findings of this research will contribute to the development of crop varieties that are resilient to drought. The project will enhance research mentoring and training of students, as well as provide fundamental insights in plant biology that will be incorporated into teaching and disseminated to the general public.A fundamental issue in plant ecophysiology is how plants protect themselves from cavitation. Challenges to the plant vascular system associated with soil drying occur slowly, such that stomatal closure, root shrinkage, and leaf shedding are effective means of regulating xylem potentials. In contrast, excursions in transpiration rate have the potential to be fast relative to the ability of stomata to close, and thus to expose leaves to potentially damaging water potentials. This is especially the case for angiosperms in which stomatal aperture does not passively track leaf water potential and increases in transpiration lead initially to opening, rather than closing. A recent discovery shows that in the smallest leaf veins, which are the ones in closest contact to the sites of evaporation and thus experience the most negative pressures, the conduits deform (collapse) rather than cavitate and the change in shape is completely (and rapidly) reversible. The goals of this project are to investigate (1) whether the reduction in hydraulic conductivity due to terminal conduit collapse protects upstream xylem from experiencing water potentials that cause cavitation and (2) how collapse contributes to stomatal regulation of transpiration. The study provides a new perspective on the coordination of liquid and vapor phase transport, with implications for both plant productivity and drought response. Greater understanding of the role of xylem mechanical properties may provide new targets for phenotyping crop varieties and understanding plant diversity.