Integrating physiology, anatomy, and modelling to understand xylem embolism spreading in angiosperms based on gas diffusion kinetics

水在植物中的流动对我们生物圈和人类的功能至关重要，但大多数人会非常惊讶地听到植物中的水运输机制知之甚少。虽然有大量证据表明植物中存在张力水运输，但主要缺点之一是缺乏对为什么这种运输系统不会因吸入大气泡（栓塞）而不断失败的机械理解，这在干旱胁迫期间尤其会发生。一种新的“气动技术”，它测量在部分真空中的压力变化的动力学，同时从切开的木质部组织中提取气体，表明摩尔空气放电对应于基于其他方法的木质部的栓塞阻力。这一方法启发我们研究气体在木质部中的运动和木质部汁液中溶解的大气气体。通过结合气动和液压实验与解剖观察和建模，该建议将调查气体运输在不同程度的脱水下的被子植物木材的导水细胞。主要的问题，将被解决包括气体进入木质部血管，木质部汁液和栓塞形成的溶解气体浓度之间的潜在联系，以及与血管的多孔细胞壁之间的气体运动的解剖特征。我们预计气体扩散是ca。比质量流快200倍，因为容器间凹坑膜的多孔性质和大表面积，其代表水和气体流动的多孔介质。气体扩散的速率将取决于管间小坑膜的厚度和管间小坑膜面积的总量，但也取决于在被水力分段的位置处的管的非随机布置。此外，我们还推测，栓塞的形成并不完全是由压力驱动的，而是可能取决于木质部汁液中溶解气体的浓度，而温度的变化对溶解气体浓度的影响比压力的变化更大。并基于光学方法目视观察栓塞扩散。实验将辅以解剖观察，包括坑膜的透射电子显微镜，并在坑膜和血管水平的气体扩散模型的发展，这将是对所获得的实验数据进行测试。基于乌尔姆在功能木质部解剖学方面的长期专业知识，这个创新项目将有助于我们对植物木质部干旱诱导栓塞的理解。该项目的产出对我们理解植物水分利用和植物对干旱的反应具有影响，考虑到目前对气候变化的担忧，这一点尤其重要。

Water flow through plants has utmost importance for the functioning of our biosphere and the human population, but most people would be extremely surprised to hear that the mechanism of water transport in plants is poorly understood. While there is massive evidence for tensile water transport in plants, one of the major shortcomings is the lack of a mechanistic understanding why this transport system is not constantly failing by pulling in large air-bubbles (embolism), which would especially occur during drought stress. A new “pneumatic technique”, which measures the kinetics of pressure change in a partial vacuum, while extracting gas from a cut open xylem tissue, demonstrates that the molar air discharge corresponds to embolism resistance of xylem based on other methods. This method inspired us to investigate gas movement in xylem and dissolved atmospheric gas in xylem sap. By combining pneumatic and hydraulic experiments with anatomical observations and modelling, this proposal will investigate gas transport across water conducting cells of angiosperm wood under different levels of dehydration. Major questions that will be addressed include gas entry into xylem vessels, the potential link between dissolved gas concentrations of xylem sap and embolism formation, and the anatomical features associated with gas movement between porous cell walls of vessels. We expect that gas diffusion is ca. 200 times faster than mass flow because of the porous nature and large surface area of intervessel pit membranes, which represent porous media for water and gas flow. The rate of gas diffusion will depend on the thickness of intervessel pit membranes and the total amount of intervessel pit membrane area, but also on the non-random arrangement of vessels at places that are hydraulically segmented. Moreover, it is hypothesised that embolism formation is not exclusively pressure-driven, but may depend on the dissolved gas concentration of xylem sap, which is more affected by changes in temperature than pressure.Significant progress will be made in understanding gas transport within the three-dimensional vessel network of angiosperms by conducting pneumatic experiments, centrifuge-flow experiments, and visual observation of embolism spreading based on the optical method. The experiments will be complemented with anatomical observations, including transmission electron microscopy of pit membranes, and the development of a gas diffusion model at the pit membrane and vessel level, which will be tested against the experimental data obtained. Based on the long-standing expertise at Ulm on functional xylem anatomy, this innovative project will contribute to our understanding of drought-induced embolism in plant xylem. Output from this project has implications for our understanding of plant water use and plant responses to drought, which is especially relevant given current concerns about climate change.