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

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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.