The role of gas movement between xylem conduits in embolism propagation based on modelling and flow-centrifuge experiments

Chapter 1: The review ‘Gas Diffusion Kinetics in Relation to Embolism Formation and Propagation in Angiosperm Xylem: A Mini-Review’ provides an in-depth analysis of the development of models and methods addressing the relationship between gas diffusion and embolism formation in xylem. Previous studies often presented model equations, assumptions, and simplifications in supplementary materials, which, while focusing on the biological significance, may obscure critical information. This paper aimed to clarify these aspects by summarizing the current understanding of gas diffusion kinetics' functional importance in embolism formation and propagation, drawing on experimental and modelling evidence. The development of new methods and models for investigating gas movement through xylem tissue has enabled a comprehensive exploration of the relationship between gas diffusion and embolism propagation in angiosperms. Several studies have suggested that diffusion drives an increase in gas concentration within a recently embolized vessel. This increase in gas concentration is experimentally shown to be closely associated with embolism formation, as the recently embolized vessel may become a source that increases the likelihood of embolism formation in interconnected neighbouring conduits. Given that gas diffusion is influenced by anatomical features such as pit membrane thickness, total intervessel pit membrane area in a vessel, and vessel dimensions, several studies have used these parameters to model radial and axial gas diffusion in various species and attempted to correlate them with embolism resistance. While radial gas diffusion could be considered the primary determinant for embolism initiation as it introduces additional air into the system, axial gas diffusion does not appear to correlate directly with embolism formation. Modelling the mechanistic process of embolism formation has proven to be challenging due to the current lack of a fully comprehensive understanding of this phenomenon. Considering that gas movement may be linked to embolism resistance, modelling gas movement through xylem tissue may be a feasible alternative to investigate the mechanisms of embolism formation. However, some simplifications in these models, such as considering vessels as cylindrical structures connected at their ends, can overestimate the rate of gas diffusion by misrepresenting the actual pathways and overestimating the distances for gas movement. These limitations in the models call for further investigation to provide more accurate insights into embolism dynamics.

Chapter 2: The paper ‘The potential link between gas diffusion and embolism spread in angiosperm xylem: evidence from flow-centrifuge experiments and modelling’ delves into the fundamental mechanisms underlying embolism propagation in angiosperm xylem. While it is generally accepted that embolism propagation is primarily a pressure-driven process—whereby more negative xylem sap pressures increase the likelihood of embolism—there is emerging evidence suggesting that embolism formation may also depend on the proximity to a gas source and local changes in gas pressure and concentration. If this association holds true, embolism propagation may be dynamic because gas diffusion in xylem can be relatively slow, from seconds to many hours. To explore this hypothesis, the study employs a combination of gas diffusion modelling and a hydraulic apparatus method for locating embolisms, integrated with the flow-centrifuge technique. The flow-centrifuge offers the advantages of precise control over temperature and water potential (induced by rotational speed) while providing highly accurate measurements of hydraulic conductivity (Kh). The study compares temporal changes in Kh at different temperatures (5, 22, and 35°C) over one hour of spinning, and under various pressures, to modelled gas concentration changes in a recently embolized vessel at the centre of a centrifuge sample under different conditions. The findings reveal that changes in Kh are logarithmic and species-specific, increasing at low centrifugal speeds (< 3250 RPM) and decreasing at higher speeds. The nature of the unexpected increase in Kh can only be speculated, possibly as a result of the rearrangement and/or dissolution of embolism. On the other hand, the observed reductions in Kh are experimentally linked to a temporal rise in embolism at the sample's centre, likely due to increasing gas concentrations in recently embolized vessels. The transition of a water-vapoured filled conduit to a conduit filled with gas at atmospheric pressure increases the likelihood of embolism formation in interconnected neighbouring conduits, thus reducing hydraulic conductivity over time even when water potential remains constant. This study challenges the traditional view that embolism is solely pressure-driven, as indicated by the x-axis of vulnerability curves. It suggests that embolism propagation is a dynamic process that requires time to develop as gas diffuses into embolized vessels. The results highlight the complex interplay of pressure, temperature, and vessel traits in influencing embolism spread, requiring a re-evaluation of current models and assumptions regarding xylem function under stress conditions.

Chapter 3: The paper ‘Embolism propagation does not rely on pressure only: time-based shifts in xylem vulnerability curves of angiosperms determine the accuracy of the flow-centrifuge method’ explores the practical implications of the spin-time effect on measurements of embolism resistance, which is detailed in the previous chapter. This study specifically quantifies the time-based shifts in flow-centrifuge vulnerability curves (VCs) and their parameter estimations across six angiosperm species. Instead of relying solely on traditional methods that consider the loss of hydraulic conductivity only as a function of xylem water potential (Ψ), this paper incorporates spin-time into the VC analysis. Our experiments show that VCs shift towards more positive Ψ values with increasing cumulative spin time, making the values corresponding to 50% loss of hydraulic conductivity (P50) less negative. This VCs shift is due to the increasing gap between hydraulic conductivity (Kh) and maximum hydraulic conductivity (Kmax) over time. As time progresses, Kmax values measured at lower centrifugal speeds tend to increase, while Kh values measured at higher centrifugal speeds decrease. This divergence stretches the difference between Kh and Kmax over time, leading to overestimations of embolism resistance when percentage loss of hydraulic conductivity is calculated. Therefore, although pressure remains the major determinant of embolism formation, incorporating spin-time into flow-centrifuge VCs is crucial for avoiding these overestimations. To address this, the paper proposes using time-stable Kh measurements in VC analysis by modelling the relationship between spinning time and Kh. This approach enables the derivation of more reliable metrics for constructing VCs and reduces the error when making comparisons between different conditions (such as Ψ or temperature), species, or samples. The dynamic nature of vulnerability curves, particularly concerning time-based factors influencing hydraulic conductivity measurements in flow-centrifuge experiments, underscores the need to explore and address potential limitations in current methodologies, as demonstrated through the proposed improvements in this paper.