A Unit Pipe Pneumatic model to simulate gas kinetics during measurements of embolism in excised angiosperm xylem

Abstract

The Pneumatic method has been introduced to quantify embolism resistance in plant xylem of various organs. Despite striking similarity in vulnerability curves between the Pneumatic and hydraulic methods, a modeling approach is highly needed to demonstrate that xylem embolism resistance can be accurately quantified based on gas diffusion kinetics. A Unit Pipe Pneumatic (UPPn) model was developed to estimate gas diffusion from intact conduits, which were axially interconnected by interconduit pit membranes. The physical laws used included Fick's law for diffusion, Henry's law for gas concentration partitioning between liquid and gas phases at equilibrium, and the ideal gas law. The UPPn model showed that 91% of the extracted gas came from the first two series of embolized, intact conduits, and only 9% from the aqueous phase after 15 s of simulation. Embolism resistance measured with a Pneumatic apparatus was systematically overestimated by 2 to 17%, corresponding to a typical measuring error of 0.11 MPa for P50 (the water potential equivalent to 50% of the maximum amount of gas extracted). Because results from the UPPn model are supported by experimental evidence, there is a good theoretical and experimental basis for applying the pneumatic method to research on embolism resistance of angiosperms.

A User Manual to Measure Gas Diffusion Kinetics in Plants: Pneumatron Construction, Operation, and Data Analysis

“…Relatively fast Effervescence; vessel length exceeds cavitation chamber length; hydraulic artefacts; destructive Cochard et al, 1992;Gullo and Salleo, 1992;Yin and Cai, 2018 Pereira et al, 2016Pereira et al, , 2020aBittencourt et al, 2018;Zhang et al, 2018;Paligi et al, 2021;Yang et al, 2021 (Schenk et al, 2020), xylem sap will come out at the cut-open end, which will not only be problematic for pneumatic measurements, but also for hydraulic methods. Other forms of wounding response are likely slow and unlikely to affect the vessel dimensions during dehydration.…”

“…The removal of incorrect Ψ 50 values in the above references considerably improves the correlation between P 50 values based on the pneumatic method and other methods. Moreover, the Unit Pipe Pneumatic model indicates that 91% of the gas volume extracted in 15 s of simulation comes from the first two series of intact and embolised vessels, while only 9% of the gas extracted results from xylem sap ( Yang et al, 2021 ), which is saturated or supersaturated with gas ( Schenk et al, 2016 ). This modelling exercise also showed that the Pneumatron may underestimate embolism resistance by 2–17%, with a typical measuring error of 0.11 MPa for Ψ 50 values.…”

“…After reaching the target pressure, it will stop and pressure values will be recorded for 1 min, which is the default measuring time. However, it should be emphasized that the best results for constructing vulnerability curves are obtained for an extraction period of 15 s based on modelling ( Yang et al, 2021 ) and experimental work ( Paligi et al, 2021 ). Users should change the duration of the gas extraction, depending on the experimental requirements, and should consider that long extraction periods (>30 s) increase the proportion of gas from other sources than embolised, intact conduits, such as gas dissolved in xylem sap, or gas extracted radially across cell walls.…”

“…The amount of gas extracted can be calculated from pressure measurements, using the ideal gas law. As gas extraction from embolised, intact vessels is fast and delays in measuring pressure changes can increase the measuring error ( Paligi et al, 2021 ; Yang et al, 2021 ), the Pneumatron has been shown to provide a major improvement of the manual pneumatic apparatus. The main advantages of the Pneumatron are its automated approach and high speed in milliseconds of creating a partial vacuum, taking pressure measurements, storing data, and opening or closing the valves between the discharge tube and the atmosphere.…”

“…This paper aims to provide such a user manual, which may avoid common mistakes in pneumatic experiments and misinterpretation of data. Besides its importance for measuring xylem embolism resistance, the Pneumatron can also be used to study gas kinetics of plants in vivo , and vessel length distributions ( Pereira et al, 2020b ; Yang et al, 2021 ). These methods require minor modifications of the software programme and tube connections.…”