In situ measurements of dissolved gases in xylem sap as tracers in plant physiology

Abstract

Abstract Trees transport gases from the ground into the atmosphere through the process of transpiration. Tracing gases transported through this mechanism continuously and under field conditions remains an experimental challenge. Here we measured gases dissolved in the tree sap in situ and in real time, aiming to simultaneously analyse the transport of several gases (He, Ar, Kr, N2, O2 and CO2) from the soil, through the trees, into the atmosphere. We constructed and inserted custom-made semi-permeable membrane probes in the xylem of a fir tree and measured gas abundances at different heights using a portable gas equilibrium membrane-inlet mass spectrometer (‘miniRUEDI’). With this method, we were able to continuously measure the abundances of He, Ar, Kr, N2, O2 and CO2 in sap over several weeks. We observed diurnal variations of CO2 and O2 concentrations that reflected tree physiological activities. As a proof of the concept that trees do uptake dissolved gases in soil water, we irrigated the tree with He-enriched water in a tracer experiment and were able to determine upward sap flow velocity. Measurements of inert gases together with reactive species, such as CO2 and O2, allowed separation of the physical transport and exchange of gases derived from the soil or atmosphere from biological reactions. We discuss the opportunities that our technique provides for continuous in situ measurements of gases in the tree sap.