Microelectrode technique for in situ measurement of carbon dioxide concentrations in xylem sap of trees

We developed a new microelectrode technique for measuring CO2 concentration ([CO2]) in xylem sap of trees. This technique enabled us to make rapid and continuous measurements of xylem sap [CO2] in situ. In this report, we discuss the methodology and establish the feasibility of the technique. We also describe calibration procedures, temperature sensitivity, field use and other characteristics of the microelectrodes. An example of data collected in the field is provided. Microelectrode calibration was accomplished at constant temperature in air of known [CO2]. When sampling temperature differed from calibration temperature, correction was necessary. We developed an equation to correct for temperatures between 15 and 35 degrees C when calibration was conducted at 25 degrees C. Equations based on Henry's Law were used to convert measured gas phase [CO2] (%) to concentration of all products of CO2 dissolved in sap (mmol l(-1)). We inserted microelectrodes into stems of three tree species to measure diurnal changes in [CO2] in the xylem sap. A diurnal pattern with depression during the day and elevation at night was observed. Mean daily [CO2] ranged from 1.6 to 10.3 mmol l(-1). Microelectrodes were suitable for making diurnal measurements for up to 7 days without recalibration. We also used the microelectrodes to measure [CO2] of soil in situ. Soil [CO2] ranged from 1 to 4% (gas phase), with little diurnal variation.