PARENT SESSION
Poster Session #28: Gas Exchange.
Tuesday, August 6. Presentation from 5:00 PM to 6:30 PM. Exhibit Hall B & C, TCC

Microelectrode technique for measuring xylem sap CO₂ concentration in tree stems.

McGuire, Mary Anne^*,1, Teskey, Robert¹, ¹ University of Georgia, Athens, Georgia

ABSTRACT- We developed a new method for in situ measurement of the CO₂ concentration ([CO₂]) of xylem sap in trees. Carbon dioxide microelectrodes were tested in laboratory and field conditions to determine their characteristics and feasibility for this application. Electrodes were calibrated at 25°C in gas at known [CO₂]. Response was curvilinear with [CO₂]. We created electrode-specific equations for converting electrode mV output to [CO₂]. To correct for temperature sensitivity of the electrodes, we developed a linear equation by monitoring response at different temperatures. After our calibration experiments were complete, electrodes were placed in stems of yellow-poplar (Liriodendron tulipifera L.), white oak (Quercus alba L.), and loblolly pine (Pinus taeda L.) trees at a field site near Athens, Georgia, USA. Carbon dioxide concentration ranged from ~1 to 20 % and varied diurnally and seasonally. We used equations based on Henry's Law to convert [CO₂] to the concentration of all products of CO₂ dissolved in water ([CO₂*], mmol l^-1). Our results indicate that microelectrodes are suitable for measuring [CO₂*] of xylem sap in tree stems in real time over a period of days. Our results also suggest that much of the CO₂ released by respiration of woody tissues is relocated in the transpiration stream rather than diffusing into the atmosphere. The microelectrode technique has advantages over other methods. Microelectrodes are sensitive enough to detect fluctuations in [CO₂] within minutes, and can easily be placed at different depths and heights in the stem. We found that microelectrodes are useful for obtaining information about xylem sap [CO₂*] that has not been measurable by any other means.

KEY WORDS: carbon dioxide microelectrode, xylem sap, respiration