PARENT SESSION
Poster Session # 13: Biogeochemistry, Photosynthesis, and Respiration.

Tuesday, August 5 Presentation from 5:00 PM to 6:30 PM. SITCC Exhibit Hall B.

Release of CO₂ from xylem sap is responsible for apparent wound respiration in tree stems.

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

ABSTRACT- A rapid increase in CO₂ efflux from plant tissues after they have been disturbed or damaged has usually been considered a respiratory wound response. However, the CO₂ concentration ([CO₂]) in tree stems is normally very high (3-10%, or 30,000 to 100,000 mol mol^-1) relative to atmospheric concentration (370 mol mol^-1). The high CO₂ gradient from xylem to atmosphere is maintained by significant barriers to diffusion in the cambium and bark layers. This suggests that any disturbance to the stem that affects these barriers, for example, cutting through bark, will increase the rate of efflux of CO₂ from the xylem to the atmosphere. We examined the role of [CO₂] in the xylem of trees on apparent wound respiration by experimentally manipulating xylem CO₂ concentration over a range of 1 to 17% [CO₂] in saplings of two tree species, sycamore (Platanus occidentalis) and sweetgum (Liquidambar styraciflua) and measuring CO₂ efflux from the stems before and after wounding. In all cases (n=18), pre-wound and post-wound stem CO₂ efflux was linearly related to stem [CO₂] (R² = 0.83 and 0.87, respectively). Wounding caused a rapid increase in CO₂ efflux that was approximately four-fold greater than pre-wound rates of efflux, and could be attributed almost entirely to the release of CO₂ from the xylem. These results indicate that the rapid increase in CO₂ efflux from tree stems observed after bark disturbance has been misinterpreted as a wound response, when actually much of the effect can be explained simply as CO₂ degassing from the xylem. This finding has important implications for studies of stem respiration if the bark was disturbed prior to measurement, for example, to facilitate attachment of a cuvette.

Key words: xylem CO₂ concentration , wound respiration