CO2 transported in xylem sap affects CO2 efflux from Liquidambar styraciflua and Platanus occidentalis stems, and contributes to observed wound respiration phenomena

The [CO2] in the xylem of tree stems is typically two to three orders of magnitude greater than atmospheric [CO2]. In this study, xylem [CO2] was experimentally manipulated in saplings of sycamore (Platanus occidentalis L.) and sweetgum (Liquidambar styraciflua L.) by allowing shoots severed from their root systems to absorb water containing [CO2] ranging from 0.04% to 14%. The effect of xylem [CO2] on CO2 efflux to the atmosphere from uninjured and mechanically injured, i.e., wounded, stems was examined. In both wounded and unwounded stems, and in both species, CO2 efflux was directly proportional to xylem [CO2], and increased 5-fold across the range of xylem [CO2] produced by the [CO2] treatment. Xylem [CO2] explained 76-77% of the variation in pre-wound efflux. After wounding, CO2 efflux increased substantially but remained directly proportional to internal stem [CO2]. These experiments substantiated our previous finding that stem CO2 efflux was directly related to internal xylem [CO2] and expanded our observations to two new species. We conclude that CO2 transported in the xylem may confound measurements of respiration based on CO2 efflux to the atmosphere. This study also provided evidence that the rapid increase in CO2 efflux observed after tissues are excised or injured is likely the result of the rapid diffusion of CO2 from the xylem, rather than an actual increase in the rate of respiration of wounded tissues.