JXB Advance Access published online on May 8, 2007
Journal of Experimental Botany, doi:10.1093/jxb/erm069
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RESEARCH PAPER |
CO2 fluxes and respiration of branch segments of sycamore (Platanus occidentalis L.) examined at different sap velocities, branch diameters, and temperatures
1Warnell School of Forestry and Natural Resources, University of Georgia, Athens, Georgia 30602, USA
2Instituto Superior de Agronomia, Universidade Tecnica de Lisboa, Tapada da Ajuda, 1349-017 Lisboa, Portugal
* To whom correspondence should be addressed. E-mail: mmcguire{at}uga.edu
Respiration of stems and branches of trees (RS) has typically been estimated by measuring radial CO2 efflux from woody tissue (EA) and rates of efflux are often scaled temporally using a temperature relationship (Q10). High concentrations of CO2 in xylem sap ([CO
]) have been shown to affect EA, and the transport of CO2 in the xylem stream has been suggested as a mechanism to explain field observations of temperature-independent fluctuations in EA. Sap velocity and temperature were manipulated in detached branch segments of sycamore (Platanus occidentalis L.) under controlled conditions to quantify these effects. Within individual branches of similar size, EA and [CO] were greater at low sap velocity, while the amount of respired CO2 transported in sap (transport flux, FT) was greater at high sap velocity. EA was linearly correlated with [CO]. In branches of three diameter classes (1, 2, and 3 cm), volume-based EA, FT, and RS did not differ, but surface-area based CO2 fluxes increased with diameter class. Regardless of diameter, EA accounted for only 30% of respired CO2 at high sap velocity, while at low sap velocity, EA accounted for 71% of respired CO2. EA, FT, and RS measured at 5, 20, and 35 °C at the same sap velocity showed a typical exponential response to temperature. However, at the lowest temperature, EA accounted for only 18% of the CO2 released from respiring cells compared with 44% at the highest temperature, perhaps due to the effect of temperature on the solubility of CO2 in water. These results directly demonstrate the transport of respired CO2 in the xylem stream and may help to explain inconsistencies in stem and branch respiration measurements made in situ.
Key words: Branch respiration, Henry's law, Q10, stem respiration, xylem CO2 concentration
Received 19 December 2006; Accepted 2 March 2007
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