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The influence of CO 2  transported in the transpiration stream on measurements of leaf photosynthesis and stem respiration was investigated. Measurements were made on trees in a temperate forest in Scotland and in a tropical rain forest in Cameroon, and on shrubs in the Sahelian zone in Niger. A chamber was designed to measure the CO 2  partial pressure in the gas phase within the woody stems of trees. High CO 2  partial pressures were found, ranging from 3000 to 9200 Pa. Henry's Law was used to estimate the CO 2  concentration of xylem sap, assuming that it was in equilibrium with the measured gas phase partial pressures. The transport of CO 2  in the xylem sap was calculated by multiplying sap CO 2  concentration by transpiration rate. The magnitude of aqueous transport in the studied species ranged from 0.03 to 0.35 μmol CO 2  m −2  s −1 , representing 0.5 to 7.1% of typical leaf photosynthetic rates. These values strongly depend on sap pH. To examine the influence of aqueous transport of CO 2  on stem gas exchange, we made simultaneous measurements of stem CO 2  efflux and sap flow on the same stem. After removing the effect of temperature, stem CO 2  efflux was positively related to sap flow. The apparent effect on measurements of stem respiration was up to 0.7 μmol m −2  s −1 , representing ∼12% of peak stem respiration rates.

The effect of aqueous transport of CO 2 in xylem sap on gas exchange in woody plants