Inside out: efflux of carbon dioxide from leaves represents more than leaf metabolism

High concentrations of inorganic carbon in the xylem, produced from root, stem, and branch respiration, travel via the transpiration stream and eventually exit the plant through distant tissues as CO(2). Unlike previous studies that focused on the efflux of CO(2) from roots and woody tissues, we focus on efflux from leaves and the potential effect on leaf respiration measurements. We labeled transported inorganic carbon, spanning reported xylem concentrations, with (13)C and then manipulated transpiration rates in the dark in order to vary the rates of inorganic carbon supply to cut leaves from Brassica napus and Populus deltoides. We used tunable diode laser absorbance spectroscopy to directly measure the rate of gross (13)CO(2) efflux, derived from inorganic carbon supplied from outside of the leaf, relative to gross (12)CO(2) efflux generated from leaf cells. These experiemnts showed that (13)CO(2) efflux was dependent upon the rate of inorganic carbon supply to the leaf and the rate of transpiration. Our data show that the gross leaf efflux of xylem-transported CO(2) is likely small in the dark when rates of transpiration are low. However, gross leaf efflux of xylem-transported CO(2) could approach half the rate of leaf respiration in the light when transpiration rates and branch inorganic carbon concentrations are high, irrespective of the grossly different petiole morphologies in our experiment.