Efflux and assimilation of xylem-transported CO in stems and leaves of tree species with different wood anatomy.

Determining the fate of CO respired in woody tissues is necessary to understand plant respiratory physiology and to evaluate CO recycling mechanisms. An aqueous C-enriched CO solution was infused into the stem of 3-4 m tall trees to estimate efflux and assimilation of xylem-transported CO via cavity ring-down laser spectroscopy and isotope ratio mass spectrometry, respectively. Different tree locations (lower stem, upper stem and leafy shoots) and tissues (xylem, bark and leaves) were monitored in species with tracheid, diffuse- and ring-porous wood anatomy (cedar, maple and oak, respectively). Radial xylem CO diffusivity and xylem [CO ] were lower in cedar relative to maple and oak trees, thereby limiting label diffusion. Part of the labeled CO was assimilated in cedar (8.7%) and oak (20.6%) trees, mostly in xylem and bark tissues of the stem, while limited solution uptake in maple trees hindered the detection of label assimilation. Little label reached foliar tissues, suggesting substantial label loss along the stem-branch transition following reductions in the radial diffusive pathway. Differences in respiration rates and radial xylem CO diffusivity (lower in conifer relative to angiosperm species) might reconcile discrepancies in efflux and assimilation of xylem-transported CO so far observed between taxonomic clades.