Efflux and assimilation of xylem-transported CO2 in stems and leaves of tree species with different wood anatomy.

ABSTRACT

Determining the fate of CO2 respired in woody tissues is necessary to understand plant respiratory physiology and to evaluate CO2 recycling mechanisms. An aqueous 13 C-enriched CO2 solution was infused into the stem of 3-4 m tall trees to estimate efflux and assimilation of xylem-transported CO2 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 CO2 diffusivity and xylem [CO2 ] were lower in cedar relative to maple and oak trees, thereby limiting label diffusion. Part of the labeled 13 CO2 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 CO2 diffusivity (lower in conifer relative to angiosperm species) might reconcile discrepancies in efflux and assimilation of xylem-transported CO2 so far observed between taxonomic clades.