Carbon isotope composition of respired CO2 in woody stems and leafy shoots of three tree species along the growing season: physiological drivers for respiratory fractionation

The carbon isotope composition of respired CO2 (d(13)CR ) and bulk organic matter (d(13)CB) of various plant compartments informs about the isotopic fractionation and substrate of respiratory processes, which are crucial to advance the understanding of carbon allocation in plants. Nevertheless, the variation across organs, species and seasons remains poorly understood. Cavity Ring-Down Laser Spectroscopy was applied to measure d(13)CR in leafy shoots and woody stems of maple (Acer platanoides L.), oak (Quercus robur L.) and cedar (Thuja occidentalis L.) trees during spring and late summer. Photosynthesis, respiration, growth and non-structural carbohydrates were measured in parallel to evaluate potential drivers for respiratory fractionation. The CO2 respired by maple and oak shoots was C-13-enriched relative to d(13)CB during spring, but not late summer or in the stem. In cedar, d(13)CR did not vary significantly throughout organs and seasons, with respired CO(2 )being C-13-depleted relative to d(13)CB. Shoot d(13)CR was positively related to leaf starch concentration in maple, while stem d(13)CR was inversely related to stem growth. These relations were not significant for oak or cedar. The variability in d(13)CR suggests (i) different contributions of respiratory pathways between organs and (ii) seasonality in the respiratory substrate and constitutive compounds for wood formation in deciduous species, less apparent in evergreen cedar, whose respiratory metabolism might be less variable.

Automated summary

The carbon isotope composition of respired CO2 and bulk organic matter informs about the isotopic fractionation and substrate of respiratory processes in plants. This study investigated the variation in carbon isotope composition across different organs, species, and seasons using cavity ring-down laser spectroscopy. The results showed different patterns in d(13)CR among the studied plant species, with maple and oak shoots exhibiting C-13 enrichment during spring and cedar showing no significant variation. These findings suggest different contributions of respiratory pathways between organs and seasonality in respiratory substrate and constitutive compounds.