13CO2/12CO2 exchange fluxes in a clamp-on leaf cuvette: Disentangling artefacts and flux components

Abstract

Leaks and isotopic disequilibria represent potential errors and artefacts during combined measurements of gas exchange and carbon isotope discrimination (Δ). This paper presents new protocols to quantify, minimize, and correct such phenomena. We performed experiments with gradients of CO2 concentration (up to ±250μmolmol-1) and δ13CCO2 (34‰), between a clamp-on leaf cuvette (LI-6400) and surrounding air, to assess (1) leak coefficients for CO2, 12CO2, and 13CO2 with the empty cuvette and with intact leaves of Holcus lanatus (C3) or Sorghum bicolor (C4) in the cuvette; and (2) isotopic disequilibria between net photosynthesis and dark respiration in light. Leak coefficients were virtually identical for 12CO2 and 13CO2, but ∼8 times higher with leaves in the cuvette. Leaks generated errors on Δ up to 6‰ for H.lanatus and 2‰ for S.bicolor in full light; isotopic disequilibria produced similar variation of Δ. Leak errors in Δ in darkness were much larger due to small biological:leak flux ratios. Leak artefacts were fully corrected with leak coefficients determined on the same leaves as Δ measurements. Analysis of isotopic disequilibria enabled partitioning of net photosynthesis and dark respiration, and indicated inhibitions of dark respiration in full light (H.lanatus: 14%, S.bicolor: 58%). © 2015 John Wiley