The influence of (photo)respiration on carbon isotope discrimination in plants
The contribution which (photo)respiration makes to carbon isotope discrimination (Δ 13 C) was examined by conducting simultaneous gas exchange measurements and isotopic analysis of carbon dioxide passing over leaves of Triticum aestivum and Phaseolus vulgaris, via manipulations of the carbon isotope composition (δ 13 C) of source CO 2 during growth and measurement. Dark respiration only altered net Δ 13 C (Δ obs ) at low CO 2 assimilation, and was sensitive to source CO 2 δ 13 C during measurement. Photorespiration reduced Δ obs relative to Δ 13 C predicted from p i /p a (Δ i ) over the full range of CO 2 assimilation, to a greater degree under elevated oxygen partial pressure (pO 2 ), indicating fractionation during photorespiration (f) in T. aestivum. For P. vulgaris, Δ obs was insensitive to elevated pO 2 at higher assimilation rates, suggesting thatf was minimal. A model was developed to calculate gross discrimination (Δ ps ), independent of (photo)respiration, from which estimates off were obtained for T. aestivum (3.3‰) and P. vulgaris (0.5‰). Because photorespiratory fractionation varies interspecifically, and influences net Δ 13 C which is directly reflected in leaf δ 13 C, consideration of (photo)respiratory fractionation is necessary when interpreting δ 13 C of leaf material, especially under conditions where (photo)respiratory CO 2 losses make a large relative contribution to total plant carbon budgets.