The influence of (photo)respiration on carbon isotope discrimination in plants

The contribution which (photo)respiration makes to carbon isotope discrimination (Delta(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 (delta(13)C) of source CO2 during growth and measurement. Dark respiration only altered net Delta(13)C (Delta(obs)) at low CO2 assimilation, and was sensitive to source CO2 delta(13)C during measurement. Photorespiration reduced Delta(obs) relative to Delta(13)C predicted from p(i)/p(a) (Delta(i)) over the full range of CO2 assimilation, to a greater degree under elevated oxygen partial pressure (pO(2)), indicating fractionation during photorespiration (f) in T. aestivum. For P. vulgaris, Delta(obs) was insensitive to elevated pO(2) at higher assimilation rates, suggesting that f was minimal. A model was developed to calculate gross discrimination (Delta(ps)), independent of (photo)respiration, from which estimates of f were obtained. for T. aestivum (3.3 parts per thousand) and P. vulgaris (0.5 parts per thousand). Because photorespiratory fractionation varies interspecifically, and influences net Delta(13)C which is directly reflected in leaf delta(13)C, consideration of (photo)respiratory fractionation is necessary when interpreting delta(13)C Of leaf material, especially under conditions where (photo)respiratory CO2 losses make a large relative contribution to total plant carbon budgets.