Abstract
ABSTRACTThe contribution which (photo)respiration makes to carbon isotope discrimination (Δ13C) 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 (δ13C) of source CO2 during growth and measurement. Dark respiration only altered net Δ13C (Δobs) at low CO2 assimilation, and was sensitive to source CO2δ13C during measurement. Photorespiration reduced Δobs relative to Δ13C predicted from pi/pa (Δi) over the full range of CO2 assimilation, to a greater degree under elevated oxygen partial pressure (pO2), indicating fractionation during photorespiration (f) in T. aestivum. For P. vulgaris, Δobs was insensitive to elevated pO2 at higher assimilation rates, suggesting that f was minimal. A model was developed to calculate gross discrimination (Δps), independent of (photo)respiration, from which estimates of f were obtained for T. aestivum (3.3‰) and P. vulgaris (0.5‰). Because photorespiratory fractionation varies interspecifically, and influences net Δ13C which is directly reflected in leaf δ13C, consideration of (photo)respiratory fractionation is necessary when interpreting δ13C of leaf material, especially under conditions where (photo)respiratory CO2 losses make a large relative contribution to total plant carbon budgets.
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