Carbon isotope discrimination during photosynthetic CO2 assimilation has been extensively studied and rigorous models have been developed, while the fractionations during photorespiratory and dark respiratory processes have been less well investigated. Whilst models of discrimination have included specific factors for fractionation during respiration (e) and photorespiration (f), these effects have been considered to be very small, i.e. not significantly modifying the net discrimination expressed in organic material. On this paper we consider the fractionation effects associated with specific reactions, set against the overall discrimination which occurs during source-product transformations. We review the studies which have recently shown that discrimination occurs during respiration at night in intact C3 leaves, leading to the production of CO2 enriched in 13C (i.e. e = -6 ‰), and modifying the signature of the remaining plant material. Under photorespiratory conditions (i.e. increased oxygen concentration and high temperature), the photorespiratory fractionation factor may be high (with f around +10 ‰), and significantly alters the observed net photosynthetic discrimination measured during gas exchange. Fractionation factors for both respiration and photorespiration have been shown to be variable among species and with environmental conditions, and we suggest that the term “apparent fractionation” be used to describe the net effect for each process. In this paper we review the fractionations during photorespiration and dark respiration and the metabolic origin of the CO2 released during these processes, and we discuss the ecological implications of such fractionations.

Key words: Carbon isotope, C3 plants, discrimination, fractionation, isotope effect, photorespiration, respiration