Carbon isotope discrimination in terrestrial plants: carboxylations and decarboxylations
The process of CO2 fixation by all terrestrial plants modifies the ratio of 13C/12C between plant material and atmospheric CO2. Changes in the isotopic composition of plant material, δ13C , relative to source CO2, δ13Ca, represent discrimination, or Δ13C, which is normally positive, reflecting organic material depleted in 13C (Farquhar et al., 1982; O’Leary, 1981; Vogel, 1980). Primarily, the predominant photosynthetic pathway (C3, C4 or crassulacean acid metabolism (CAM)) governs large scale discrimination (+30 to −6%o) via the operation of varying proportions of the two carboxylating enzymes, ribulose-1, 5-bisphosphate carboxylase/oxygenase (RUBISCO) and phos-phoenolpyruvate carboxylase (PEPC). Within these types, discrimination varies between the extremes of these enzymatic limits of fractionation and that for diffusion through air (+ 4.4%o), as regulated by diffusive supply of CO2 to the sites of carboxy-lation. The contribution of CO2 from respiration may represent a further source of discrimination (± 4%o), if the processes have discernible isotope effects. More fine-scale shifts in Δ13C (± 2%o) may be the result of interspecific variation within the intrinsic fractionation expressed by both carboxylations and decarboxylations in leaves.