C-isotope composition of CO2 respired by shoots and roots:: fractionation during dark respiration?

The CO2 respired by leaves is C-13-enriched relative to leaf biomass and putative respiratory substrates (Ghashghaie et al., Phytochemistry Reviews 2, 145-161, 2003), but how this relates to the C-13 content of root, or whole plant respiratory CO2 is unknown. The C isotope composition of respiratory CO2 (delta(R)) from shoots and roots of sunflower (Helianthus annuus L.), alfalfa (Medicago sativa L.), and perennial ryegrass (Lolium perenne L.) growing in a range of conditions was analysed. In all instances plants were grown in controlled environments with CO2 of constant concentration and delta(13)C. Respiration of roots and shoots of individual plants was measured with an open CO2 exchange system interfaced with a mass spectrometer. Respiratory CO2 from shoots was always C-13-enriched relative to that of roots. Conversely, shoot biomass was always C-13-depleted relative to root biomass. The delta-difference between shoot and root respiratory CO2 was variable, and negatively correlated with the delta-difference between shoot and root biomass (r(2) = 0.52, P = 0.023), suggesting isotope effects during biosynthesis. C-13 discrimination in respiration (R) of shoots, roots and whole plants (e(Shoot), e(Root), e(Plant)) was assessed as e = (delta(Substrate) - delta(R))/(1 + delta(R)/1000), where root and shoot substrate is defined as imported C, and plant substrate is total photosynthate. Estimates were obtained from C isotope balances of shoots, roots and whole plants of sunflower and alfalfa using growth and respiration data collected at intervals of 1 to 2 weeks. e(plant) and e(Shoot) differed significantly from zero. e(plant) ranged between -0.4 and -0.9%, whereas e(Shoot) was much greater (-0.6 to -1.9%). e(Root) was not significantly different from zero. The present results help to resolve the apparent conflict between leaf- and ecosystem-level C-13 discrimination in respiration.