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

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Abstract
The CO2 respired by leaves is 13C‐enriched relative to leaf biomass and putative respiratory substrates (Ghashghaie et al., Phytochemistry Reviews 2, 145–161, 2003), but how this relates to the 13C content of root, or whole plant respiratory CO2 is unknown. The C isotope composition of respiratory CO2 (δR) 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 δ13C. 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 13C‐enriched relative to that of roots. Conversely, shoot biomass was always 13C‐depleted relative to root biomass. The δ‐difference between shoot and root respiratory CO2 was variable, and negatively correlated with the δ‐difference between shoot and root biomass (r2 = 0.52, P = 0.023), suggesting isotope effects during biosynthesis. 13C discrimination in respiration (R) of shoots, roots and whole plants (eShoot, eRoot, ePlant) was assessed as e = (δSubstrate − δR)/(1 + δR/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. eplant and eShoot differed significantly from zero. eplant ranged between −0.4 and −0.9‰, whereas eShoot was much greater (−0.6 to −1.9‰). eRoot was not significantly different from zero. The present results help to resolve the apparent conflict between leaf‐ and ecosystem‐level 13C discrimination in respiration.

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