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

Abstract

ABSTRACT The CO 2 respired by leaves is 13 C‐enriched relative to leaf biomass and putative respiratory substrates (Ghashghaie et al ., Phytochemistry Reviews 2, 145–161, 2003), but how this relates to the 13 C content of root, or whole plant respiratory CO 2 is unknown. The C isotope composition of respiratory CO 2 ( δ 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 CO 2 of constant concentration and δ 13 C. Respiration of roots and shoots of individual plants was measured with an open CO 2 exchange system interfaced with a mass spectrometer. Respiratory CO 2 from shoots was always 13 C‐enriched relative to that of roots. Conversely, shoot biomass was always 13 C‐depleted relative to root biomass. The δ ‐difference between shoot and root respiratory CO 2 was variable, and negatively correlated with the δ ‐difference between shoot and root biomass ( r 2 = 0.52, P = 0.023), suggesting isotope effects during biosynthesis. 13 C discrimination in respiration ( R ) of shoots, roots and whole plants ( e Shoot , e Root , e Plant ) was assessed as e = ( δ Substrate − δ R )/(1 + δ 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 13 C discrimination in respiration.