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

• Published in: Plant, cell and environment cell and environment, 2005-02, Vol.28 (2), p.241-250
• Main Authors: Klumpp, K, Schaufele, R, Lotscher, M, Lattanzi, F.A, Feneis, W, Schnyder, H
• Format: Article
• Language: English
• Subjects: 13C discrimination; Agricultural and forest climatology and meteorology. Irrigation. Drainage; Agricultural and forest meteorology; Agronomy. Soil science and plant productions; alfalfa; allocation; Biological and medical sciences; carbon dioxide; cell respiration; Climatic adaptation. Acclimatization; CO2 exchange; forage crops; Fundamental and applied biological sciences. Psychology; General agronomy. Plant production; grasses; Helianthus annuus; Helianthus annuus L; Lolium perenne; Lolium perenne L; mathematical models; Medicago sativa; Medicago sativa L; partitioning; photosynthesis; respiration; roots; shoots
• ISSN: 0140-7791; 1365-3040
• DOI: 10.1111/j.1365-3040.2004.01268.x

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 (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 delta13C. 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 delta-difference between shoot and root respiratory CO2 was variable, and negatively correlated with the delta-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 (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 per thousand, whereas e(Shoot) was much greater (-0.6 to -1.9 per thousand). e(Root) was not significantly different from zero. The present results help to resolve the apparent conflict between leaf-and ecosystem-level 13C discrimination in respiration.