δ13C of CO2 respired in the dark in relation to δ13C of leaf metabolites:: comparison between Nicotiana sylvestris and Helianthus annuus under drought

The variations of delta C-13 in leaf metabolites (lipids, organic acids, starch and soluble sugars), leaf organic matter and CO2 respired in the dark from leaves of Nicotiana sylvestris and Helianthus annuus were investigated during a progressive drought. Under well-watered conditions, CO2 respired in the dark was C-13-enriched compared to sucrose by about 4 parts per thousand in N. sylvestris and by about 3 parts per thousand and 6 parts per thousand in two different sets of experiments in H. annuus plants. In a previous work on cotyledonary leaves of Phaseolus vulgaris, we observed a constant C-13-enrichment by about 6 parts per thousand in respired CO2 compared to sucrose, suggesting a constant fractionation during dark respiration, whatever the leaf age and relative water content. In contrast, the C-13-enrichment in respired CO2 increased in dehydrated N. sylvestris and decreased in dehydrated H. annuus in comparison with control plants. We conclude that (i) carbon isotope fractionation during dark respiration is a widespread phenomenon occurring in C-3 plants, but that (ii) this fractionation is not constant and varies among species and (iii) it also varies, with environmental conditions (water deficit in the present work) but differently among species. We also conclude that (iv) a discrimination during dark respiration processes occurred, releasing CO2 enriched in C-13 compared to several major leaf reserves (carbohydrates, lipids and organic acids) and whole leaf organic matter.