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ABSTRACT The variations of δ 13 C in leaf metabolites (lipids, organic acids, starch and soluble sugars), leaf organic matter and CO 2 respired in the dark from leaves of Nicotiana sylvestris and Helianthus annuus were investigated during a progressive drought. Under well‐watered conditions, CO 2 respired in the dark was 13 C‐enriched compared to sucrose by about 4‰ in N. sylvestris and by about 3‰ and 6‰ in two different sets of experiments in H. annuus plants. In a previous work on cotyledonary leaves of Phaseolus vulgaris , we observed a constant 13 C‐enrichment by about 6‰ in respired CO 2 compared to sucrose, suggesting a constant fractionation during dark respiration, whatever the leaf age and relative water content. In contrast, the 13 C‐enrichment in respired CO 2 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 CO 2 enriched in 13 C compared to several major leaf reserves (carbohydrates, lipids and organic acids) and whole leaf organic matter.
Ghashghaie et al. (Tue,) studied this question.