• Substantial evidence has been published in recent years demonstrating that postphotosynthetic fractionations occur in plants, leading to 13C-enrichment in heterotrophic (as compared with autotrophic) organs. However, less is known about the mechanism responsible for changes in these responses during plant development.
• The isotopic signature of both organic matter and respired CO2 for different organs of French bean (Phaseolus vulgaris) was investigated during early ontogeny, in order to identify the developmental stage at which isotopic changes occur.
Isotopic analyses of metabolites and mass balance calculations helped to constrain the metabolic processes involved.
• At the plant scale, apparent respiratory fractionation was constantly positive in the heterotrophic phase (c. 1‰) and turned negative with autotrophy acquisition (down to –3.08‰). Initially very close to that of the dry seed (–26.83 ± 0.69‰), isotopic signatures of organic matter and respired CO2 diverged (in opposite directions) in leaves and roots after onset of photosynthesis. Respired CO2 reached values up to –20‰ in leaves and became 13C-depleted down to –29‰ in roots.
• It was concluded that isotopic differences between organs occurred subsequent to metabolic changes in the seedling during the transition from heterotrophy to autotrophy. They were especially related to respiration and respiratory fractionation.

Key words:
autotrophy, carbon isotope, dark respiration, discrimination, germination, heterotrophy, Phaseolus