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Phyllostachys edulis is a spectacularly fast-growing species that completes its height growth within 2 months after the shoot emerges without producing leaves (fast-growing period, FGP). This phase was considered heterotrophic, with the carbon necessary for the growth being transferred from the mature culms via the rhizomes, although previous studies observed key enzymes and anatomical features related to C 4 -carbon fixation in developing culms. We tested whether C 4 -photosynthesis or dark-CO 2 fixation through anaplerotic reactions significantly contributes to the FGP, resulting in differences in the natural abundance of δ 13 C in bulk organic matter and organic compounds. Further, pulse- 13 CO 2 -labelling was performed on developing culms, either from the surface or from the internal hollow, to ascertain whether significant CO 2 fixation occurs in developing culms. δ 13 C of young shoots and developing culms were higher (-26.3 to -26.9 ‰) compared to all organs of mature bamboos (-28.4 to -30.1 ‰). Developing culms contained chlorophylls, most observed in the skin tissues. After pulse- 13 CO 2 -labelling, the polar fraction extracted from the skin tissues was slightly enriched in 13 C, and only a weak 13 C enrichment was observed in inner tissues. Main carbon source sustaining the FGP was not assimilated by the developing culm, while a limited anaplerotic fixation of respired CO 2 cannot be excluded and is more likely than C 4 -photosynthetic carbon fixation., Competing Interests: None declared., (© The Author(s) 2023. Published by Oxford University Press on behalf of the Annals of Botany Company.)