Sources of carbon supporting the fast growth of developing immature moso bamboo (Phyllostachys edulis) culms: inference from carbon isotopes and anatomy.

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 C₄-carbon fixation in developing culms. We tested whether C₄-photosynthesis or dark-CO₂ fixation through anaplerotic reactions significantly contributes to the FGP, resulting in differences in the natural abundance of δ¹³C in bulk organic matter and organic compounds. Further, pulse-¹³CO₂-labelling was performed on developing culms, either from the surface or from the internal hollow, to ascertain whether significant CO₂ fixation occurs in developing culms. δ¹³C 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-¹³CO₂-labelling, the polar fraction extracted from the skin tissues was slightly enriched in ¹³C, and only a weak ¹³C 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 CO₂ cannot be excluded and is more likely than C₄-photosynthetic carbon fixation.