CO 2 concentration was monitored during three 15-day subculturing cycles in vessels containing actively proliferating plum cultures of Prunus cerasifera, clone Mr.S. 2/5. The effects of two photosynthetic photon flux density regimes: 50 ± 5 μmol m −2 s −1 and 210 ± 5 μmol m −2 s −1 were compared. Three distinct phases in the CO 2 trend were distinguished during each culturing cycle of both light treatments. In the first, occurring at the beginning of the culture cycle, the amount of CO 2 emitted by the cultures during dark periods was greater than that assimilated during the light periods. In the second phase, the opposite trend was detected, while in the third, the range of CO 2 day–night fluctuations increased or remained stable according to the number of explants per vessel. The treatment with 210 ± 5 μmol m −2 s −1 did not modify the CO 2 phase trend but induced more pronounced fluctuations in day–night CO 2 concentration. Under this light treatment, cultures reached CO 2 compensation point for a period as long as 48% of the total number of light hours, while under 50 ± 5 μmol m −2 s −1, it was only 8%. The different range in CO 2 day–night fluctuations monitored throughout a subculturing cycle, appeared to be mainly induced by changes in culture growth dynamics.