By ultrathin serial sectioning, morphological and volumetric changes of the plastidome and chondriome have been observed in Euglena gracilis Z during bleaching in darkness with addition of sodium acetate to the culture medium. In order not to introduce any modification to the synchronization pattern during bleaching, green cells were previously grown photoautotrophically on Cramer & Myers medium under continuous illumination and synchronized by temperature cycles and (2) of sodium acetate and darkness on the plastidome and chondriome of photoautotrophic cells synchronized by light-dark cycles as described previously. In photoautotrophic cells, the plastidome, consisting of about ten diskoidal chloroplasts, occupies 15% of the cell volume. The chondriome, in the form of one single giant mitochondrion branched throughout the cell, represents 6% of the cell volume. The synchronization by temperature cycles in continuous illumination does not change the morphology and volume of these organelles. However, pyrenoids disappear. In photoheterotrophic culture with sodium acetate added, the plastidome fine structure does not vary but its volume decreases by 19–25%. At that time, the plastidome thus occupies 12–13% of the cell volume. Sodium acetate provokes, on the contrary, hypertrophy of the delicate threads of the mitochondrial reticulum which appears as a network with narrow meshes around other organelles. The chondriome thus comes to occupy 10–11% of the cell volume. In heterotrophic cells, the combined effects of sodium acetate and darkness emphasize the regression of the plastidome while the chondriome appears as a fenestrated parietal shell occupying 15–16% of the cell volume. Maximal hypertrophy is obtained in 24 h. Total dedifferentiation of chloroplasts requires 6–9 successive generations in heterotropic conditions. These results are discussed in relation to numerous light-microscopic and ultrastructural observations. It has been demonstrated, as in photoautotrophic Euglena cells synchronized by light-dark cycles, that the plastidome of heterotrophic cells consists of about ten organelles, whereas the chondriome contains one single giant mitochondrion. Contrary to the opinion that the variations of the plastidome and chondriome are reciprocally related, it is proved here that dedifferentiation of chloroplasts and hypertrophy of the chondriome occur at different rates, and may be independent of one another.

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