Analysis and Characterization of 3-(3,4-Dichlorophenyl)-1,1-Dimethylurea (DCMU)-resistant Euglena: II. Modifications Affecting Photosynthesis during Adaptation to Different Doses of DCMU.

When grown in medium containing dl-lactate at 27 C in the light, Euglena gracilis Z populations underwent modifications of the pigment system in response to 0.05 to 250 micromolar 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU).Chlorophyll content dropped dramatically, the only remaining form being Chl a(673). Light-driven O(2) evolution was no longer detectable for the two highest DCMU concentrations tested. The energy-capture cross-section of detectable photosystem II units remained unchanged, although intersystem energy transfer no longer occurred. Euglena at this stage had chloroplast membranes destacked and swollen. A recovery phase then occurred, marked by enhanced photosynthetic properties. The initial forms of chlorophyll which were accumulated were highly efficient for O(2) evolution. The newly formed photosystem II antennae were connected and of small size. Finally, the third phase involved the recovery of photosynthetic capacity similar to that of the controls as the thylakoids regained their normal structures.Since these modifications occurred in the entire population and DCMU resistance persisted through successive cell generations, these adapted Euglena were considered to be a variant of the Z strain, designated ZR.