Events surrounding the early development of Euglena chloroplasts : II. Normal development of fine structure and the consequences of preillumination

Abstract

Proplastids of cells of dark-grown, nondividing Euglena gracilis var. bacillaris contain a number of girdlelike thylakoids which are located close to the outer plastid membranes and connected to one or more prolamellar bodies. On illumination, plastid structure remains unchanged during the first 6 hr, a period of slow chlorophyll accumulation, and no new membranes are formed. Structural changes appear after about 12 hr of illumination, and their beginning coincides with the commencement of the rapid rise in chlorophyll content. These involve consecutive fusion of the girdle thylakoids, formation of new thylakoids, and conversion of girdle thylakoids into straight thylakoids. The prolamellar body disappears during this process, but we have not observed its direct transformation into new thylakoids. In cells which received a 90-min light pretreatment 12 hr before exposure to continuous light, the lag period in chlorophyll accumulation is eliminated. No structural changes occur during the preillumination or during the following dark period, but they begin 6 hr rather than 12 hr after subsequent exposure to continuous light. The enhancement of chlorophyll accumulation, CO 2 fixation, and structural development require oxygen during the preillumination, but are not affected when the cells are kept during the dark period under a nitrogen atmosphere.