Abstract: | Both Class I (intact) and Class II (without the outer plastid membrane) chloroplasts of Spinacea oleracea exhibit a shrinkage of the thylakoid volume under conditions which lead to the well known light-induced light scattering increases. In the present report this shrinkage has been measured on micrographs prepared by the freeze-etch technique. In cloroplasts kept in darkness through the freezing or in those treated with DCMU prior to exposure to red light, the thylakoids are in a slightly swollen condition: in plastids exposed to red light and no inhibitor, the thylakoid membranes are closely appressed, giving the thylakoid a shrunken appearance relative to the control. It is further shown that Class I chloroplasts which are actively fixing CO2 do not give appreciable light scattering changes, but lowering the pH away from the optimum for ATP formation (and CO2 fixation) or adding the uncoupler quinacrine restores the light-induced scattering increases. |