Freeze-Fracture Ultrastructure of Thylakoid Membranes in Chloroplasts from Manganese-Deficient Plants

Abstract

Leaves from spinach (Spincia oleracea L. cv Hybrid 102) plants grown in Mn-deficient nutrient solution were characterized by chlorosis, lowered chlorophyll a/b ratio and reduced electron transport. There were characteristic changes in room temperature fluorescence induction kinet-ics with increased initial yield (F.) and decreased variable fluorescence (F,). The fluorescence yield after the maximum fell rapidly to a level below Fo. The shape of the rise from F. to the maximum was altered and the size of photosystem II units increased, as measured by half-rise time of F, in the presence of 343,4-dichlorophenyl)-1,1-dimethylurea. The Mn-deficient leaves were harvested before necrosis, when thin section electron microscopy revealed no disorganization of the thylakoid system. Thylakoid membranes were examined by freeze-fracture electron micros-copy. The effect of Mn-deficiency was the specific loss of three-quarters of the particles from the endoplasmic fracture face of appressed thyla-koids (EFs). Mn-deficient leaves were restored to near normal 2 days after application of exogenous Mn to the nutrient solution. It is concluded that the loss of most, but not all, functional photosystem II reaction centers from grana, with no alteration in light-harvesting complex or photosystem I, is responsible for the fluorescence and functional prop-erties observed. The response of thylakoids to Mn deficiency shows that there is a fundamental difference in composition and function of stacked and unstacked endoplasmic fracture particles. The stacked endoplasmic fracture particle probably contains, in close association, the photosystem II reaction center and also the Mn-containing polypeptide, the 343,4-dichlorophenyl)-1,1-dimethylurea-binding protein, and all electron