Dissociation of supramolecular complexes in chloroplast membranes. A manifestation of heat damage to the photosynthetic apparatus.

High temperature-induced alterations to membrane structure were investigated for chloroplast thylakoid membranes isolated from leaves of Nerium oleander grown at a 20/15 degrees C or 45/32 degrees C day/night temperature regime and pretreated at temperatures from 40 to 55 degrees C. Quantitative analysis of micrographs of freeze-fractured membranes revealed a progressive loss of exoplasmic fracture face (EF) particles from the larger particle size classes as the temperature of the pretreatment was increased. This loss indicates that the components of the EF particles, presumed to be the chlorophyll a/b light-harvesting complex and the photosystem II core complex become physically dissociated as a result of the heat pretreatment. The high-temperature stability of this supramolecular complex is enhanced in the samples from the plants grown at the higher temperature regime. These results demonstrate that the heat-induced damage to the photosynthetic apparatus involves not only a functional dissociation of the chlorophyll a/b light-harvesting complex from the photosystem Ii complex, but a physical dissociation as well.