ULTRASTRUCTURE OF CHLOROPLASTS AND CHROMOPLASTS IN CAPSICUM ANNUUM I. THYLAKOID MEMBRANE CHANGES DURING FRUIT RIPENING

Developing chromoplasts in the fruit of Capsicum annuum were examined by electron microscopy. Special attention was given to changes in the thylakoid system. All grana and some intergranal thylakoids in the mature chromoplasts of the seven cultivars studied underwent lysis. The particulate nature of the granal membranes disappeared during lysis before the relationship between the partitions and locules was obscured. The changes during lysis support the globular concept of membrane structure. The selective lysis of the synaptic membranes of the granal partitions may be attributed to their distinctive composition and structure. Lipid globules (osmio-philic) did not accumulate in the immediate region of granal lysis, indicating that they are not directly derived from membranes undergoing degradation. During and following granal lysis a profuse development of intergranal thylakoid membranes occurred in several cultivars. In some instances a thylakoid plexus (prolamellar body) was formed. This specialized structure of the thylakoid system occurs in the chromoplasts of other species as well as in other types of plastids. Extensive, concentrically arranged thylakoid sheets with specific interspaced membrane relationships were frequently associated with the plexus. Several types of membrane associations and interrelationships in the plastid are described. An analysis of one type of membrane configuration, the thylakoid sheets, indicated that one method of growth may be through intussusception into the original membrane. The development of thylakoid plexes and of extensive thylakoid sheets during or after granal lysis indicates that dynamic synthetic activities occur in the chromoplasts of some cultivars of pepper during fruit ripening.