Ultrastructure of giant plastids in a radiation induced mutant of Osmunda regalis

Abstract

A small proportion of Osmunda regalis prothalli, grown from irradiated spores, were observed to exhibit chloroplast mutants, one of which was termed “spot”. Spot mutant cells have their chloroplasts aggregated into one or more large masses, shown at the ultrastructural level to originate from fusion of pre-existing chloroplasts. These aggregates contain no internal separation, and were therefore termed giant chloroplasts. The giant plastids may contain over 1000 grana of normal proportions per section, and numerous osmiophilic droplets and starch grains. Peripheral reticulum and vacuoles were also observed within the giant plastids. Apart from some cytoplasmic vacuolation, the remainder of the cell contents did not appear to be altered in structure.