Membrane-bound Plastid Inclusions and Chloroplast Thylakoid Formation in Sunflower (Helianthus annuus L.)

Abstract

The ultrastructure, distribution and frequency of membrane-bound plastid inclusions present in the epidermal cells of leaves of intact sunflower plants (Helianthus annuus L.) and in the epidermal and mesophyll cells of sunflower leaf discs cultured in darkness have been studied. These inclusions appear to be dilated thylakoids containing a granular material which, under normal conditions, is probably involved in chloroplast membrane formation. It is suggested that this material accumulates, and inclusions form, in the chloroplasts of sunflower leaves in two specific situations. Firstly, in the completely differentiated cells of the epidermis where the chloroplasts, although at a relatively immature stage, have nevertheless reached a terminal stage of development. Secondly, in the mesophyll cells of young leaves when chloroplast development has been arrested at an immature stage by a 5-day dark period. In the latter situation the material can be remobilized if plastid development is restimulated. The plastids of sunflower leaf discs cultured in darkness contain both membrane-bound inclusions and prolamellar bodies, indicating that they are separate and distinct structures possibly containing different membrane components.