Differentiation of Chromoplasts in Cucumis sativus Petals

The development of fibrillar-type chromoplasts, with particular emphasis on the morphogenesis and structural characteristics of the fibrils, was studied in Cucumis sativus petals by light and electron microscopy. Chromoplast differentiation was characterized by the progressive accumulation of fibrils that arranged in a parallel fashion to form birefringent bundles. Immediately before flower opening, the fibrillar bundles in the petals lost their compact structure, and at anthesis, the majority of them partially or completely disorganized. In the plastids of senescing petals, most fibrils were arranged in parallel again. Ultrastructural studies indicated that the formation of at least some of the fibrils started within plastoglobules. The outer diameter of most fibrils ranged from 13 to 18 nm. When transversely sectioned, they were polygonal in shape, often resembling a pentagon or hexagon. In addition to these most common fibrils, different forms that appeared to be composed of two to several ‘‘incompletely separated’’ single fibrils were also frequently present. When standard preparation methods were used, the coat of the fibrils did not appear as a uniformly stained layer but consisted of darkly stained roundish subunits interposed with more lightly stained areas. The morphological observations are discussed with regard to the fine structural changes of chromoplasts during flower development, as well as to the structure of fibrils and the mechanisms of their formation.