Fine structure of mesophyll cells in senescing leaves of Phaseolus.

1. In fully senescent mesophyll cells from Phaseolus almost all the cytoplasmic contents had been lost apart from the resistant plasmalemma and some small empty vesicles. In some parts of the cell groups of plastids, together with occasional distorted mitochondria, spherosomes and a few ribosomes also remained. Such plastids were much smaller than those of normal cells and had a spherical shape. The surface membrane had remained intact and enclosed a group of spherical globules with dense structureless contents. All the thylakoids and the stromal ribosomes and starch, shown to be present in the stroma of normal cells, had been broken down. The globules are formed by the accumulation of lipids from thylakoid breakdown, and contain the lipid-soluble carotenoids freed at the same time. 2. The earliest stage of senescence examined showed more or less intact cells apart from the accumulation of some globules within each chloroplast. Distortion of the thylakoids in the vicinity of these globules is considered to be evidence for their localized breakdown in the chloroplast. 3. Because the well-ordered sequence in early senescence it is thought that wholesale release of hydrolytic enzymes would not account for this. At a later stage however, wholesale release of materials from various organelles would constitute a mopping-up process. Of particular significance here would be the vacuolar contents. 4. It would seem from the findings of this investigation that the primary cause of the senescence sequence would be consistent with the appearance, either by synthesis or activation, of a specific enzyme complex which is capable of decomposing thylakoids in the chloroplast. It is thought that this enzyme would increase in quantity and move from the plastids to affect membranes around other organelles at a later stage.