Ultra-thin sections of Nicotiana glutinosa L. leaves inoculated with a concentrated solution of tobacco mosaic virus were made at short intervals from 0 to 78 hours after inoculation. Eight hours after inoculation, the size of starch grains increased. This was followed by rupture of cytoplasmic and chloroplast membranes. At about 24 hours there was a great increase in number of mitochondria, which persisted until about 60 hours, when some became electron opaque while others appeared to disintegrate. Finally, the cell contents were compressed into one area of the cell, where they became electron opaque. This was accompanied by collapse of the rest of the cell and tearing away of the cell walls from adjacent cells. The nucleus remained stable and intact for as long as observations could be made. No identifiable virus particles were seen.
Mesophyll cells of Nicotiana glutinosa exposed to starvation in the light after leaf excision showed the following major changes: vacuolation of the cytoplast; rearrangement of chloroplast and nuclear material; accumulation of cytoplasmic crystals and of starch; evagination and increased perforation of the nuclear membrane; and lysis of the tonoplast. Rearrangement of chloroplast material resulted in extended, 3-dimensional arrays (pseudocrystalline bodies), composed of electron-dense globules 85–100 Å in diameter. The ultrastructural degeneration of cells within a leaf was unsynchronized, and the sequence of degenerative intracellular events was not fixed. These observations are compatible with an unequal distribution of "essential" metabolites among those cells. Cells suffering from starvation stress showed ultrastructural changes not observed in cells of aging attached leaves and vice versa. Thus, starvation stress does not merely accelerate the process of cell-aging, as it operates in attached leaves.
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