ULTRASTRUCTURAL-CHANGES IN AGING LEAVES OF A LIGHT-GROWN ACHLOROPHYLLOUS MUTANT OF BARLEY

The pattern and sequence of cellular degradation during the course of leaf senescence remains obscure and the nature of the trigger that ind uces cell senescence is unknown. In order to probe the pre-mortem phas e of senescence temporal changes in cell ultrastucture were studied in aging leaves of light-grown achlorophyllous Hordeum vulgare L. cv. Dy an mutant seedlings. Electron microscope examination of the ultrastruc ture of mesophyll cell plastids revealed the absence of ribosomes and a highly disorganized prolamellar body. Both the number and size of pl astoglobuli increased with aging and this change coincided with deplet ion of starch grains and dilation of lamellar membranes. Aging of meso phyll cells occurred coincident with a decline in ribosome content of the cytoplasm and loss of matrix granularity. Loss of ribosomes associ ated with the outer nuclear envelope membrane and a reduction in chrom atin were also apparent. Only after 10 days was there evidence of loss of internal membrane integrity and swelling of mitochondrial cristae. Compartmentation was thus maintained during the aging process with me mbrane dissolution occurring late in senescence. These results suggest that an inability to produce chlorophyll and carotenoids and form thy lakoid stacks due to the absence of plastid ribosomes, contributes to the rapid onset of senescence in light-grown achlorophyllous seedlings . Furthermore, disruption of chloroplast ribosome synthesis/assembly m ay constitute part of the plastid signal involved in triggering cell s enescence.