Ak. Cowan et al., ULTRASTRUCTURAL-CHANGES IN AGING LEAVES OF A LIGHT-GROWN ACHLOROPHYLLOUS MUTANT OF BARLEY, Physiologia Plantarum, 94(3), 1995, pp. 391-398
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.