Abstract
Anatomical features in the chloroplasts of palisade cells in peach (Prunus persica) leaves during senescence were observed by transmission electron microscopy and epifluorescent microscopy. Changes in the chloroplast DNA (cp-DNA) were examined as were the contents of protein, DNA, RNA and nuclease activity. In chloroplasts, the indications of naturally senescing leaves were characterized by the dilation of the thylakoid system and a decrease in the number of stacked grana. During such changes, chloroplast size was much smaller than those of normal cells and the chloroplast gradually became spherical with the appearance of osmiophilic globuli. In the fully senescent mesophyll cells, the chloroplast envelope ruptured and the plastid contents dispersed throughout the interior of the cell appearing as numerous small empty vesicles. Epifluorescent microscopy with 4'-6-diamidino-2-phenylindole (DAPI) showed that the cp-DNA degraded during leaf senescence and disappeared completely at leaf death. Changes in cp-DNA during leaf senescence and chloroplast degradation were concomitant and correlated with cell nucleus degradation. The protein, DNA, and RNA contents of cells decreased significantly during leaf yellowing. During the decrease in DNA levels in senescing leaves the nuclease from peach leaves required Zn2+ for full activation during senescence at Stage III (yellowish-green leaves), while most nuclease activity required Ca2+ and its activity did not change during leaf senescence.
