Abstract:

Leaf senescence induces up-regulated expression of lipid degradation-related genes，and activities of lipases are found to be enhanced in vacuoles. However，it is not clear whether vacuoles are the only site for the degradation of lipid droplets. Therefore，dynamics of lipid droplets in cucumber leaves of different developmental stages were investigated by means of transmission electron microscopy to understand the sites for lipid droplets caused by leaf senescence. Some plastidial lipid droplets in senescent leaves were demonstrated to undergo degradation inside chloroplasts. Degradation began from one side and then expanded to the other. In the cytoplasm some cytosolic lipid droplets were found to be attached to the surface of chloroplasts，indicating that their occurrence might have a close relation to chloroplasts. Some cytosolic lipid droplets revealed a close association with vacuoles，indicating that they were on the pathway to be transported into the vacuoles. Lipid droplets underwent degradation after they were transported into vacuoles. Apparently，cucumber leaf senescence caused in-situ degradation of plastidial lipid droplets and vacuolar degradation of cytosolic lipid droplets. These observations proved that vacuoles are not the only site for the degradation of lipid droplets.

Key words: Cucumis sativus, leaf；senescence, lipid droplet, in-situ degradation, vacuolar degradation