Aging and regreening in soybean cotyledons. 1 Ultrastructural changes in plastids and plastoglobuli

Ultrastructural changes were studied in plastids from mature, partially senescent, senescent, and regreened soybean cotyledons. Tissues were fixed for electron microscopy and also plastoglobuli were isolated from 14-, 16-, and 23-day-old tissues as well as from tissues which had been regreened. In mature tissues, the mesophyll, as compared to the palisade parenchyma, plastids have a poorly developed lamellar system. At the beginning of senescence there occurred an increase in the size of granal stacks and in the size and number of plastoglobuli. At late stages of senescence there is a breakdown of thylakoids. During regreening there is a recovery of the chloroplast fine structure. Plastoglobuli isolated from senescent cotyledons were large and appeared wrinkled when observed with a scanning electron microscope unlike those isolated from mature tissues which were spherical. Chloroplasts in regreened tissues originate either from proplastids or from intermediate stages of slightly senescent plastids.