Cotyledons of Light-grown soybean (GbvJne mar L. var Wayne) seedlings were used as a model system to study the possibility that aging requires qualitative changes in protein synthesis. Cotyledons reached a final stage of senescence and then abscised about 22 days after imbibition. Cotyledon senescence was reversed at 20 days after germination by epicotyl removal. Thereafter, the cotyledons regained much of the chlorophyll, RNA, protein, and polyribosomes lost during aging.Total poly(A)mRNA was extracted from 4-, 12., 20-day-old, and rejuvenated cotyledons and translated in a wheat germ system. Comparison of translation products on two-dimensional O'Farrell gels showed that many translation products increased in quantity during aging, while roughly half as many decreased. Rejuvenation returned the translation products to approximately 4-day-old levels in roughly half of those products which were diminished with age. Conversely, almost one-third of the products which had increased with age decreased with rejuvenation. None of the translation products were totally lost nor were newly synthesized products detected during aging. Therefore, aging in this system probably does not involve complete gene repression or depression. The observation that epicotyl removal causes a reversal in the levels of various proteins synthesized in vitro was corroborated by similar observations following in vivo labeling of cotyledon sections and analysis by SDS-polyacrylamide gel electrophoresis and fluorography. Densitometric scans of fluorograms revealed a gradual shift in profiles of both in vitro and in vivo translation products during aging. Rejuvenated cotyledon proteins had a proffle resembling that of 4-day-old cotyledons. The oVerall level of 135Simethionine incorporation into protein in vivo declined gradually during aging but was restored to 4-dayold levels within 2 days after epicotyl removal.Many theories have been advanced to explain aging in plants and animals. A major concern is whether aging is controlled by genetically programmed mechanisms. Indirect evidence supports the concept that aging in plants is programmed (15, 24) while direct evidence is available for human cells (9,16 during aging (3,22). Changes in translation at these levels, as well as in rates of mRNA transcription, could reduce the synthesis of vital proteins or even favor the production of destructive proteins. The initial agent which triggers the senescence process is unknown. Cytokinins appear to regulate the decline in various macromolecules (protein, nucleic acid, etc.) associated with aging in plants (6,10,13,18), and the loss of membrane permeability (20,23,25). We have examined changes in the spectrum of proteins synthesized during the aging and rejuvenation of soybean cotyledons. The aging of soybean cotyledons can be reversed simply by epicotyl removal, even after roughly 90%o of the nucleic acids and proteins have been lost (11). Following epicotyl removal, Chl, RNA, protein, and polyribosome levels are increased (Skadsden and Cherry,...