1. Protein synthesis in subcellular fractions from young, mature and old ("starved") leaves of tobacco plants was compared with protein synthesis in the same fractions from detached senescent leaves, To this purpose, leaf-disks have been infiltrated with solutions of methionine-35S or leucine-U-14C and exposed in light or darkness, then the tissues have been homogenized, and the specific activities of the proteins of chloroplast, mitochondria, ribosomes and soluble fractions have been determined.
2. The mitochondria fraction showed the highest specific activity (c. p. m./mg protein) in all of the investigated fractions, both exposing the disks in light or darkness.
3. Light promoted the incorporation of methionine-35S into leaf proteins. This promotion could be referred to the increase of the incorporated radioactivity into the proteins of the chloro, plast fraction.
4. In detached leaves a steady decrease of the specific activity in all fractions ran parallel to the decay of the tissues. But the mitochondria fraction seemed to be more influenced during senescence than the other fractions.
5. The occurence of roots at the petioles prevented the decrease of the specific activity of the proteins. Moreover, after rooting a significant increase of the specific activity has been observed in all fractions, especially in the mitochondria.
6. A comparison of protein synthesis in young and old leaves from the same plant indicated that the synthesis was in old > mature > young in terms of the specific activity. However, the reverse was obtained in terms of absolutely incorporated radioactivity into the proteins.
7. During 14CO2-photosynthesis young leaves synthesized more 14C-protein than the old ones. In the following dark period, however, the increase of the radioactivity in the proteins, expressed as per cent of the acid-insoluble substances, was higher in the old tissues.
8. The differences of protein synthesis in detached, ageing leaves and in old leaves from the plants have been discussed in connection with the problem of senescence.
Mein Dank gilt Herm Prof. Dr. K. Mothes für die vielseitige Förderung der Arbeit. Fräulein M. Klein hat mir bei der Durchführung der Versuche wertvolle technische Hilfe geleistet.
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