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Protoplasts isolated from lettuce (Lactuca sativa L.) leaves underwent progressive senescence when incubated in 0.7 M mannitol with various content of polyamines. This senescence was manifested by morphological deterioration and ultimate lysis of protoplasts, by a decrease in viability of protoplasts and incorporation of [³H]- uridine into macromolecules. Protoplasts incubated with putrescine (0.1, 1 mM) spermidine (0.1, 1, and 10 mM) and spermine (0.1 mM) were stabilized, and the incorporation of [³H]-uridin into RNA was higher than control protoplasts after 12 hrs. Especially, protoplasts incubated with 0.1 mM putrescine were most stable, and showed an inhibition of [³H]-uridin incorporation for initial 1, 2, 3 hrs, but with time, the incorporation was restored to levels greater than others.
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