The mechanism whereby L-serine specifically promotes the dark senescence of detached oat (Avena) leaves has been examined. The fact that this promotion is strong in darkness but very weak in white light has been explained, at least in part, by the finding that added serine is partly converted to reducing sugars in light. Labeled serine gives rise to 14C-sugars and 14CO2. In the absence of C02, serine does cause chlorophyll loss in light and undergoes a decreased conversion to sugar.As to the large promotion of protease activity which accompanies senescence in the dark, reported earlier, careful purification of the proteases shows that the L4-4(Cserine is not incorported into these enzymes, although it is incorporated into the total protein. Cycloheximide decreases the overall synthesis both of protease and of total protein, but again [C4iserine does not impart radioactivity to the purifed acid proteases. Even when serine is simply added to the protease assay the proteolysis is significantly increased. It is concluded that serine promotes the protease activity by synergizing with the enzyme, or by activating an apoenzyme.One of the early findings about leaf senescence in Avena was that senescence in darkness is accelerated by the amino acid L-serine (12). Two other amino acids, related to serine in its synthetic pathway by the glycolytic system, had similar but much smaller effects (6). Although senescence is characterized by rapid proteolysis, nevertheless, in order for senescence to occur, the synthesis of some protein(s) appears to be necessary. For this reason, it was suggested that the role ofserine might be to become incorporated into the active site of a protease. One of the two major proteinases of the oat leaf does appear to have serine at its active site (2), but this does not, of course, prove the serine to be the limiting factor. Because it is rare for protein synthesis in tissues to be controlled by the availability ofone particular amino acid, a further study of the fate of serine in senescing oat leaves has been undertaken. The results show that the behavior ofserine in the light differs from that in the dark; in the light it does not promote Chl loss but is partly converted to reducing sugars. In the dark, where it does promote the loss of both Chl and protein, i.e. true senescence, no evidence could be found for its direct incorporation into the proteases of the leaf, but rather the data To follow senescence, eight 3-cm subapical segments were floated on 10 ml of the test solution in a Petri dish under white light of the same intensity as above, or in total darkness. After (usually) 3 d in darkness or 6 d in light, the leaves were extracted for 15 min in boiling 80% ethanol and the Chl, a-amino nitrogen, and reducing sugars determined in the extract in the usual way (12).For metabolism experiments, six or seven subapical segments (about 120 mg fresh weight) were floated on water for 15 min, then incubated at 25C in 2 ml of 25 mM unlabeled L-serine containing 0.1 ml L-[U-'4C]serine (New En...
