Metabolism of activated oxygen in detached wheat and rye leaves and its relevance to the initiation of senescence.

The activities of several enzymes either generating or decomposing O 2 (-) or H2O2, were investigated during the course of senescence of detached wheat (Triticum aestivum L.) and rye (Secale cereale L.) leaves in light and in darkness. Most of the activities, although not in full synchrony, declined with the degradation of chlorophyll and protein. The decline was slower in light than in darkness (e.g. glycolate oxidase, EC 1.1.3.1; urate oxidase, EC 1.7.3.3.; catalase, EC 1.11.1.6) and was further retarded after application of kinetin. The activity of superoxide dismutase (EC 1.15.1.1) declined only very little or, in detached rye leaves, even remained unchanged. For lipoxygenase (EC 1.13.11.12) the decline was enhanced in light and not affected by kinetin. Total peroxidase (EC 1.11.1.7) activity strikingly increased after excision of the leaves. The increase was higher in the dark than in light and further enhanced by kinetin. Activity of L-amino-acid oxidase (EC 1.4.3.2) was not detected. The peroxide content of the detached leaves slowly increased during senescence, being higher in light than in darkness. The malondialdehyde content strongly increased in light, but not in darkness. Application of several chemicals known as scavengers for oxygen radicals (1,4-diazobicyclo(2,2,2)octane, α-tocopherol acetate, p-benzoquinone, D-penicillamine copper, 2-amino-2-(hydroxymethyl)-1,3-propanediol, formate) did not notably retard chlorophyll degradation in senescencing leaves. Thiourea and urate retarded chlorophyll breakdown in light, obviously because they were used as nitrogen sources. Chlorophyll breakdown was greatly accelerated by D2O, particularly in light, presumably by enhancing photooxidative damage. The results indicate that increased peroxide metabolism accompanies the senescence of detached leaves. They do not, however, support the free-radical theory that an accumulation of activated oxygen initiates leaf senescence.