Ethylene as a Regulator of Senescence in Tobacco Leaf Discs

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Abstract
The regulatory role of ethylene in leaf senescence was studied with excised tobacco leaf discs which were allowed to senesce in darkness. Exogenous ethylene, applied during the first 24 hours of senescence, enhanced chlorophyll loss without accelerating the climacteric-like pattern of rise in both ethylene and CO2, which occurred in the advanced stage of leaf senescence. Rates of both ethylene and CO2 evolution increased in the ethylene-treated leaf discs, especially during the first 3 days of senescence. The rhizobitoxine analog, aminoethoxy vinyl glycine, markedly inhibited ethylene production and reduced respiration and chlorophyll loss. Pretreatment of leaf discs with Ag+ or enrichment of the atmosphere with 5 to 10% CO2 reduced chlorophyll loss, reduced rate of respiration, and delayed the climacteric-like rise in both ethylene and respiration. Ag+ was much more effective than CO2 in retarding leaf senescence. Despite their senescence-retarding effect, Ag+ and CO2, which are known to block ethylene action, stimulated ethylene production by the leaf discs during the first 3 days of the senescing period; Ag+ was more effective than CO2. The results suggest that although ethylene production decreases prior to the climacteric-like rise during the later stages of senescence, endogenous ethylene plays a considerable role throughout the senescence process, presumably by interacting with other hormones participating in leaf senescence.