The use of ethylene-suppressed lines to assess differential sensitivity toethylene of the various ripening pathways in Cantaloupe melons

Physiological characterization of ethylene-suppressed Cantaloupe Charentais melons (Cucumis melo var. cantalupensis Naud ev. Vedrantais) revealed that some ripening-associated events, like degreening of the rind and cell sepa ration in the peduncular abscission zone, are totally dependent on ethylene . By contrast, some other ripening events, like softening and membrane dete rioration, depend only partially on ethylene and display some ethylene-inde pendent components. Application of increasing levels of exogenous ethylene on these antisense 1-aminocyclopropane-1-carboxylic acid oxidase fruits ena bled the determination of the gradual sensitivity of various ripening pathw ays to the hormone. The threshold level of ethylene capable of physiologica l activity varied from 1 ppm for degreening of the rind to 2.5 ppm for soft ening, membrane deterioration and cell separation in the peduncular absciss ion zone. Up to a saturating dose of 5 ppm, the extent of rind degreening w as proportionally related to the level of applied ethylene. The saturating levels of ethylene for flesh softening (2.5 ppm) and for membrane deteriora tion and cell separation in the peduncular abscission zone (5 ppm) were muc h lower than the internal ethylene found at the climacteric peak of wild-ty pe fruit (over 100 ppm). The cessation of ethylene treatment resulted in a complete arrest of the rind degreening and peduncular cell separation indic ating that both ripening pathways are completely dependent on ethylene. On the contrary, softening and membrane deterioration, though significantly sl owed upon removal of ethylene treatment, continued to proceed in the absenc e of the hormone, thereby unmasking the ethylene-independent component of s oftening and membrane deterioration. The presence of ethylene-independent c omponents in the regulation of individual pathways indicates that the ripen ing of climacteric fruit involves a substantial portion of non-climacteric regulation.