Regulation of ethylene production and ripening by saskatoon (Amelanchier alnifolia Nutt.) fruit

Changes in respiration and ethylene production were characterized during maturation and ripening of saskatoon (Amelanchier alnifolia Nutt.) fruit. On a per fruit basis, respiration and ethylene production increased 78 and 400%, respectively, as fruit ripened on the plant and trends were consistent with those for climacteric fruits. When estimated on a fresh and dry weight basis, increased rates of ethylene production were still apparent during ripening; however, respiration rate declined. Trends in respiration rates and endogenous ethylene levels of harvested fruit of nine maturity classes, from immature green (class one) to fully ripe and purple (class nine), were consistent with those of fruit growing on the plant. Tissue prints showed that ACC (1-aminocyclopropane-1-carboxylic acid) oxidase was distributed throughout the pericarp of fruit at all nine maturity stages and that the enzyme was most concentrated in the immature stages on a per fruit basis. On a protein basis, ACC oxidase increased progressively with development of cv. Smoky fruit but remained relatively constant over the nine maturity classes of cv. Northline fruit. In contrast, ACC oxidase levels were relatively low in cv. Pembina fruit over the first four maturity classes, increased substantially as fruits developed from class four to five, then remained constant as fruit ripened to maturity class nine. Treating immature harvested Pembina fruit (maturity classes one to three) with ACC effected a 28- to 108-fold increase in ethylene production, compared with an average of only 7-fold for ACC-treated fruit of maturity classes four through nine. Preharvest treatment of class-three fruit with ACC induced ripening to maturity class eight within 5 days, while untreated fruit required 15 days to reach class eight. Vacuum infiltrating class four fruit with alpha-aminooxyacetic acid (AOA) or aminoethoxyvinyl glycine (AVG) inhibited ethylene production and color development substantially. The inhibiting effect of AOA on ripening was eliminated when fruit were infiltrated with equimolar concentrations of AOA and ACC. Cobalt, an ACC oxidase inhibitor, also inhibited ethylene production and ripening. Collectively, our results indicate that ethylene synthesis by preclimacteric fruit is limited by the availability of ACC, ethylene is responsible for initiating ripening, and thus, saskatoon fruit are climacteric.Key words: Amelanchier alnifolia, saskatoon, fruit ripening, ethylene.