ABA and ethylene treatments were applied to preclimacteric `Granny Smith' apples [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] harvested at three different maturity stages. Ethylene production rates, ethylene-forming capacity (EFC), free and conjugated ACC contents, presence of ACC oxidase (ACO) and ripening-related ACC synthase (ACS) proteins, and endogenous ABA levels were monitored at harvest and during 3 weeks thereafter. ABA treatment resulted in a specific accumulation of ACO protein and of ACS-related polypeptides in fruit collected ≈2 months before commercial harvest, whereas the same tissues showed no response to exogenous ethylene. In contrast, fruit harvested 1 month later proved more sensitive to ethylene but not to ABA, in accordance with evolution of endogenous ABA levels, which were highest at this maturity stage and were enhanced in response to exogenous ethylene. A possible role for ABA as an inductor of the competency to ripen is discussed. Chemical names used: abscisic acid (ABA); 1-aminocyclopropane-1-carboxylic acid (ACC).
I. Lara and M. Vendrell
Endogenous ABA, free and conjugated ACC concentrations, ethylene-forming capacity (EFC), and presence of ACC oxidase (ACO) and ACC synthase (ACS) proteins were monitored during the preharvest maturation period of `Granny Smith' apple fruit (Malus sylvestris L. Mill. var. domestica (Borkh.) Mansf. `Granny Smith'). Total proteins from peel and pulp tissues were also extracted at different maturity stages and separated by sodium dodecyl sulphate polyacrylamide gel electrophoresis, providing evidence of differential protein accumulation during fruit development. Endogenous ABA concentration in the peel tissue was higher than in pulp, the highest level occurring ≈2 months before commercial harvest. In the pulp tissue, concomitant increases in ACC and ABA concentrations were observed, preceded by a peak in EFC. However, no ACO or ripening-related ACS proteins were detectable throughout the period considered, suggesting that very low levels of both enzymes are present during the preclimacteric stage of `Granny Smith' apples. A hypothesis on the possible interaction between ABA and ethylene during maturation of `Granny Smith' apples is proposed. Chemical names used: abscisic acid (ABA); 1-aminocyclopropane-1-carboxylic acid (ACC).