Melting and NMF are two main phenotypes of peach fruit and are characterized by different softening patterns in the final stage of ripening. MF cultivars carry the dominant, wild-type allele of the (M) locus that controls flesh firmness, whereas NMF cultivars possess a mutant, homozygous recessive (mm) allele (Peace et al., 2005). MF peaches become extremely soft (i.e., “melting”) quickly after ripening is triggered. Thus, they need to be harvested unripe, at a “"firm-mature” stage, to minimize mechanical injuries, but consequently have considerably lower eating quality than tree-ripened fruit (Cascales et al., 2005; Delwiche and Baumgardner, 1983; Williamson and Sargent, 1999). NMF peaches lose firmness slowly and remain firmer even at full ripeness; thus, they should be ideal for fresh consumption because harvesting can be delayed until fruit reach a more advanced stage of ripeness.
The initiation of peach fruit ripening is accompanied by a climacteric surge of respiration and ethylene production and the peaks of both can occur simultaneously (Amoros et al., 1989; Ferrer et al., 2005). MF fruit are typically harvested at the preclimacteric stage. NMF peaches can be harvested when autocatalytic ethylene production and ripening have already begun, but the ideal harvest maturity and the quality of NMF fruit have not been extensively studied. SSC is one of the most commonly used quality indicators for peaches. SSC in ripe peaches was reported to have a high correlation with perceived sweetness (Crisosto et al., 2006). Consumer acceptance was found to increase constantly as SSC increased (Crisosto and Crisosto, 2005). TA also plays an important role in consumer acceptance of peaches because it was found to correlate strongly with perception of sourness (Crisosto et al., 2006). TA gradually decreases as ripening progresses (Bakshi and Masoodi, 2009; Kwon et al., 2007; Moing et al., 1998), whereas SSC changes little (Robertson et al., 1991, 1992). Interaction between SSC and TA may also affect consumer acceptance of peaches and plums (Crisosto and Crisosto, 2005; Crisosto et al., 2004). For example, peaches with TA greater than 0.80 and SSC less than 10% were perceived by consumers to be of low quality (Crisosto and Crisosto, 2005; Robertson et al., 1990).
Textural changes during peach fruit ripening are achieved by the concerted action of several cell wall modification enzymes (Brummell et al., 2004; Hadfield and Bennett, 1998). Endo-polygalacturonase [endo-PG; electrical conductivity (EC) 126.96.36.199] mRNA is highly expressed after the initiation of climacteric ethylene and the increased activity is accompanied by a conversion of water-insoluble to water-soluble pectin during the melting phase (Orr and Brady, 1993; Pressey and Avants, 1978). Because endo-PG is encoded by a multigenic family, the relatively firmer texture of ripe NMF peaches has been suggested to be the result of deletion or mutation of one or more of the members resulting in virtually no endo-PG activity (Callahan et al., 2004; Lester et al., 1994, 1996; Pressey and Avants, 1978). However, quantitative correlation between the flesh firmness and the levels of the endo-PG polypeptide has not been demonstrated (Morgutti et al., 2006). Exo-PG (EC 188.8.131.52) removes monomer units from the non-reducing end of the pectin chain and its activity increases only after extensive fruit softening for MF peaches (Downs and Brady, 1990; Orr and Brady, 1993). Exo-PG activity can be similar or higher in NMF than in MF peaches; thus, the contribution of exo-PG to texture changes during ripening of NMF peaches is also not clear (Manganaris et al., 2006; Pressey and Avants, 1973, 1978).
Before the initiation of PG-mediated pectin degradation, de-methylesterification by PME (EC 184.108.40.206) is required (Fischer and Bennett, 1991). PME activity has been shown to increase sharply at an early stage of peach ripening and remains constant or decreases throughout the melting phase in MF cultivars (Brummell et al., 2004; Glover and Brady, 1995). PME activity of NMF ‘Andross’ peaches was reported to be significantly lower than that of MF ‘Caldesi 2000’ before and after ripening (Manganaris et al., 2006). Therefore, PME activity may be related to softening of peaches more directly than endo-PG activity.
In this study, physiological and biochemical properties of MF and NMF peaches harvested during the commercial harvest period were evaluated. The objectives included 1) to determine the postharvest respiration rate and ethylene production of MF and NMF peach cultivars at various maturity stages during storage at 20 °C; 2) to quantify the qualities of the fruit objectively after postharvest ripening at 20 °C for 5 d; and, 3) to investigate the relationship between PG, PME, and softening of MF and NMF cultivars.
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