Melting of ‘Big Top’ Nectarine Fruit : Some Physiological, Biochemical, and Molecular Aspects

The ‘Big Top’ nectarine [Prunus persica (L.) Batsch] has appreciable keeping quality since it resembles, at ripening, the stony hard (SH) peaches in firmness and crispness, but melts at a slow speed at full ripening. We have characterized the postharvest behavior of ‘Big Top’ fruit, treated or not with ethylene for 5 d after harvest (DAH), and compared it with that of a SH peach (‘Ghiaccio’). Pp-ACS1 expression, ethylene evolution, endo-PG production and softening were evaluated and compared to those of the physiologically ripe melting flesh (M) cultivar ‘Bolero’. Like ‘Bolero’, ‘Big Top’ fruit did express Pp-ACS1 and evolve ethylene, but with a 5-d delay. Pp-endo-PG expression, production of an active endo-PG and fruit melting showed a parallel behavior; ethylene treatment enhanced all the above features. In SH ‘Ghiaccio’ Pp-ACS1 expression, ethylene evolution, endo-PG production and softening were absent during the first 5 DAH in air. ‘Ghiaccio’ neither expressed Pp-ACS1 nor evolved ethylene even after ethylene treatment, but responded by accumulating Pp-endo-PG transcripts and an active endo-PG protein, with flesh melting. A ‘Big Top’ Pp-endo-PG clone showed several single nucleotide (SNPs) and insertion-deletion (InDels) polymorphisms in comparison to the M Pp-endo-PG clone of ‘Bolero’ and substantial similarity with the Pp-endo-PG clone of ‘Ghiaccio’. In ‘Big Top’ we identified a peculiar SNP [base pair (bp) 348] and InDels shared with ‘Ghiaccio’, possibly suitable for discriminating among different genotypes. Overall, the data confirm the pivotal role of ethylene in the regulation of endo-PG production and in the determination of peach flesh texture, and support the evidence that ‘Big Top’ could be classified as a melting (slow melting) phenotype.