Peach is the main cultivated stone fruit species, with an estimated world production of over 25 million tons (FAOSTAT, 2018). China is by far the first country in terms of production volumes (almost 15 Mt), followed by Italy and Spain (both around 1.5 Mt). Most of the production is intended for fresh consumption, while a portion is destined to processing into juices, jams and canning. Peach cultivation has an extraordinarily wide and constantly evolving varietal landscape. Different peach types are available on the market and are easily recognized from peculiar characteristics of the fruit, mainly the fuzziness of the epidermis, which distinguishes peaches from nectarines; the glabrous epidermis is a recessive monogenic trait controlled by a mutation in the MYB gene PpeMYB25, which regulates the production of trichomes, thereby determining the ‘nectarine’ phenotype (Vendramin et al., 2012). The color of the flesh is another typical classification criterion: the yellow trait originated from at least 3 distinct mutation events affecting an ancestral gene responsible for cleavage of carotenoids and hence white color (PpCCD4) (Falchi et al ., 2013). The ‘blood flesh’ character is also present in some varieties, although without commercial relevance, at least so far. The blood flesh is due to the activation of a NAC gene (called ‘Blood’, BL) which induces the synthesis of cyanidins (anthocyanins) (Zhou et al., 2015). This character is expressed in conjunction with the basic color (white or yellow). Although most peaches have a round or elongated shape, there is also a flattened shape (commonly called ‘flat’) initially spread only locally (in China, or elsewhere, such as the ‘tabacchiera’ in some areas of Sicily, Italy). This trait is due to a dominant allele (lethal when homozygous) and probably depends on a mutation of a LRR-RLK gene involved in the pathway of brassinosteroids, a class of plant hormones regulating cell division (López-Girona et al., 2017).

Flesh texture is another fundamental trait from a technological and commercial point of view, and at least three different types are described in peach: melting (M), non-melting (NM) and Stony- Hard (SH).

The NM varieties retain a firm flesh even when fully ripe and are mainly intended for canning. This trait is regulated by a copy number variation for an endo-polygalacturonase (endoPG) gene, an enzyme involved in cell wall degradation: one copy (PpendoPGM, H1), two copies (PpendoPGF and PpendoPGM, H2) and none (H3). Cultivars carrying one of H1H1, H1H2, or H1H3 combinations display melting flesh not adherent to the endocarp (‘freestone’) or semi-adherent; those with H2H2 and H2H3 are melting but with adherent flesh (‘clingstone’) while H3H3 are NM with adherent flesh (Gu et al. 2016). The SH type is instead characterized by an almost crunchy texture, due to very low ethylene production in the fruit, in turn caused by a reduced activity of the auxin biosynthesis gene PpYUC11-like (Pan et al. 2015). This monogenic recessive trait is controlled by the hd locus, distinct from the locus controlling M/NM trait. Nevertheless, SH is difficult to distinguish from the NM type, and masks its expression (epistasis): only treatment with ethylene of the SH fruits can reveal the presence of the NM phenotype. Despite a certain commercial potential, SH are not of particular importance in the current market. Fruits for fresh consumption are generally of the melting type, being at physiological maturity soft and juicy; however, the speed and/or extent of flesh melting varies widely. In general, commercial varieties tend to have a firm to very firm flesh. An important variant (with Mendelian inheritance, most probably dominant) has been identified in the ‘Big Top’ nectarine (defined as ‘slow softening’), although the physiological mechanisms are still unknown (Ghiani et al., 2011).

A remarkable variability is also present in terms of flavor, due to the content of aromatic and phenolic compounds, soluble sugars (fructose, glucose, sorbitol and, mainly, sucrose) and organic acids (malic, citric, quinic, in quantitative order). Total sugar content, generally expressed through refractometric measures, ranges between 10 – 15% in fresh market cultivars, but can exceed 20 – 25% in some accessions. Sugar content depends on complex interactions between genetic, environmental and cultural factors, still poorly known and only partially exploited in breeding (Cirilli et al., 2016). Besides absolute contents, flavor is strongly influenced by the ratio between sugars and acids: fruits tend to be perceived with greater sweetness as this ratio increases. The monogenic dominant ‘low-acid’ trait determines a strong reduction in total acidity (values lower than 5.5 g /l), not affecting sugar content, while causing an increase in the sugar/acid balance. This character confers to peaches a higher sweetness than those with normal acidity (irrespective of comparable amount of sugars). However, when acid content is too low (together with sugars concentration below 12%) the perception of the typical ‘peach’ aroma is reduced, also causing a certain ‘flattening’ of the organoleptic characteristics.