Peach is the most widely cultivated fruit (summer) tree in the temperate regions, particularly in the Mediterranean basin, Americas, Far East, and China. It was the first woody species to be targeted for intensive cultivation. At the end of the 19th century Bonvicini pioneered modern cultivation practices in his family farm in Massa Lombarda (Ravenna county, northern Italy). Following his successful trials, intensive orchard cultivation became the flagship of the Italian fruit industry and has been ‘copied’ all over the world. Fruit growers, technicians and scholars have studied the intensive orchard management practices developed for peach and applied them to many other fruit species.
Peach (Prunus persica L. Batsch) is a diploid species (2n = 16, estimated genome size of 265 million bases) belonging to the Rosaceae family, genus Prunus, subgenus Amygdalus (Bailey, 1927). Results from genomic analyses place the origin of peach about 2.5 million years ago in the glacial refuges generated by the lifting of the Tibetan plateau, in southwest China (Yu et al., 2018). This hypothesis is also corroborated by the discovery of fossil residues (stones, or endocarp) in the Kunming region dated at that time (Su et al., 2015). The wild ancestor of peach is still unknown (perhaps extinct) and probably shared with other related species such as almond (P. dulcis). The progenitor’s drupe was likely not edible and similar to an almond fruit (i.e. dry and fibrous flesh, and drupe opening along the suture when ripe). The fleshy and edible fruit evolved only later in peach, as well as in P. ferganensis, P. mira and P. kansuensis but not in others, such as P. davidiana, P. mongolica and P. tangutica. These related species are characterized by poor fruit quality and are prevalently used as rootstocks, although they may represent a source of resilience traits to biotic and abiotic stress.
P. mira is a spontaneous tree of eastern Tibet with a very long life cycle (up to 1000 years); the edible fruits are highly variable in shape, size and color; the stone is generally smooth although, in some types, it resembles P. persica.
P. kansuensis is a spontaneous species from the north-west of China with an astringent fruit and a parallel grooved stone, without holes.
P. davidiana is a hardy, spontaneous tree, with a fleshy fruit and stone with numerous small holes; some characteristics are of particular interest, including resistance to Sharka disease (Plum Pox Virus), some nematodes (Meloidogyne incognita) and other diseases, or adaptability to different soil conditions and soil sickness’ ( Foulongne et al., 2003).
P. ferganensis has recently been re-classified as a peach subspecies; fruits are characterized by high variability, with the stone exhibiting parallel grooves and kernel can be free of the cyanogenetic glucoside which gives the typical bitter taste.
Despite its origins, most probably from the highlands of Tibet, for centuries peach was mistakenly believed to be of Persian origin. Archaeological findings of stones dating back to the Neolithic age (6000 BC) in the Hemudu villages on the Yangtze river (Zheng et al., 2014) are a recent evidence. Peach ‘domestication’ seems to begin around 3000 BC, while its arrival in Europe could have occurred through various routes: perhaps through Persia, noticed by Alexander the Great in the gardens of King Darius III; or through Egypt, according to Diodorus Siculus. The most widely accepted hypothesis, however, is that peach reached the Mediterranean basin through the Silk Road, and spread in the Roman Empire, as reported by Pliny the Elder (79 AD) and other Latin authors (Faust & Timon, 1995). After the fall of the Roman Empire, peach seems to have disappeared from the countryside in Europe, and was saved by Charlemagne in the gardens of the monasteries. It then showed up again during the Renaissance in the horti (gardens) of the noble families. It is then exported to the New World by the Hispanic colonizers.
The peach golden age (in modern times) begins in the mid-1800s, with the arrival in the American state of Delaware of ‘Chinese Cling’, a white-fleshed Chinese peach with red speckled skin (Hedrick, 1917). In the early 1900s, American varieties derived from ‘Chinese Cling’ and ancient Hispanic peaches (the ‘Indian peaches’) began to spread first in North America and then, at the beginning of the last century, in Europe, where in the meantime only local varieties were sporadically cultivated (descendants of the first genetic materials that had reached the Mediterranean basin). Finally, the new varieties were imported in China. Very curious is the case of nectarines, maybe originating in the Tarim basin, but developed by the modern breeding in California (USA) in the second half of the last century (and later in Europe) and ‘reborn’ in Chinese breeding thanks to American varieties. In China the peach industry has been led almost exclusively by small fruit growers, using local accessions, mainly white fleshed and low-acid taste.
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.
Many of the peach types known today have an ancient origin. Around the year 1000, Zhou ShiHou (1081 AD) noted in his book ‘Luo Yang Hua Mu Ji’ over 30 types of peaches, including nectarines and low-acid flat peaches (Peen-to) also described as ‘honey taste’. Peach fruit variability was known in Roman age, reported by Latin authors (Virgil, Columella, Palladio) who described some of its types, such as Duracinus, referred to the clingstone peaches; this term is still used today to define the non-melting texture (in fact this character is associated with an adherent flesh). The first experiences of peach breeding in Italy date back to the 16 – 17th century in Tuscany, testified by the works of pomologists and painters (such as Bimbi, who worked at the Medici court) who portrayed the cultivars growing in the gardens (pomari) of the Florentine noble villas. However, the twentieth century is the golden age of peach breeding, with the creation of thousands of varieties.
In addition to improving fruit aesthetic and commercial characteristics, the main objectives were the extension of maturity calendar and adaptation to different growing environments. The ripening calendar presently covers over 6 months, from the end of April (in protected cultivation) with extra-early varieties, until the beginning of November with the late cultivars (at the northern latitudes). The first milestone of modern breeding was ‘Elberta’, obtained in the USA in 1889, from which most of today’s yellow-fleshed varieties descend. Until its introduction, the white flesh type dominated the peach industry. ‘J. H. Hale’, released in 1912 and probably deriving from ‘Elberta’, was the second relevant introduction, as characterized by some improved fruit quality traits (firmness and size, other than a delicious taste). ‘J. H. Hale’ has been widely used in breeding programs of the early 1900s in Italy and can be traced as a founder in the pedigrees of over 200 released varieties. The third major US introduction was ‘Redhaven’ (1940s), which marked a substantial improvement in aesthetic and commercial peach quality (Giovannini et al., 2014).
The release of ‘Le Grand’ (1942) established an important advance in the ‘nectarine’ selection, which began in the early 1900s from seedlings of various origins (such as the Pakistan accession ‘Quetta’ and the New Zealand ‘Goldmine’ and ‘Lippiatt’) (Vendramin et al., 2012). The area of origin of the nectarine character is not yet known, although the widespread presence of nectarines in Turkestan (inspirers of the famous essay ‘The Golden Peach of Samarcanda’) suggested the Tarim oases in the Fergana valley (Hedrick, 1917). In Europe, the nectarine ‘Sbergio’ (from the Arabic al-berchiga) was introduced in Sicily perhaps from the 10th century, while its cultivation in the Niceto valley might date from the sixteenth century (Venuti, De agricultura opusculum, 1516) . The name ‘nectarine’ appears around 1660, inspired by the German nektarpfirsich = peach-nectar, later translated into English as ‘nectarine’.
Breeding programs continued in the 1960s and 1970s, with the release of ‘O’ Henry’, a yellow peach variety characterized by a red overcolour extended to almost the whole skin, and the ‘Red Gold’ nectarine. The release of ‘Big Top’ (1983) revolutionized the varietal landscape, leading to the progressive supremacy on the market of the nectarine type, following a combination of characters such as low-acid taste, the complete and early coloring of the fruit and the characteristic texture (recently defined as ‘slow softening’ trait). The 1980s also saw the development of other fruit types, such as flat (starting with the ‘improved’ Stark Saturn, 1985) and low-acid peaches (first of all the yellow peach ‘Royal Glory’, 1987). The last few decades, on the other hand, show a growing interest for peach breeding in China. Although the improved cultivars of western origin are the starting point of such new programs, the only partially explored Chinese diversity could play a fundamental role in the future development of the crop worldwide.
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