Plum Breeding

Worldwide, plum is one of the main species, occupying an area of about 2,600,000 ha and ensuring production about 11,700,000 tons. Even though there are over 6000 plum cultivars belonging to 19 – 40 species, there is still the need to create new cultivars due to the demands of growers and consumers. In addition, the large plum-growing countries (Romania, Serbia, Germany, Bulgaria, etc.) have decreased production due to plum pox virus (PPV) attack. Therefore, these countries developed breeding programs with the following objectives: resistance/ tolerance to PPV, productivity, fruit quality, late blooming, self-fertility, different ripening times, short growing period, spur fructification, etc. Using different breeding methods (controlled hybridization, open pollination, selection in wild population on Prunus sp., and mutagenesis), in the last years, over 450 plum cultivars were released, from which 70% represent European cultivars and 30% Japanese cultivars.

Plums are appreciated for fresh consumption and also for dehydration and processing in the different forms (jams, compotes, jellies, candied fruits, frozen fruits, liqueurs, brandy, etc.) [6].
Plums are the fruits with the highest nutritional value, having a high content in carbohydrates, minerals, and vitamins that stimulate the body's health [3,7].
Recent studies at Tuft University, Boston, have shown that dehydrated plums have the highest antioxidants content, contributing to the neutralization of free radicals and thus to the prevention of cancer.
The therapeutic and prophylactic value of plums has been known since ancient times; they have alkalizing, mineralizing, laxative, diuretic actions.
Chemical composition of plums.
China is the country with the fastest development in the plum production, but the major producers of plums, from Prunus domestica and Prunus insititia species, are Romania, Serbia, the Republic of Moldova, Spain, Russia, Poland, France, Germany, and Bulgaria alarming declines in plums production. These decreases are due to the competition of the citrus, peaches, bananas, and other fruits but especially to the viral diseases that have destroyed the plum orchards and depreciated fruit quality in those countries.  Table 2.
Area and production of plums in the world and in major growing countries.
A large number of breeding programs are developed in many countries from Europe ( Table 4). Some of plum breeding programs have been reduced or stopped in the countries where production has declined or funding is no longer available. At the same time, some breeding programs became more private with less public funding [22].
The breeding objectives are general and specific. The general objectives include the following: • Productivity • Fruit quality • Disease resistance, especially to plum pox virus.
The special objectives concern the following (

Breeding methods
There are several breeding methods: A. Conventional methods: • Selection Conventional breeding methods are still largely used in the majority plum breeding programs, and the most important of these are intra and interspecific hybridization and open pollination. These methods contributed in a substantially way to modify the genetic structure of quantitative traits of new plum cultivars and rootstocks [3,10,20,28].
Intraspecific hybridization steps are parents' choice according to the breeding objectives, isolation and emasculation of flowers from the mother parent, collecting pollen from father parent, artificial pollination, control of fruits set, pick up of hybrids fruits, extracting the hybrids stones, and cultivation of seedlings.
In the last years, the interest for obtaining complex genotypes has increased, and for this reason, the interspecific hybridizations have been used.
Within the European FP 7 project, a breeding program for the creation of rootstocks with resistance to plum pox virus was carried out. Thus, at the Technical Sources: [3,10,26,27]. Table 4.
Mutagenesis was applied also in Romania on buds and seeds, using gamma radiations (Co À60 ) and X-rays (Roentgen). For examples, the Romanian plum cultivars 'Alina' and 'Tita' have been obtained through irradiation of 'Tuleu gras' seeds with X-rays [19,29].
In recent years, genetic engineering and biotechnologies have an important role in plum breeding.
Thus, in vitro culture techniques are used to obtain rootstocks virus free. Also, the protocol for pollination and fertilization in vitro of some plum varieties was elaborated during the many years of investigations. For pollination in vitro, it is important to take the non-pollinated flowers at the stage of ovule receptivity for pollen tube. The ovules of 'Sweet Common Prune' were pollinated with pollen of 'Stanley.' Excised ovules were placed on white medium with 15% of sucrose and pollinated with pollen extracted from anthers. The fertilized ovules should be cultivated in the test tubes, in complete darkness, at a temperature of 25°C, for 60 days. After 7 days, percentage of growing embryos was determined [33][34][35].
Embryo culture is used to create varieties with very early ripening. Results in this direction had Gercheva and Zhivondov [36] taking immature embryos from the 'Burmosa' (P. salicina) and 'Ruth Gerstetter' (P. domestica) varieties and cultivating them on a culture medium Murashige and Skoog. Burmosa's embryos had a very good germination. In the last time, due to a very low germination percentage of some plum hybrid seeds in the breeding process, a new research using embryo culture method was started in Romania [36].
Molecular markers have a wide range of possible applications in plum breeding using markers such as RAPD and AFLPs [5]. In plum, gene transfer was applied especially for resistance to plum pox virus in order to create plum cultivars resistant to PPV. The PPV-CP (coat protein) gene was isolated, sequenced, cloned, and used for Agrobacterium tumefaciens-mediated transformation of plum [37]. Transgenic European plums were obtained that were grafted on the rootstocks of P. domestica, and their behavior at PPV was studied in the greenhouse. After 2 years of testing in greenhouse, a transgenic line C5 was resistant to PPV. This clone was registered as 'Honeysweet' cv. Later, this clone was also tested in the field, in Poland, Romania, and Spain under high-pressure infections and had a high level of PPV resistance [38][39][40][41][42].

Genetic sources of breeding
Germplasm collections are a major source of plant genetic diversity, which have an effect to improve crop. Collection, conservation, and evaluation of plant genetic resources are the most important conditions for breeding program. Plant breeders use these genetic resources in hybrid combinations because they are looking for new traits to be included into new varieties [1,2,[43][44][45][46][47].
The existing ex situ collections can be a source of genes potentially useful as material in breeding work or sources of cultivars for a sustainable production.
In Europe there are a lot of plum genetic resources preserved in ex situ collections from about 30 countries.
A large number of plum accessions (estimated to be 4500) are kept in Russia and adjacent states, in four experimental stations located in different climatic conditions coordinated by Vavilov Institute of Plant Industry, Saint Petersburg. From this total, 2325 accessions (about 1600 original Russian cultivars) belong to P. domestica (2n = 48), and 2175 accessions are diploid plums (2n = 16), from which 500 genotypes belong to P. cerasifera [1].
From the ex situ collections, local varieties are very valuable, and many of them are characterized with good adaptability to environmental factors, high productivity, high resistance to pests and diseases, and fruits rich in nutrients necessary for human nutrition and different diets [54,55].
The observations and determinations carried out over the years in the germplasm collections have revealed potential genitors with valuable traits for achieving the objectives from plum breeding programs.
Yield potential is a result of several factors, such as growth vigor, precocity, self-fertility, etc. Frost resistance. The frost resistance of the plum varieties is in accordance with their origin and the place where they were created. Thus, the varieties originating in Western Europe have a low or medium resistance ('Reine Claude Verte' and 'd'Agen'), while those originating in North America and China are very resistant ('Opata,' 'Sapa,' and 'Waneta') [56]. The species P. americana and P. nigra transmit to the offspring the frost resistance [32]. P. ussuriensis species have a very high resistance to low temperatures, but its use in breeding is limited due to the fact that it blooms very early, a trait that is transmitted to the offspring; there is a risk of flower destruction if low temperatures occur during flowering [56]. The European varieties that have good resistances to frost are the following: 'Bonne de Bry,' 'Mount Royal,' 'Pozegača,' 'Stanley,' etc. Considering that the plum blooms early, immediately after the almond and apricot, the goal is to improve the resistance to the late frosts in the spring which, in recent years, have been more and more frequent.  [6,53,57].
Fruit characters. Fruit quality represents an important breeding objective, as the consumer requirements are constantly increasing [59-62]. The new variety will be successful only if certain characteristics are met, such as fruit size, skin color, commercial aspect, sweetness, acidity, firmness, flavor, and juiciness [26,[63][64][65].
• Fruit size. Generally, regarding this trait, the most influence on the progenies is exercised by the mother parent and the cumulative effect of both parents [6]. Therefore, in order to obtain hybrids with large fruits, it is desirable for parents to have large fruits. At the European plum, the fruit size is absolutely necessary, especially since most varieties with special tasting qualities have small fruits (e.g., 'Tuleu gras'). They are of interest as genitors of fruit size for the following varieties: 'Jubileum' from Sweden; 'Tophit'; 'Haganta' from Germany [16]; 'Valor' and 'Vision' from Canada; 'Čacčanska Najbolja' from Serbia; and 'Record,' 'Vâlcean,' 'Tita,' 'Carpatin,' and 'Romanța' from Romania [6,57].
• Fruit color. For fresh market, the fruit color is an important trait. The fruit color of plums ranges from dark blue to blue, purple, red and yellow. At P. domestica and P. salicina species, the skin color is determined by an allelic series, which the allele for the yellow color is recessive to the allele for the blue, red, or purple colors [32]. In the European fruit market, consumers prefer two skin colors: green with different shades (France and parts of Germany) and blue in most countries. The following can be used as parents for the blue color of the skin: 'Stanley,' 'Standard,' 'Oneida,' 'Valor,' 'Vision,' 'Kirke,' 'Hackmann,' 'Bluebell,' 'Bluefree,' 'Negre de Seini,' 'Negre de Bilceşti,' 'Vinete româneşti,' 'Tămâioasă de Bistriţa,' 'Piteştean,' 'Pescăruş,' 'Centenar,' 'Dâmboviţa,' etc. [6,53,57].
• Fruit taste. The taste is the most important aspect of fruit quality. The fruit taste, expressed as a ratio between the main components (sugar, acidity, tannins, vitamins, and aroma) has a great variability, according to the consumers' requirements but also on the origin of the varieties. For example, people from Southern Europe and those from Asia prefer sweet fruits. In other countries, people prefer fruit with a balance between sugar and acidity [16]. Also, a balanced taste has varieties originating from P. domestica and P. insititia [6,57]. Thus, the following varieties are recommended as genitors for good and very good taste: 'Tuleu gras,' 'Centenar,' 'Gras ameliorat,' 'Grase de Becs,' 'Uriașe de Sibiu,' 'Agent,' and 'Andreea' in Romania [6]; 'Bijelica sitna' and 'Prskulja' in Bosnia and Herzegovina; 'Moravka,' 'Metlaš,' 'Obični piskavac,' and 'Čačanska Najbolja' in Serbia; 'Auerbacher,' 'Ortenauer,' and 'Wangenheims' in Germany; 'Italian prune,' 'President' in the USA, and 'd'Agen' in California and France [3].
Sharka (PPV) resistance. One of the major objectives in plum breeding, both in our country and worldwide, is the resistance to viral diseases, especially to plum pox virus, one of the most damaging pathogens causing yield losses over 70%, especially to susceptible cultivars [66,67]. The yield of sensitive varieties, which externalize disease's symptoms on fruit, loses commercial value for fresh market. The fruits affected are blemished, misshapen, and distorted with sunken lesions in flesh. If don't drop prematurely and rich harvest maturity, the fruits have poor flavor, small size, low sugar content and anthocyanin. These fruits can be sold at low price, only to distilleries for brandy processing [67].
The only efficient way to fight against this virus is to develop resistant or tolerant cultivars. The following can be used as a gene source for the resistance to In Germany, at Hohenheim University, Stuttgart, Professor Hartmann achieved very good results on the PPV resistance line, surprisingly, by using the 'Stanley' variety as a parent. An "absolute resistance" through hypersensitivity (HR-hypersensitive reaction) was identified in the 'Jojo' variety obtained from the cross between the 'Ortenauer' and 'Stanley' varieties [16,68].
Zagrai et al. [42] showed that transgenic plants, with incorporated coatprotein (CP) genes, through mediation with the bacterium Agrobacterium tumefaciens, confer protection against Sharka [42]. A transgenic clone, C5 (named 'Honeysweet'), was also studied under the conditions of different countries from Europe, remaining free of PPV after years of testing [42].

Achievements and prospects
A large number of breeding programs are developed in different countries from Europe. Some of the plum breeding programs have been reduced or stopped in the countries where production has declined or funding is no longer available. At the same time, some breeding programs became more private with less public funding [10,22].
In France, the plum breeding program started in 1947 at INRA Bordeaux, and eight plum varieties (6 varieties for drying (prunes) and 2 varieties for fresh consumption (plums)) were registered: