**5. Breeding methods**

There are several breeding methods:

	- Selection
	- Hybridization
	- Mutagenesis
	- In vitro cells and tissue culture
	- Induction of somaclonal variations
	- Somatic hybridization
	- Genetic engineering


#### *Plum Breeding DOI: http://dx.doi.org/10.5772/intechopen.92432*

A large number of breeding programs are developed in many countries from Europe

• Late blooming and frost resistance in United Kingdom, Bulgaria, Moldova, and

• Winter hardiness in Latvia, Belarus, Moldova, Russia, Sweden, and Norway

Eighty percent of all breeding activities are carried on by *Prunus domestica* and

(**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

• Disease resistance, especially to plum pox virus.

• Short period of vegetation in Sweden

• Self-fertility in Latvia and Romania

There are several breeding methods:

• Good storage in Norway

**5. Breeding methods**

A.Conventional methods:

• Hybridization

• Mutagenesis

B. Biotechnological methods:

• Somatic hybridization

• Genetic engineering

**8**

• In vitro cells and tissue culture

• Induction of somaclonal variations

• Selection

The special objectives concern the following (**Table 3**):

• Extended ripening period in Bulgaria and Romania

only twenty percent by *Prunus salicina* [2, 13, 23–25].

the following:

Prunus

• Productivity

• Fruit quality

Belarus


University of Munich, the 'Dospina 235' (*P. domestica P. spinosa*) and 'Docera 6'

In Romania, there were crossed varieties belonging to the *P. domestica* and *P. insititia* species with the same number of chromosomes, and several cultivars were named: 'Silvia,' 'Ialomita,' 'Diana' ('Renclod Althan' 'Early Rivers'), 'Renclod de Caransebes' ('Renclod Althan' 'Wilhelmina Spath'), 'Doina,' 'Zamfira' ('Anna Spath' 'Renclod Althan'), 'Romaner,' and 'Iulia' ('Tuleu gras' 'Renclod

Hybridization between diploid species (*P. cerasifera*, *P. salicina*, *P. simonii*, *P. besseyi*, *P. americana*, *P. nigra*, *P. munsoniana*, *P. angustifolia*, and *P. hortulana*) can be very easy. For example, 'Santa Rosa' variety with American origin is a mixture

In recent years, fruits of plum hybrids obtained from interspecific crosses have

• Interspecific hybrids between *P. domestica* and *P. armeniaca* called 'Plumcot®' (e.g., 'Red Velvet,' 'Royal Velvet,' 'Flavor Supreme,' 'Flavor Queen,' 'Rutland,' 'Plum Parfait,' 'Spring Satin,' and 'Yiksa'). In Bulgaria, Argir Zhivondov made crosses between *P. domestica* ('Stanley' cv). and *P. armeniaca* ('Modesto' cv.)

• Interspecific hybrids between (*P. domestica P. armeniaca*) *P. domestica* and (*P. salicina P. armeniaca*) *P. salicina*. The name of these hybrids is

• Interspecific hybrids between (*P. domestica P. armeniaca*) and *P. armeniaca* called 'Aprium®' (e.g., 'Flavor Delight,' 'Flavor Candy,' and 'Honey Rich

Regarding clonal selections, these have been done into the old plum orchard from Romania with 'Vinete romanesti' cv., and the following clones were obtained: 'Vinete românești 300,' 'Vinete românești 303,' and 'Vinete românești 4.' Clonal selections produced also several rootstocks: 'Oteșani 8,' 'Oteșani 11,' and 'Voinești

Mutagenesis was applied also in Romania on buds and seeds, using gamma radiations (Co60) and X-rays (Roentgen). For examples, the Romanian plum cultivars 'Alina' and 'Tita' have been obtained through irradiation of 'Tuleu gras'seeds

In recent years, genetic engineering and biotechnologies have an important role

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.

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

After 7 days, percentage of growing embryos was determined [33–35].

(*P. domestica P. cerasifera*) rootstocks were obtained [31].

*DOI: http://dx.doi.org/10.5772/intechopen.92432*

between *P. salicina*, *P. simonii*, and *P. americana* [32].

and obtained the cultivar named 'Standesto' [21].

Althan') [2, 6].

*Plum Breeding*

'Pluot®'.

Aprium').

with X-rays [19, 29].

in plum breeding.

B' [7, 29].

**11**

appeared on the world market:

#### **Table 4.**

*Plum breeding centers and objectives in Europe.*

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.

Out of artificial sexual hybridization, several variants have been utilized: simple crossing, double crossing, pyramid type crossing, recessive crossing (back cross), and diallel crossing [29].

Between 1950 and 2000, most plum varieties have been created by intraspecific simple crossing (A B): 'Stanley' ('d'Agen' 'Grand Duke') in the USA; 'Valor' ('Imperial Epineuse' 'Grand Duke') in Canada; 'Čačanska Lepotica' ('Wagenheim' 'Pozegača'), 'Krina' ('Wagenheim' 'Italian Plum') in Serbia; 'Jojo' ('Ortenauer' 'Stanley') in Germany; 'Centenar' ('Tuleu gras' 'Early Rivers') and 'Roman' ('Tuleu gras' 'Early Rivers') in Romania, etc. [2, 29].

Using double crossing, in Romania, the 'Valcean' plum cultivar was obtained ['H8-12' ('Renclod Althan' 'Wilhelmina Spath') 'H 5-23' ('Renclod Althan' 'Early Rivers')] [2, 29].

In the last 25 years, the pyramidal type cross [(A B) C] was more utilized. Thus, in Romania, using this type of crossing, several varieties were registered: 'Minerva,' 'Sarmatic,' 'Pitestean' ['Tuleu timpuriu' ('Tuleu gras' 'Early Rivers') 'Early Rivers'] [2, 29].

In the last years, the interest for obtaining complex genotypes has increased, and for this reason, the interspecific hybridizations have been used.

Initially, interspecific hybridization was used to improve plum rootstocks. Thus, the plum rootstock 'Ishtara' is a complex interspecific hybrid between *P. domestica*, *P. cerasifera*, and *P. armeniaca* [30]. 'Jaspi' plum rootstock was obtained from the crossing between *P. salicina* Methley and *P. spinosa.* 'Marianna' rootstock was obtained from the combination between *P. cerasifera* and *P. munsoniana* [3].

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

#### *Plum Breeding DOI: http://dx.doi.org/10.5772/intechopen.92432*

University of Munich, the 'Dospina 235' (*P. domestica P. spinosa*) and 'Docera 6' (*P. domestica P. cerasifera*) rootstocks were obtained [31].

In Romania, there were crossed varieties belonging to the *P. domestica* and *P. insititia* species with the same number of chromosomes, and several cultivars were named: 'Silvia,' 'Ialomita,' 'Diana' ('Renclod Althan' 'Early Rivers'), 'Renclod de Caransebes' ('Renclod Althan' 'Wilhelmina Spath'), 'Doina,' 'Zamfira' ('Anna Spath' 'Renclod Althan'), 'Romaner,' and 'Iulia' ('Tuleu gras' 'Renclod Althan') [2, 6].

Hybridization between diploid species (*P. cerasifera*, *P. salicina*, *P. simonii*, *P. besseyi*, *P. americana*, *P. nigra*, *P. munsoniana*, *P. angustifolia*, and *P. hortulana*) can be very easy. For example, 'Santa Rosa' variety with American origin is a mixture between *P. salicina*, *P. simonii*, and *P. americana* [32].

In recent years, fruits of plum hybrids obtained from interspecific crosses have appeared on the world market:


Regarding clonal selections, these have been done into the old plum orchard from Romania with 'Vinete romanesti' cv., and the following clones were obtained: 'Vinete românești 300,' 'Vinete românești 303,' and 'Vinete românești 4.' Clonal selections produced also several rootstocks: 'Oteșani 8,' 'Oteșani 11,' and 'Voinești B' [7, 29].

Mutagenesis was applied also in Romania on buds and seeds, using gamma radiations (Co60) 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 [33–35].

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

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

I. Hjalmarsson, V. Trajkovski





S.H. Hjeltnes - Fruit quality

T. Lakatos - Fruit quality

**Country Breeding centers Breeder Objectives**

**1 2 3 4**

Krymsk Far East Experiment Station, Pavlovsk, St. Petersburg Maicop Experiment Station Krasnodar Region Volgograd Experiment Station,

Krasnoslobodsk

Balsgard

Njos, Lofthus

*Plum breeding centers and objectives in Europe.*

Sweden University of Agricultural Sciences,

Norway Ullensvang Research Centre Division

Hungary Research and Extension Centre for Fruit Growing, Ujfeherto

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.

Out of artificial sexual hybridization, several variants have been utilized: simple crossing, double crossing, pyramid type crossing, recessive crossing (back cross),

Between 1950 and 2000, most plum varieties have been created by intraspecific simple crossing (A B): 'Stanley' ('d'Agen' 'Grand Duke') in the USA; 'Valor'

('Wagenheim' 'Pozegača'), 'Krina' ('Wagenheim' 'Italian Plum') in Serbia; 'Jojo' ('Ortenauer' 'Stanley') in Germany; 'Centenar' ('Tuleu gras' 'Early Riv-

Using double crossing, in Romania, the 'Valcean' plum cultivar was obtained

In the last 25 years, the pyramidal type cross [(A B) C] was more utilized. Thus, in Romania, using this type of crossing, several varieties were registered:

In the last years, the interest for obtaining complex genotypes has increased, and

Initially, interspecific hybridization was used to improve plum rootstocks. Thus, the plum rootstock 'Ishtara' is a complex interspecific hybrid between *P. domestica*, *P. cerasifera*, and *P. armeniaca* [30]. 'Jaspi' plum rootstock was obtained from the crossing between *P. salicina* Methley and *P. spinosa.* 'Marianna' rootstock was obtained from the combination between *P. cerasifera* and *P. munsoniana* [3]. 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

'Minerva,' 'Sarmatic,' 'Pitestean' ['Tuleu timpuriu' ('Tuleu gras' 'Early Riv-

('Imperial Epineuse' 'Grand Duke') in Canada; 'Čačanska Lepotica'

ers') and 'Roman' ('Tuleu gras' 'Early Rivers') in Romania, etc. [2, 29].

['H8-12' ('Renclod Althan' 'Wilhelmina Spath') 'H 5-23' ('Renclod

for this reason, the interspecific hybridizations have been used.

rootstocks [3, 10, 20, 28].

*Sources: [3, 10, 26, 27].*

**Table 4.**

Prunus

and diallel crossing [29].

Althan' 'Early Rivers')] [2, 29].

ers') 'Early Rivers'] [2, 29].

**10**

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].

members within the Genres CT95 No. 61 project titled "International network on

Bordeaux, France, the manager being Emilie Balsamin [1].

human nutrition and different diets [54, 55].

the objectives from plum breeding programs.

Most of these genetic resources have been evaluated and included in a European *Prunus* database (2254 accessions). In 1994, the European Cooperative Programme for Crop Genetic Resources Networks (ECP/GR) had the initiative to maintain this European *Prunus* database at the National Institute of Agronomic Research (INRA)

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

The observations and determinations carried out over the years in the germplasm collections have revealed potential genitors with valuable traits for achieving

**Yield potential** is a result of several factors, such as growth vigor, precocity,

• *Growth vigor.* For, plum there are no sources of genes for low vigor or dwarf type. The following are sources of genes for low vigor: 'Stanley,' 'Marry Mather,' 'd'Ente 698,' 'Sugar,' 'Ealta Dorata di Coe,' 'Early Golden,' 'Belle de September,' 'Sticolase de Voinesti,' 'Prun negru,' 'Perje de toamna,' 'Galbene

• *Precocity*. The growers require varieties with precocious. Good donors for precocity are 'Stanley,' 'Bluefree,' 'Čačanska Lepotica,' 'Čačanska Rodna,'

• *Self-fertility*. Good donors for self-fertility are *'*Stanley,' *'*Anna Späth,' 'Standard,' 'Ontario,' 'Ialomița,' 'Diana,' 'Romanța,' etc. [6, 16].

**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. For example, in April 2009 and 2017 in Pitesti, Mărăcineni, Romania, temperatures of 1.7°C in the air and 3.4°C (2009) and 4.2°C (in 2017) in the soil were recorded. At these temperatures in 2009, the flowers of the varieties 'd'Agen' and 'Anna Späth' were destroyed in a proportion of 70–80%. In 2017, the low temperatures destroyed the young fruits. In this regard, the resistance of flowers to the late frosts can be achieved by creating late-flowering varieties. The late-flowering varieties that transmit this trait to the descendants can be mentioned: 'De Bistriţa,' 'Vinete româneşti 300,' 'Bistriţene de Haţeg,' 'Prune roşii,' 'Busuioace de Geoagiu,' 'Perje de toamnă,' 'Tuleu timpuriu,' 'Pescăruş,' 'Tămâioasă de Bistriţa,' 'Albe de Bilceşti,' 'Superb,' 'Prune roşii,' 'Prun de stepă,' 'Tuleu gras,' 'Pozegača,'

de Aninosani,' 'Grase de Becs,' 'Grase de Pesteana,' etc. [2, 6].

'Verity,' 'Valor,' 'Centenar,' 'Minerva,' etc. [6, 16].

*Prunus* genetic resources".

*DOI: http://dx.doi.org/10.5772/intechopen.92432*

*Plum Breeding*

self-fertility, etc.

**13**

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 [38–42].

#### **6. 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, 43–47].

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].

Other European countries that have a large number of plum accessions are Belgium (616), Hungary (579), France (555), Italy (506), Bulgaria (377), the United Kingdom (380), Switzerland (326), Nordic countries (324), the Czech Republic (276), Portugal (263), Serbia (249), Turkey (232), Latvia (223), Germany (165), etc. [48, 49].

In Romania there are plum collections in two centers: RIFG Pitesti and UCv-SCDP Valcea. Ex situ conservation of accessions is done by different methods: conservation in the field collections (at RIFG Pitesti and UCv-SCDP Vâlcea), conservation in plastic containers, and cryoconservation, at 196°C, in liquid nitrogen (at UCv-SCDP Valcea) [50–53]. The plum cultivar collection at RIFG Pitesti, established in 1997, includes 550 accessions, and the plum rootstock collection, established in 2009, includes 92 accessions. The plum collections at SCDP Vâlcea, established in 1989, 1993, and 1996, include 361 accessions (species, cultivars, and rootstocks). So, from the total of 1003 accessions (cultivars and rootstocks), 34 are species and interspecific hybrids, 407 are local cultivars, 476 are foreign cultivars and rootstocks, and 86 are other genotypes (biotypes, hybrids, mutants, etc.). Most of the accessions belong to *Prunus domestica*, *Prunus insititia*, *Prunus cerasifera*, *Prunus spinosa*, and *Prunus salicina*.

The observations and measurements in ex situ collections were done according to the IBPGR *Prunus* descriptors updated by the ECP/GR *Prunus* Working Group

#### *Plum Breeding DOI: http://dx.doi.org/10.5772/intechopen.92432*

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

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

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

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 collec-

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

In Romania there are plum collections in two centers: RIFG Pitesti and UCv-SCDP Valcea. Ex situ conservation of accessions is done by different methods: conservation in the field collections (at RIFG Pitesti and UCv-SCDP Vâlcea), conservation in plastic containers, and cryoconservation, at 196°C, in liquid nitrogen (at UCv-SCDP Valcea) [50–53]. The plum cultivar collection at RIFG Pitesti, established in 1997, includes 550 accessions, and the plum rootstock collection, established in 2009, includes 92 accessions. The plum collections at SCDP Vâlcea, established in 1989, 1993, and 1996, include 361 accessions (species, cultivars, and rootstocks). So, from the total of 1003 accessions (cultivars and rootstocks), 34 are species and interspecific hybrids, 407 are local cultivars, 476 are foreign cultivars and rootstocks, and 86 are other genotypes (biotypes, hybrids, mutants, etc.). Most of the accessions belong to *Prunus domestica*, *Prunus insititia*,

The observations and measurements in ex situ collections were done according to the IBPGR *Prunus* descriptors updated by the ECP/GR *Prunus* Working Group

Other European countries that have a large number of plum accessions are Belgium (616), Hungary (579), France (555), Italy (506), Bulgaria (377), the United Kingdom (380), Switzerland (326), Nordic countries (324), the Czech Republic (276), Portugal (263), Serbia (249), Turkey (232), Latvia (223), Germany (165),

culture method was started in Romania [36].

**6. Genetic sources of breeding**

tions from about 30 countries.

genotypes belong to *P. cerasifera* [1].

etc. [48, 49].

**12**

traits to be included into new varieties [1, 2, 43–47].

*Prunus cerasifera*, *Prunus spinosa*, and *Prunus salicina*.

[38–42].

Prunus

members within the Genres CT95 No. 61 project titled "International network on *Prunus* genetic resources".

Most of these genetic resources have been evaluated and included in a European *Prunus* database (2254 accessions). In 1994, the European Cooperative Programme for Crop Genetic Resources Networks (ECP/GR) had the initiative to maintain this European *Prunus* database at the National Institute of Agronomic Research (INRA) Bordeaux, France, the manager being Emilie Balsamin [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. For example, in April 2009 and 2017 in Pitesti, Mărăcineni, Romania, temperatures of 1.7°C in the air and 3.4°C (2009) and 4.2°C (in 2017) in the soil were recorded. At these temperatures in 2009, the flowers of the varieties 'd'Agen' and 'Anna Späth' were destroyed in a proportion of 70–80%. In 2017, the low temperatures destroyed the young fruits. In this regard, the resistance of flowers to the late frosts can be achieved by creating late-flowering varieties. The late-flowering varieties that transmit this trait to the descendants can be mentioned: 'De Bistriţa,' 'Vinete româneşti 300,' 'Bistriţene de Haţeg,' 'Prune roşii,' 'Busuioace de Geoagiu,' 'Perje de toamnă,' 'Tuleu timpuriu,' 'Pescăruş,' 'Tămâioasă de Bistriţa,' 'Albe de Bilceşti,' 'Superb,' 'Prune roşii,' 'Prun de stepă,' 'Tuleu gras,' 'Pozegača,'

'd'Agen,' 'Late d'Agen,' 'Drjanovska Sliva,' 'Korai Besztercei,' 'Troianskaia Sliva,' 'Hamanova Svetska,' 'R.C. Violet,' 'OK,' 'Belle de Liege,' 'Belle de Louvain,' 'Mohawk,' 'R.C. d'Oullins,' etc. [6, 53, 57].

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,'

**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

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 plum pox virus (Sharka): 'Popesti,' 'Vieneze,' 'Roman,' 'Cisnadie,' 'Lachi,' 'Alina,' 'Ungurești,' 'Cioraști de Prahova,' 'Flora,' 'Negre de Saru,' 'Negre de Bilcești,' 'Krimska,' 'Peche,' 'Chabat,' 'Hüttner,' 'Belle de Liege,' 'Kirke,' 'Marry Mather,'

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

Zagrai et al. [42] showed that transgenic plants, with incorporated coatprotein (CP) genes, through mediation with the bacterium Agrobacterium tumefaciens,

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

In *Romania*, during 60 years of breeding work, over 2,000,000 plum flowers were pollinated at different centers: Pitești, Vâlcea, Bistrița, and Strejești. From the hybrid seeds, approximately a thousand hybrids were obtained which, thanks to their variability, enabled the selection of many new valuable hybrids. As a result 40 new plum cultivars were registered. In the first two breeding stages (1955–1970), the cultivars 'Tuleu timpuriu' (1967, 'Tuleu gras' 'Peche'), 'Superb' (1968, 'Tuleu gras' 'Abbaye d'Arton'), 'Gras ameliorat' ('Grase românești'—self-pollination), 'Vinete românești 300' (1970, 'Vinete românești'—selection), and 'Tuleu dulce' (1968, 'Tuleu gras' 'd'Agen') were selected. Among the basic genitors, 'Tuleu

'Honeysweet'), was also studied under the conditions of different countries from

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].

*DOI: http://dx.doi.org/10.5772/intechopen.92432*

*Plum Breeding*

low price, only to distilleries for brandy processing [67].

between the 'Ortenauer' and 'Stanley' varieties [16, 68].

Europe, remaining free of PPV after years of testing [42].

confer protection against Sharka [42]. A transgenic clone, C5 (named

'Giant plum,' and 'Condata' [53].

**7. Achievements and prospects**

[10, 22].

**15**

**Ripening time**. In the plum breeding program, it is desirable to extend the ripening season, creating very early varieties, but also very late. Donors for earliness are recommended: 'Ersinger,' 'Petrovača' [3], 'Early Rivers,' 'Ruth Gerstetter,' 'Čačanska Lepotiča,' 'Čačanska Rana,' 'Ialomița,' 'Diana,' 'Scolduș de vară,' 'Boboloase,' etc. [2, 6]. Potential genitors for lateness are: 'Grand Duke' [58], 'Anna Späth,' 'President,' 'Record,' 'Vinete românești,' 'Grase românești,' etc. [2, 6, 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, 63–65].


#### *Plum Breeding DOI: http://dx.doi.org/10.5772/intechopen.92432*

'd'Agen,' 'Late d'Agen,' 'Drjanovska Sliva,' 'Korai Besztercei,' 'Troianskaia Sliva,' 'Hamanova Svetska,' 'R.C. Violet,' 'OK,' 'Belle de Liege,' 'Belle de Louvain,'

**Ripening time**. In the plum breeding program, it is desirable to extend the ripening season, creating very early varieties, but also very late. Donors for earliness are recommended: 'Ersinger,' 'Petrovača' [3], 'Early Rivers,' 'Ruth Gerstetter,' 'Čačanska Lepotiča,' 'Čačanska Rana,' 'Ialomița,' 'Diana,' 'Scolduș de vară,' 'Boboloase,' etc. [2, 6]. Potential genitors for lateness are: 'Grand Duke' [58], 'Anna Späth,' 'President,' 'Record,' 'Vinete românești,' 'Grase românești,' etc.

**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

• *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

• *Fruit shape*. At *Prunus domestica* species, the ellipsoidal fruit shape is dominant over the spherical one, whereas at *Prunus salicina* there is no dominance for the spherical or elongated shape [32]. On the fruit market in Middle Europe elongated shape fruits are preferred. The following varieties can be used as parents for the elongated shape: 'Stanley,' 'Pozegača,' 'Vinete românești,' 'Tuleu gras,' 'Centenar,' etc. [6]. The following varieties can be used as parents for the spherical shape: 'Anna Späth,' 'Kirke,' 'California Blue,' Gras ameliorat,'

• *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.

• *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

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

'Mohawk,' 'R.C. d'Oullins,' etc. [6, 53, 57].

[2, 6, 57].

Prunus

[26, 63–65].

[6, 57].

[6, 53, 57].

**14**

'Grase de Becs,' etc. [6, 57].

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 plum pox virus (Sharka): 'Popesti,' 'Vieneze,' 'Roman,' 'Cisnadie,' 'Lachi,' 'Alina,' 'Ungurești,' 'Cioraști de Prahova,' 'Flora,' 'Negre de Saru,' 'Negre de Bilcești,' 'Krimska,' 'Peche,' 'Chabat,' 'Hüttner,' 'Belle de Liege,' 'Kirke,' 'Marry Mather,' 'Giant plum,' and 'Condata' [53].

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].
