**7. Varieties and cultivars**

There are apparently no named cultivars, but there are local preferences according to fruit color. Tamarillo have been described as three strains yellow, red (red skin, yellow-orange flesh), and purple (red-purple skin, light orange flesh), with the red being more popular and more common but no named varieties have been analyzed. In Europe and in the USA, the red and purple cultivars are the preferred by consumers due to its attractive color, flavor and nutritional properties. In Malaysia, the red variety of tamarillo can be easily grown at Cameron Highlands, Pahang. It is an egg-shaped bright red fruit with yellow-orange flesh and black seeds that are surrounded by purple seed coat. In Kenya the main varieties grown are the Gold-mine, Inca red, Rothamer, Solid gold and Ruby red. Red fruits are chosen for the fresh fruit markets because of their appealing color. Different selections have been developed in New Zealand from time to time. Commercial plantings of New Zealand's current dominant "black" type were selected in the early 1920s as a variation on the yellow and purple variants that had been used in the past. A reselection process resulted in the creation of this massive, higherquality red variety. New Black was chosen by William Bridge in 1927 for its huge fruit. Ruby Red has been a staple of the New Zealand economy for decades. Heart-shaped

*Tamarillo (*Cyphomandra betacea *(Cav.)) Origin, Cultivation, Breeding and Management DOI: http://dx.doi.org/10.5772/intechopen.106601*

fruit with a deep crimson color and a savory flavor were selected in 1970. Inca Gold is a canning favorite because of its amber hue and oval shape. If you are looking for something a little more vibrant, Ecuadorian Orange is a great choice. In 1979, Kaitaia Yellow was chosen for its flavor and sweetness. New Zealand is testing a new cultivar named Goldmine. Rothemer is an 85-gram fruit from San Rafael, California, with a bright red exterior and a golden yellow pulp. Solid Gold is an orange, luscious fruit with a sweet flavor. Red Delight, Yellow, and Oratia Round are also made in New Zealand.

#### **8. Breeding and crop improvement**

Tamarillo's chromosomes are diploid (2n = 2x = 24). However, spontaneous tetraploids, aneuploids, and triploids have all been documented. The flowers are autogamous and self-compatible but flowers need to be shaken for pollination. Traditional plant breeding has largely ignored the tamarillo, and few crop improvement techniques have been applied. Most attempts to transfer this crop into new areas have failed since they have relied on a single cultivar. As a result, the ability to use variety for local adaptation has been limited. On the other hand, the loss of genetic diversity in this crop and allied wild species is a major worry [40, 41]. Plantation management, fruit quality, and postharvest management are all improved through research and breeding. Breaking seed dormancy, improving fruit taste, and increasing yields are all key breeding goals. Little "stones" of sodium and calcium that occasionally form in the fruit skin offer a difficulty for industrial applications and must be eradicated through breeding. The following are the most important breeding procedures in this crop.

#### **8.1 Selection**

Although the tamarillo is essentially autogamous, there is some variation among plants of the same accession, particularly in traditional growing methods. This variance is caused by spontaneous mutations or by the inadvertent introduction of genetic material from different populations through genotype crossover [42]. This variation could be used to find a favorable variant that could be used in a future breeding effort for crop improvement. As a result, screening valuable genetic variety in traditional growing areas for conservation, selection, and crop improvement for this underutilized fruit crop is critical.

#### **8.2 Hybridization**

Pollen tubes pierce the ovary and set fruits in most interspecific matings with *Cyphomandra*, according to [30], but seeds do not mature. Pringle and Murray [43] experimented with interspecific hybridization of *Cyphomandra* with nine different Cyphomandra species and discovered that the majority of the crosses failed after fertilization. However, a species hybrid between the two Brazilian species *C. corymbiflora* and *C. diploconos* was obtained very easily. Most species' combinations had reciprocal variances in compatibility. The results of these crosses indicate that the S locus is not involved in the regulation of interspecific incompatibility.

#### *8.2.1 Intraspecific hybridization*

There are no technical issues in crossing between different varieties of *C. betacea*. Tamarillo individuals have a high degree of homozygosity due to their autogamy,

and crossing genetically dissimilar individuals produces homogeneous offspring. Commercial production of the hybrid tamarillo is simple because each fruit contains more than 300 seeds [44]. The agronomic behavior of F1 hybrids, on the other hand, is completely unknown. Most solanaceous horticulture crops have heterotic yield features [45], and the tamarillo is no exception. Obtaining segregant generations allows for recombination and segregation, allowing for the emergence of new superior genetic combinations. There are, however, a scarcity of investigations on the degree of diversity in segregant generations. Despite the fact that it takes several years to evaluate, tamarillo cannot be regarded a standard fruit crop, as individual genotype evaluation can take up to 20 years. Nonetheless, evaluating each individual takes 3–5 years, making yearly species breeding schemes impracticable, as it could take up to 10 generations before an improved cultivar is released. It will be impossible to determine the best effective breeding strategy for this crop until studies are conducted to determine the corresponding value of the additive and dominant components of genetic variation for each of the primary interest traits. Breeders are interested in developing pure lines or F1 hybrids that maximize heterozygosis's. Vegetative propagation, on the other hand, allows for the propagation of the most valuable genotypes that may arise in segregant generations, such as an F2 produced by complimentary or transgressive crossings. Cloning permits the entire genotype to be preserved, which might be useful in the production of novel cultivars. Micropropagation for tamarillo [46] provides for vegetative propagation without the risk of viral transmission.

#### *8.2.2 Interspecific hybridization*

Interspecific hybridization could be effective for transferring desirable traits from wild species to cultivated forms, such as disease and worm resistance. Crossings between *C. acuminata* Rusby and *C. uniloba* Rusby have proven successful. These species are morphologically quite similar to *C. betacea*. Fertility is low in Tamarillo hybrids with *C. acuminata*. Those obtained using *C. uniloba*, on the other hand, are vigorous and prolific [30]. Other Cyphomandra species, such as *C. hartwegii* (Miers) Sendt. Ex Walp. and *C. sibundoyensis* Bohs, are edible as well, and may have some future potential as stand-alone plants or sources of genetic variety for tamarillo breeding. There is no publicly available enhanced breeding material for tree tomatoes. Nonetheless, numerous hybrids between *S. betaceum* and *S. unilobum* are being tested in Colombian fields as a result of breeding operations [47].

#### **8.3 Polyploidy breeding**

Triploid and tetraploid seedlings are found at low frequency among commercial plantings of the diploid tamarillo and may arise from the union of unreduced gametes. Further, [48] reported successful induction of tetraploids in the tamarillo (*Cyphomandra***,** *betacea* (Cav.) Sendt.) by the application of colchicine to the germinating seed. Triploids and aneuploids are produced from interploidy crosses. Most aneuploids produce primary trisomics (2n = 2x + 1 = 25), but some possesses 26 chromosomes and few may be hyperpolyploids. Aneuploidy with 25 chromosomes, instead of 24 as the diploid types, show good fertility and give a similar or even higher yield and fruit size than their diploid counterparts. The pollen fertility of these plants can vary from 10 to 90%. The morphological traits of aneuploids and diploids were the same. For commercial use, aneuploids have fruit and seed sizes comparable to

*Tamarillo (*Cyphomandra betacea *(Cav.)) Origin, Cultivation, Breeding and Management DOI: http://dx.doi.org/10.5772/intechopen.106601*

those of diploids. Tamarillo polyploids have been proven to have low fertility and poor agronomic properties, whether they are spontaneous or manufactured [12].

#### **8.4 Biotechnological methods**

Traditional techniques have revealed themselves inadequate for improving cultivars because of the low success of cross-pollination, the high incidence of incompatibility and phytosanitary issues [49]. A valid alternative for this plant's breeding are biotechnological methods as *in vitro* cloning and genetic transformation [46].

#### *8.4.1 In vitro regeneration systems*

Tamarillo micropropagation methods have been described in various assays [49]. *In vitro* cloning can be achieved through (1) axillary shoot proliferation, which was the first method to be applied [10] (2) organogenesis, obtained on leaf explants [50] and (3) somatic embryogenesis, first obtained from mature zygotic embryos and hypocotyls cultures, and later from other explants [51, 52].

#### *8.4.1.1 Somatic embryogenesis*

Plant multiplication has the potential to be revolutionized by somatic embryogenesis, a powerful biotechnology tool. Auxin rich media is used to induce somatic embryogenesis in tamarillo, where embryogenic callus is first generated (induction phase) and subsequently developed into embryos after being switched to a medium free of auxin (development phase). Several explants of tamarillo have the potential to initiate embryogenic cultures including, mature zygotic embryos, young leaves, cotyledons and hypocotyls. Tamarillo mature zygotic embryos were one of the first explants tested for somatic embryogenesis. The formation of embryogenic tissue offers a great potential for large-scale production of plantlets [53] and is also useful in plant genetic transformation. Somatic embryos pass through different morphological phases similar to those occurring during zygotic embryogenesis [54]. The subculturing of somatic embryos, on auxin-free medium for a further 4–5-week period give rise to green normal plantlets.

#### *8.4.2 Genetic transformation*

A number of viral infections impact the health and vigor of tamarillo trees, as well as the appearance of the fruit. The tamarillo mosaic virus, the most important pathogenic virus, has no known resistance in Cyphomandra species (TaMV). There has been little success in using traditional breeding programmes to increase tamarillo's virus resistance. Plants can now be genetically modified to be resistant to a variety of viruses, thanks to recent advances in molecular biology, including the tobacco mosaic virus. *Agro bacterium* mediated transformation of tamarillo has been successfully achieved and protocols for transformation are now available [55]. The use of genetic transformation offers the great opportunity for the improvement of many characters for which there is not enough genetic variation in the local germplasm. Genetic transformation is also being used as a functional genomics tool, helping to better understand the process of somatic embryogenesis [6]. The most successful strategy so far has involved constitutive expression in the host plant of the coat protein gene of the target virus. Recently, the coat protein gene for TaMV has been cloned and sequenced

which may allow TaMV resistance to be engineered into tamarillo. The application of the *Agro bacterium* mediated transformation method to obtain genetically modified tamarillo plants regenerated *via*, organogenesis was used to introduce the pKIWI110 binary vector into leaf disks by using a virulent LBA4404 [56] and some of them also expressed the b-D-glucuronidase (gusA) reporter gene and chlorsulfuron resistance. *Agrobacterium* mediated transformation was also used to obtain tamarillo plants resistant to tamarillo mosaic virus (TaMV) that were regenerated by shoot proliferation [6]. The NEP25 gene has been silenced in tamarillo using Agrobacterium-mediated genetic transformation procedures [10]. After undergoing somatic embryogenesis, these plants have been regenerated and are currently being evaluated to see if they have the same capacity for somatic embryogenesis as untransformed plants.

#### **8.5 Breeding for yield**

At present, the tamarillo is being introduced as a promising crop in a variety of environments, e.g., regions with a Mediterranean climate [37, 57]. But most attempts to introduce tamarillo culture have been based on a single cultivar and this has restricted the opportunity to exploit variation for local adaptation. For this purpose, germplasm screening is essential in order to select the most adapted types in local environments. Several evaluations of tamarillo germplasm have shown a high degree of genetic variation for yield and fruit weight. Differences among different plant genetic materials can be as high as two-fold for fruit weight [37, 57]. The fruit yield is highly affected environmental components and exploitation of most adapted material could maximize the selection for yield.

#### **8.6 Breeding for quality**

Among fruit quality parameters fruit shape is of interest because it is directly related to consumer acceptance because it affects fruit attractiveness and is important in terms of packaging and presentation. The fruit shape varies from round to elongated, with a ratio length/width higher than two among accessions. Round to oval shapes appear to be preferred. Fruit color varies from yellow types to purple and it may have stripes or not. Differences among cultivars have been found organoleptic characters like soluble solids, titratable acidity, ascorbic acid and other characters [37, 58–60]. Cultivars with a higher sugar/acid ratio are probably more suited for processing industry. Sugar/acid ratio and sweetness were found to be higher in one of the seven accessions studied by [38] that had reduced acidity (between 15% and 23% and similar levels of soluble solids). Recently developed 'Oratia Red' and 'Andys Sweet Red' have a high sugar/acid ratio (Boyes and Strubi, 1997). Breeding for nutritive value is also possible as lot of variation for ascorbic acid content and vitamin A have been found among genotypes [19, 38, 58, 60]. Aromatic compounds of tamarillo have been identified [61, 62] and selection of high aromatic cultivars is of interest. However, aroma is a complex character, in which many interactions among different compounds are involved and in which environmental influence is considerable.

#### **8.7 Breeding for disease resistance**

Few studies have been devoted to tamarillo breeding for disease resistance. Most work has dealt with TaMV resistance. Resistance to TaMV has not been achieved yet, even in wild species belonging to the genus *Cyphomandra*. Genetic transformation

*Tamarillo (*Cyphomandra betacea *(Cav.)) Origin, Cultivation, Breeding and Management DOI: http://dx.doi.org/10.5772/intechopen.106601*

is being used to develop tamarillo cultivars resistant to TaMV. Strategies used up to now have mostly involved the use of genetic constructs which include sequences of the coat protein of this virus. The use of mutagenic agents, such as nitrous acid for the production of defective TaMV strains, which could be used for cross protection has not been successful [56]. Resistance to anthracnose is also being attempted by *in vitro* selection of cells capable of growing in the presence of crude filtrate of the fungus [63]. However, there are no reports of the efficacy of this strategy in developing mature plants resistant to this disease.

#### **8.8 Breeding for early harvesting**

Early harvesting is of significance since different cultivars differ in how quickly their fruit ripens [64]. This could be exploited to select the earliest cultivars. There is also quantitative variation in the response to postharvest applications of ethylene in some cultivars so it is possible to achieve successful postharvest ripening [65]. Despite being considered a non-climacteric fruit [65], ethylene applications stimulate ripening [66, 67]. Fruit can be harvested during the turning stage and allowed to ripen after harvest in materials where ethylene induces ripening. Fruits that are turning may be stored in this way and matured as needed.

#### **8.9 Breeding for parthenocarpy**

Occasionally, parthenocarpic (seedless) fruit-producing trees can be discovered in orchards. These trees require vegetative propagation because they are the result of spontaneous mutations. Fruits from parthenocarpy are ovoid-shaped, red to orange in color, and have stripes that range from green to coffee in hue. Orange is the color of the flesh. Fruit weighs normally only 20 g and is smaller than other fruit. Due to their lack of seeds, parthenocarpic tamarillos would be very intriguing; nevertheless, before this form of fruit can become well-known, low weight and poor yield issues must be resolved.
