Review on Tomato (Solanum lycopersicum, L.) Improvement Programmes in Ghana

Tomato is an important component of every Ghanaian meal, and its cultivation contributes significantly to livelihood improvement. The demand for tomato in Ghana outstrips supply, and therefore local production is augmented by imports from neighbouring countries. Despite the importance of tomato in Ghana, past tomato-breeding programmes have been unsystematic and had not led to the development of new varieties that meet the needs of consumers as well as environmental stresses. This review outlined tomato production trends, constraints and past tomato improvement programmes in Ghana, which mainly focused on germplasm collection, morphological and agronomic characterization, molecular evaluation, diversity study, as well as screening germplasm against biotic and abiotic stresses. The established variability and the outcomes of the evaluations against the various biotic and abiotic stresses have not been utilized in the development of new varieties. This work will serve as a reference for developing future tomato-breeding programmes.


Introduction
Tomato (Solanum lycopersicum, L.) belongs to the Solanaceae family also called Nightshades, which include more than 3000 species [1]. Other examples of crops within the Nightshade family include pepper, potato, eggplants and tobacco. Tomato originated from the Andean region, which is modern day Chile, Bolivia, Ecuador, Colombia and Peru; however, the original site of domestication is unclear [2]. Two hypotheses have been expressed for the original site of tomato domestication: one stipulates Peru and the other Mexico. It is, however, presumed that Mexico is probably the site of domestication and Peru is the centre of diversity [3]. Originally, tomatoes were pea-sized berries but domestication and plant breeding have resulted in increased fruit sizes [4].
Tomato continues to be the most important vegetable in the world due to increasing commercial and dietary value, widespread production as well as model plant for research [5]. Tomato is utilized as a fresh crop or processed into various forms such as paste, puree and juices. Tomato is a rich source of vitamins (A and C), minerals (iron, phosphorus), lycopene, Beta-carotene, high amount of water and low calories [6]. The five leading producers of tomato in the world are China, India, United States of America, Turkey and Egypt [7]. The world's tomato production in 2014 was 171 million tonnes with an average yield of 37 tonnes per hectare [8].
According to Norman [9], tomato (S. lycopersicum) was introduced into the geographical area considered modern day Ghana in the sixteenth century. Although the cultivation of tomato remains a subsistent farming activity, its cultivation and trade contributes significantly to livelihoods improvement [10]. Schippers [11] asserts that tomato is the most important vegetable in Ghana, compared to all the other vegetables. This view can be justified with the continuing increase in the demand for fresh and processed tomatoes in Ghana. With an average yield of about 8.1 tonnes per hectare in 2013, an estimated 340,218 tonnes of fresh tomatoes were produced locally and 5,945 tonnes was imported. In addition, 109,513 tonnes of processed tomatoes were imported within the year 2013 [12]. In the ensuing year, reported tomatoes statistics showed that there were increases in the local production volumes (366,772 tonnes), marching the increase in output per hectare of 8.6 t/ha [13]. The high volumes of tomato produced locally as well as imported are an indication of the importance of tomato in every Ghanaian meal.
Despite the importance of tomato in Ghana, tomato-breeding programmes over the years have not been systematic and therefore had not led to the development of new varieties that meet the needs of consumers as well as biotic and abiotic stresses [14]. The major goals of tomato breeding worldwide are increasing yield, tolerance to biotic and abiotic stresses and improvement in sensory and nutritional value of the crop [15]. Consequently, past Ghanaian plant breeders have focused on germplasm collection, evaluation of imported and local accession for morphological and agronomic traits as well as screening accessions for their reactions to biotic and abiotic stresses. Nonetheless, there have been little published breeding programmes in the past that focussed on improving fruit-quality traits or introgression genes that will make cultivars resilient to both biotic and abiotic stresses. In 2014, the Ghana National Tomato Federation stated that the union has been pushing government to support research in the development of high yielding and quality tomato variety suitable for local and export market [16]. This chapter therefore highlights tomato production trends in Ghana, tomato production constraints, past tomato-breeding programmes in the country and future tomato-breeding objectives, which will serve as a locus for developing future tomato-breeding programmes.

Tomato production trends and constraints in Ghana
Tomato is mostly produced in seven out of the 10 regions in Ghana. These production regions include Upper East region, Northern region, Brong Ahafo region, Ashanti region, Eastern region, Greater Accra region and Volta region. The demand for both fresh tomato and tomato products is year round although tomato production in Ghana is seasonal due to the differences in the rainfall patterns as well as water availability. In the exception of the Upper East Region where tomato is produced during the dry season under furrow irrigation system and some parts of the Greater Accra region, tomato production is generally rain fed. During the rainy season, harvest is abundant, leading to glut and wastage even though there is scarcity during the dry season. The abundance of tomato during the rainy season results in low prices and low return on investment. Tomato produced during the rainy season is supplied to the market from May to October but the varieties produced during this period are poor in colour, watery, acidic and have a shorter shelf life, making them unsuitable for processing. Due to the unavailability of processing tomato varieties, all the three state-owned tomato-processing factories had to shut down. Tomato varieties that are currently grown by Ghanaian farmers are mostly imported varieties and farmers selected varieties. A very important open-pollinated variety (OPV) grown in Ghana particularly in the Brong Ahafo region is the Power Rano (a cross between Power and Laurano varieties) which was identified by the National Research Institute (NRI) researchers in the 1990s based on its good production and local processing qualities [17].
Dry season production in Ghana on the other hand is challenging, and demand is in excess of supply. This period partially coincides with the Christmas season when demand for tomato is at its peak. In order to meet the dry season demand, there is heavy importation of fresh tomato from neighbouring countries, particularly Burkina Faso to augment local supply. Some parts of the Greater Accra region such as Ashiaman, Tema and Weija grow tomato under irrigation system and mostly supply tomato unto the market from September to December, and the Upper East region then continues tomato supply from January to April. Imported tomato from Burkina Faso supplements local production 5-6 months of the year [18] with a peak supply from February to April [19]. It has been established that, with the availability of water and favourable night temperatures, the highest quality and fruit yield of tomato is obtained in the dry season [20]. In Ghana, the capacity for dry season tomato production lies in the savannah zones, particularly the Upper East, Volta and the Greater Accra regions since water for dry season irrigation is not a limiting factor in these regions. Tomato production halted in the Upper East region in 2002 due to Tomato Yellow Leaf Curl Disease (TYLCD) and a complex of fungal pathogens [21]. In addition, over 600 tomato farmers in the Agotime-Ziope District of the Volta region were reported to have lost virtually all their investment following the TYLCD infection (in 2014) of over 1000 hectares of tomato farms in the area [22]. A high night temperature, a high prevalence of TYLCD and inadequate irrigation facilities to channel the available water are characteristics of dry season production of tomato in the Greater Accra region. Ghana's inability to produce tomato during the dry season therefore has been attributed to a lack of irrigation facility, a high incidence of Tomato Yellow Leaf Curl Disease [23,24] as well as high night temperatures [25].

Past tomato-breeding programmes in Ghana
Tomato-breeding programmes in Ghana can be traced to the 1950-1978 when cultivars like OK, MH and Wosowoso were developed. A major tomato-breeding programme led by the National Research Institute (NRI) in UK also carried out a study from 1994 to 2000. Post 2000, tomato improvement programmes focussed mainly on screening tomato germplasm for both biotic (particularly the TVLCD) and abiotic stresses as well as mutation breeding; however, none has led to the release of varieties. Robinson and Kolavalli in 2010 stated that since the NRI tomato-breeding work ended in 2000, there have been no breeding programmes and no systematic seed multiplication in the country [26]. Again, a 2013 publication indicated that the varieties developed during the 1950 to 1978 together with farmers' selection in tomatogrowing areas have led to the development of large tomato ecotypes in Ghana [27].  [28]. Recently, 13 accessions were also collected from Afari, Akumdan and Akuawu in the Ashanti region. The recent germplasm collected included accessions such as 'Atoa', 'Daagyine', 'Local 1', 'Power', Pectofake 1, Petomech, 'Akoma', Pectofake 2, Powerano, 'Bolga', 'Dwidwi' (cherry), 'Local 2' and Rano [29]. Most of the locally collected germplasm and introduced accessions have been evaluated for various agronomic and morphological traits as well as the establishment of genetic variation that exists within this germplasm. The Savanna Agricultural Research Institute evaluated three tomato varieties (ICRISIND, Petomech and Tropimech) for various agronomic traits. Variations were observed in plant height, days to flowering, number of fruits, fruit size and fruit weight [30]. Again in 2013, SARI evaluated the following accessions: S 22, Naywli, Bebi yereye, LBR 7, Keneya, LBR 17, Abhijay and Petomech for variability in various agronomic traits [31].

Germplasm collection and genetic diversity studies
S. pimpinellifolium possesses some desirable traits that can be utilized to improve cultivated varieties; however, the size of the fruit is a hindrance to domestication. In order to improve on the size and other desirable traits, a group of researchers at the Biotechnology and Nuclear  [32]. The five induced variant lines used in the previous study were also analysed for lycopene, total antioxidant properties and other quality factors such as pH, total soluble solids (TSS) and total solids. Similarly, 10 F5 tomato-breeding lines were characterized for variability in physico-chemical properties (colour, pH, total titratable acidity (TTA), TSS and vitamin C). The lines used include wosowoso (parent variety), cherry yellow, roma variant (a prolific trait), wosowoso variant (stripped, prolific and big fruit), roma variant (bicoloured fruit), S. pimpinellifolium parent, roma variant (hardened and big fruit), roma variant (yellow skin), roma variant (red skin) and wosowoso variant (big fruit, and deep red color). The lines varied in the various physico-chemical properties measured [33]. In addition, fruits of F4 lines derived from crosses between some varieties of S. lycopersicon, cherry red, cherry yellow and roma, and wosowoso with a wild tomato, S. pimpinellifolium, were analysed for physicochemical properties, and variation was seen among the lines for the traits studied [34].
In 2014, five introduced fresh market tomato varieties from the USA and Crops Research Institute of Ghana (CRI) were evaluated for genetic variability, adaptability in Ghana as well as plant and fruit attributes. The varieties included Heinz, Shasta, Op-B149, Op-B155 and CRI-P00. With the advent of molecular markers, this study used 15 Simple Sequence Repeat (SSR) primers ( Table 1) to determine the genetic diversity existing among the five introduced fresh market tomato varieties [35]. In order to establish the genetic diversity that exists in the germplasm collected in 2015, all the accessions (in exception of Rano) were evaluated in field as well as molecularly characterized using 12 SSR primers. The SSR primers include Tom 8-9-F, Tom 11-28-F, Tom 55-56-F, Tom 59-60-F, Tom 67-68-F together with seven primers listed in Table 1 [36]. In the same year, 20 tomato genotypes were evaluated in the greenhouse as well as the field at the University of Ghana Forest and Horticultural Crops Research Centre (FOHCREC), Okumaning-Kade in the Eastern Region of Ghana to determine the genetic variability in agronomic and fruit-quality traits. There was variability in almost all the traits studied [37].
The various findings of the germplasm evaluation for morphological and agronomic traits together with the variability that exists in the germplasm can be explored in the development of new varieties.

Breeding for fruit quality
Cultivars such as OK, MH series [38] and Wosowoso [39] were developed in the 1950s. Agble [40] also began breeding for processing quality traits, shelf life and heat tolerance lines by making crosses between local accessions with heat-tolerant and nonripening gene (nor A ) from exotic accessions. Nonetheless, due to lack of continuity, no variety was released despite the positive outlook [41].
The NRI focused on pure line selection of local landraces in the Brong Ahafo region of Ghana with the aim of releasing pure lines of good open-pollinated varieties. Six varieties consisting of three local and three introduced varieties were used in that study. These varieties were selected based on farmers and traders (fruit quality, good taste and longer shelf life) preferred traits. As part of this project, a tomato breeder seed production trial was then established at Wa in the Upper West region with the five selected varieties. The research was, however, not very successful because there was no long-term impact due to lack of sustainable seed distribution systems to ensure that the resource-poor farmers have access to the developed varieties [42].

Recent Advances in Tomato Breeding and Production
From 2011 to 2013, pure line selection was used to advance a locally identified cultivar commonly called petofake. From the segregating population collected from farmers, 12 progenies (P002, P005, P011, P020, P026, P035, P057, P068, P074, P077, P082 and P085) were selected based on their fruit shape, size, color, surface and yield [43]. Trials are ongoing to release these lines.
Dried seeds of SP 300/30.4.2.4, a variant line selected from second generation (M2) following the irradiation of S. pimpinellifolium at 300 Gy, were used for a study. Also, seeds (2000) of SP 300/30.4.2.4 were re-irradiated at 150 and 300 Gy and included in the study. From the study, it was found that the irradiation led to a reduction in plant height and a larger fruit size. Variation was also observed in color, plant height, architecture, number of days to flowering and fruiting. This variation can be explored in future breeding programmes [44].

Breeding for biotic stress
Post 2000 has seen some breeding efforts made in screening tomato accessions against biotic stresses. However, most of these programmes focussed on the most devastating tomato disease (TYLCD).

Screening germplasm for tomato yellow leaf curl disease resistance
TYLCD is a major tomato disease in Ghana and Africa as a whole and can lead to a massive yield loss and consequent impact on livelihood if the vector of the disease (whitefly) is not controlled and infection starts at an early stage of the plant growth [45]. The Tomato Yellow Leaf Curl Virus (TYLCV) causes the TYLCD. It was reported that the USAID West African Regional Programme identified research on Virus resistance (VR) as a priority, and Ghana was included in seven members' regional investigation of tomato virus complex [46]. The Agricultural Biotechnology Support Project II (ABSPII) aimed to improve agriculture production in the developing countries through Biotechnology, and that is why this project was initiated in 2005 to address tomato production in West Africa. This project was a partnership among researchers from AVRDC, Cornell University and University of California-Davis (UC Davis). The ABSII established the Regional Vegetable Germplasm Trailing Network that evaluated 100 putatively TYLCD-resistant tomato varieties that were adaptable to the growing conditions of West Fifteen tomato accessions (collected from AVRDC-Taiwan and CSIR-Crops Research Institute, Ghana) that have been reported to be resistant to TYLCD as well as susceptible checks were screened against the TYLCD in a greenhouse at the Kwame Nkrumah University of Science and Technology (KNUST) in Kumasi ( Table 3) [50].
Again, between 2010 and 2011, seven tomato varieties ( Table 6) were grown in the fields against the TYLCD in the University of Ghana and the Volta region of Ghana. The symptom expression of the varieties against the TYLCV was confirmed in the laboratory using the set of primers in Table 5 in addition to Beta 01/02. The study also identified Ty-3 gene in tomato that confer resistance to TYLCV using the primers in Table 7. From the field screening, it was found that Burkina (obtained from farmers in the Volta region) had the highest TYLCD incidence, followed by Petomech and the susceptible check. However, Petomech expressed higher severity than Burkina. Both severity and incidence were lower in the hybrids in exception of F1 Thorgal that showed no symptom. AC1048/AV494 detected the most viral DNA in the samples collected. The primer set T0302-F/T0302-R did not amplify the Ty-2 gene in any of the varieties evaluated. However, Primer P6-25-F/P6-25-R amplified a band size of approximately 400 bp in F1 Jaquar, F1 Nadira and S. pimpinellifolium [51].  Table 3. Tomato accessions used for the TYLCD screening in both the greenhouse and the field.

Recent Advances in Tomato Breeding and Production
Between 2011 and 2012, a group of researchers also evaluated the susceptibility of 10 accessions to TYLCD under field conditions. The accessions include S. pimpinellifolium, Wosowoso, Cherry red, Roma, Hyb−1 (Wosowoso × S. pimpinellifolium), Hyb-2 (Roma × S. pimpinellifolium), Hyb-3  Table 4. A list of tomato accessions screened against the tomato yellow leaf curl disease in Afari.
The observed symptoms on the hybrids together with the backcrosses were slight yellowing of margins of apical leaflets and moderate yellowing and slight curling of leaflet tips. The results from the phenotypic screening were verified with a molecular marker detection of  the viral DNA among the accessions. This work also deployed both triple antibody sandwich enzyme-linked immunosorbent assay (TAS-ELISA) and PCR method (using the primers in Table 5) for the TYLCV detection in order to recommend a better way of detecting TYLCV in infected samples. A TAS-ELISA kit with a known TYLCV-infected Nicotiana benthamianapositive control was used for the study. The study confirmed the superior sensitivity of the PCR technique as a TYLCV detection method compared to the TAS-ELISA technique. There were no observable TYLCV symptoms on the BC-3 (C-Red × (C-red × S. pimpinellifolium)) in the field and both methods did not detect viral DNA in the leaf samples. BC-1 (Wosowoso × (Wosowoso × S. pimpinellifolium)) behaved similarly like BC-3 in the field but there was amplification of viral DNA by the AV494/AC1048 primer set. In addition, two PCR primers detected viral DNA in the S. pimpinellifolium even though there was no TYLCV symptom observed in the field.
Recently, there was a phenotypic evaluation of 36 local tomato genotypes ( Table 8) for the source of resistance against TYLCD in two locations (University of Cape Coast and Asuansi) in Ghana. The results showed that five accessions (K005-Petomec, K100-Local 3, K213-AVTO 9804, K116-Ashanti 2 and K042-Tomatose) out of the 36 genotypes were selected for mild severity, two genotypes showed severe symptoms (K027-Local, K202-AVTO 0102) and one genotype (LV-Fadzebegye) showed moderate severity. In order to confirm the infection or otherwise of the eight tomato accessions selected for mild and severe symptom expression, two of the viral detection primers (AV494/AC1048 and PTYv787/ PTYc1121) were used for the detection of the virus in infected plant samples (

Molecular screening of tomato germplasm for root knot nematodes resistance
This study involved the use of primer Mi23/F//Mi23/R to detect the presence or absence of Mi genes in twenty eight (28) tomato cultivars ( Table 9). The primer amplified the homozygous resistant genotypes (Mi/Mi) in cultivars VFNT, FLA 505-BL 1172, 2641A, "Adwoa Deede" and Terminator FI while the heterozygous resistant genotypes (Mi/mi) were amplified in cultivars Tima and 2644A [54].  and it was found that Nkansah, King 5, 181 (CLN 2318 F) and DV 2962 cultivars were better adapted to heat stress [55].
The outcome of these various screening programmes can be utilized in a hybridization programme by crossing genotypes expressing mild symptoms to the TYLCV and nematodes as well as genotypes that are tolerant to heat with locally adapted accessions that are susceptible to these stresses to develop resilient varieties.

Potential tomato breeding objectives
Tomato varieties currently grown in Ghana are generally acidic, watery, poor in color, poor shelf life and susceptible to TYLCV as well as intolerant to heat. Future tomato-breeding programmes should focus in the short-term on introgression of Tomato Yellow Leaf Curl Disease Resistant genes into locally adapted varieties and improving the shelf life of these locally adapted tomato varieties. These will address the major constraints facing the tomato industry in Ghana. Longterm tomato-breeding objectives should encompass the improvement of fruit color, increasing brix, improving rainy season varieties with good fruit-quality traits, increasing variability through  irradiation, resistance to other biotic and abiotic stresses as well as sensory and nutritional value. Due to the pressing nature of these short-term breeding objectives, students of the West Africa Centre for Crop Improvements (WACCI), University of Ghana, are currently breeding for TYLCDresistant varieties and prolonged tomato shelf life. Other students of the same institution are also working on breeding for processing quality and Bacteria Wilt-resistant tomato varieties.

Conclusion
Tomato is indispensable in all Ghanaian recipes and contributes significantly to the economy of Ghana. Ghana has the potential to meet the country's tomato demand; however, low yield, unavailability of quality tomato varieties, pests and diseases have hindered this potential. This review presented tomato production trends in Ghana, past tomato-breeding programmes that have been carried out as well as some potential tomato-breeding objectives. Ghana will achieve self-sufficiency in tomato production if the government, Universities, Research Centres and National Research Institute (NRI) will invest more resources into tomato breeding to achieve both the short-and long-term-breeding objectives. This review will serve as a reference for improving tomato in the country.