Nigerian Root vegetables (NRVs) such as Ginger (Zingiber officinale), garlic (Allium sativum), turmeric (Curcuma longa), onion (Allium cepa) and carrot (Daucus carota) can be defined as vegetables cultivated for their edible underground parts.
This chapter describes the various facets, from agronomy to marketing, of Nigerian root vegetables including garlic, onion, turmeric, ginger and carrot being the world’s most significant and vital root vegetables which have high culinary and medicinal value.
This chapter also presents the socio-economic, market analyses, export potential of these crops in Nigeria. It would be an important reference material for researchers, agricultural and food science students at both undergraduate and postgraduate level and policy makers; and be of great interest to experts and industries involved in root vegetables and spices trade.
The in-depth information and knowledge about the genetic conservation, socio-economics, production, pests and diseases management and post-harvest technology of root vegetables in Nigeria provided in this chapter would greatly help in efforts towards improving their production and utilization for enhanced nutrition and healthy living.
Vegetables are plants cultivated for their leaves, succulent stems, young shoots, fruits or a combination of these parts. They form an integral part of everyday diet and include diverse plant species with considerable economic and medicinal properties. Vegetables are protective foods for maintenance of health and for prevention of diseases.
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True roots such as taproots can be botanically differentiated from tuberous roots from non-roots such as bulbs, corms, rhizomes, and tubers, the word “root vegetable” being applied to all of them as it pertains to their agricultural and culinary usage.
Root vegetables are rich in nutrients such as minerals, vitamins and fiber, and play important and valuable roles in nutritional, health, economic, social, cultural and ecological aspects of rural and urban communities in Nigeria and all over the world.
Root vegetables constitute an important component of the Nigerian cultural heritage where they play vital roles in the tradition, food and income security of many households. Root vegetables (RVs) are able to make significant contributions to food security and nutrition, enhance livelihoods of marginal and smallholder farmers as well as improve the wellbeing of households.
Advantages of planting NRVs include the ease of incorporating them into existing cropping systems, provide relatively higher earnings than most of the cash crops, they can be produced on small and barely productive lands, can be successfully cultivated under varying climatic conditions, and short production cycles.
In addition, they require few purchased inputs, requires few resources and produces high yields with robust nutritional values. To realize higher return and the desired impact, it will be necessary to increase production of NRVs within the major areas of production as clusters of micro-enterprises.
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Pests and diseases can have devastating effects on vegetable crops if not well managed. According to [1], damage caused by pests on vegetable at various growth stages on the field to harvesting, storage and also during conveyance can lead to 5-40% crop loss annualy which poses a devastating effect on food and nutrition security for the rising Nigerian population.
However, various approaches have been employed by researchers to tackle the menace of pests and diseases in root vegetable crops ranging from cultural practices, biological control, use of indigenous knowledge, plant extracts, pheromones, synthetic pesticides, use of improved planting materials and Integrated Pest Management (IPM) approach.
Collection and Conservation of Nigerian Root Vegetable Genetic Resources
With high rate of population growth, demand for agricultural crops worldwide is expected to increase with a higher purchasing power per person which implies higher consumption and use of agricultural produce [2, 3, 4, 5].
Genetic materials of horticultural crops and their wild types are of great importance for food and nutrition security, and also serve as good sources of fodder, fuel, shelter, as well as sources of high-value industrial products to meet the high demand of an increasing global population.
Genetic materials also provide useful sources of genetic variation required by plant breeders for crop improvement, and a broad genetic base within the gene pool is necessary to expand the scope of identifying and introgessing desirable genes underlying agronomically-important traits [6].
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Genetic diversity has huge value for present and future generations, and more efforts must be made for its conservation and sustainable utilization [7]. Despite the usefulness of genetic resources, available genetic variability including landraces is getting eroded at an alarming rate, causing an enormous reduction of variability.
This situation thus requires fast action to conserve germplasm [6]. The conservation, likewise sustainable use of germplasm is necessary in the promotion of food and nutrition security and gives room for securing diversity to respond to future challenges including the increasing climate change [8].
The Genebank of the National Centre for Genetic Resources and Biotechnology (NACGRAB) holds one of the most extensive collections in Nigeria, with a total record of over 10,500 accessions of 40 different crops, comprising of wild relatives, landraces, as well as old and more recent cultivars of crops, with germplasms of various vegetables being well represented.
Vegetables include various genera and species; and are a vital element for a balanced diet which supplies vitamins, antioxidants, minerals, fiber, amino acids, as well as other compounds that improves health, and contribute to nutrition security [9].
Roughly a million accessions of crops that are entirely or partly used as vegetables are conserved ex-situ [9]. However, only 7% of the total global ex-situ conservation are fully used as vegetables which are mostly leafy or fruit vegetables. Root vegetables are not well represented in genebanks as they are mostly conserved in the fields; thus, are potentially exposed to pest and diseases and environmental variation.
To avoid the risk of losing available germplasm, such crops are however amenable to in vitro conservation which is cost and labour intensive and requires consistent material transfer to fresh growth media. Root vegetables like ginger are conserved in vitro at the NACGRAB’s genebank in Nigeria.
Of the ca. 7.4 million accessions of Plant Genetic Resources for Food and Agriculture (PGRFA) that are ex-situ conserved globally, only 7% (i.e. ca. 518,000 accessions) are vegetables [10]. Of these global vegetable collections, only alliums are well-represented root vegetables in ex-situ collections [9].
As a result, the level of representation of root vegetables in both National and global collections, calls for the need to explore, collect and conserve more of the variabilities in root vegetables. Though we still have the benefit of a vast agrobiodiversity, there is the need to be conscious that two out of five plant species are endangered with losses occuring on a regular basis [11] due to the increasing climate change, extension of human settlements, and substitution of landraces with hybrid cultivars.
Here's a table summarizing the representation of vegetables in global ex-situ conservation:
| Category | Percentage of Total PGRFA Accessions |
|---|---|
| Vegetables | 7% (ca. 518,000 accessions) |
| Root Vegetables | Underrepresented (except for alliums) |
Biotechnology: Tool for Root Vegetables Production and Improvement in Nigeria
Biotechnology application has provided unparallel opportunities for plant production and quality improvement [13, 14, 15]. Possibilities for improving agronomically-important traits are limitless with the biotechnological applications like in vitro culture techniques, marker-assisted breeding technologies, genetic engineering, genome editing or a combination of all the novel gene technologies.
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Onions
Onion being a crop that is propagated by vegetative means with high heterozygosity, the cultivars are often low in reproductive fertility, making the breeding of this diploid species a challenging effort. Onion has a relatively long breeding cycle and genetic complexity, as well as sensitivity to inbreeding depression which determines the conventional methods adopted for onion improvement.
Traditional breeding methods for onion involve mass selection for disease/pest resistance, improving or maintaining quality traits such as a bulb color, shape and increasing yields and shelf-life. This has given rise to the development of many good clones. According to [12], the yield potential of onion has remained relatively constant over centuries despite rigorous breeding efforts.
Conventional breeding of diploids plants often involves screening and backcrossing of large number of plants in order to obtain the desired genotype. Selection of many desirable traits at the initial stage can be ineffective and/or time consuming. Hence, onion breeders have most times needed to screen a large number of seedlings (up to a million) to enable identification of a single clonal line that can pull through to the release of a successful cultivar.
To increase onion production and improvement through biotechnology, the following directions for research of this crop plant are suggested:
- ploidy manipulation as an alternative technique to induce haploids in onions [16]. More haploid regenerants should be produced by ploidy manipulation to improve onion for breeding purpose
- embryo rescue techniques to enable successful intergeneric and interspecific hybridization as has been widely reported for other species [17]. Hybrid embryo developments have been developed through embryo rescue. These hybrids could have horticulturally- and agronomically- important traits
- protoplast regeneration and fusion have been used to improve a plant’s agronomic and horticultural characteristics such as pests and diseases resistance [18]. This technique should be used to produce more somatic hybrids that are pests and diseases resistant. Somatic variation can lead to creation of additional genetic variation in onion. Tolerance to herbicides, environmental or chemical stresses have been developed via this technique [19]. Somatic hybridization is an alternative technique to overcome both intraspecific and interspecific cross incompatibility to a large extent [20], and this technique could be used to introduce horticulturally- and agronomically-important genes in onions.
As genetic diversity is the basic input for breeding programmes [21] an understanding of genetic diversity among Nigeria onion germplasm collection is imperative for onion breeding.
Ijeomah et al. [22] studied and revealed the genetic diversity of 10 cultivars of spring onions in Nigeria using one SSR and three ISSR markers. The four markers yielded a total of 26 polymorphic alleles with polymorphic information content (PIC) values ranging from 0.6402 to 0.7569. The resulting UPGMA dendrogram showed that the 10 cultivars studied formed two main clusters with one subgroup showing no genetic distance among them. This study indicated the efficiency of SSR and ISSR markers to estimate the extent of genetic polymorphisms of spring onion cultivars with potential utility towards the conservation and management of Allium species.
Ginger
Most ginger improvement efforts have been restricted to evaluation and selection of naturally-occurring clonal variants. Conventional crossing efforts have been largely ineffective as a result of rare flowering and poor seed setting. Efforts at evolving high yielding clones through mutation and polyploid breeding indicated lack of success [23, 24].
Furthermore, the seed stock (rhizomes for vegetative propagation) seriously suffers from fungal and bacterial diseases such as Pseudomonas solanacearum (bacterial wilt), Fusarium oxysporum (yellow leaf), Pythium aphanidermatum (soft rot), Phyllosticta zingiberi (leaf spot), leading to heavy crop losses [25, 26].
Underground rhizomes are usually used as vegetative propagules for ginger which accounts for its very low multiplication rate [25, 26]. Different explant types used for micropropagation of ginger and other related species include meristem, axilliary buds, shoot tips and aerial pseudostems, although the commonly used explants are rhizome buds and shoot tips which have been reported as responsive explants for large-scale micropropagation to generate pathogen-free propagules [27].
An optimum fragment (explant) size is required for initiating successful tissue cultures. Sathyagowri and Seran [28] reported that rhizome buds of 0.5 cm in length were best for initiating ginger in vitro culture and shoot multiplication among the different tested explant sizes of 0.5, 1.0 and 2 cm long.
The establishment of clean in vitro culture ginger from rhizome explants can be a daunting task as these underground explant is laden with pathogens resulting in contamination of cultures. As a general rule, absence of browning and freedom from contamination are criteria for the explants’ survival for subsequent shoot multiplication.
Surface sterilization of explants is commonly carried out with disinfectants such as ethanol (C2H5OH), sodium hypochlorite (NaOCl) and mercuric chloride (HgCl2). Ginger aseptic cultures have been obtained by surface sterilization of the rhizome buds explants with 0.1% HgCl2 solution for 10-20 min [16], turmeric [29].
Rout et al. [30] established a surface sterilization protocol for ginger sprouting buds explants using 2% (v/v) Teepol for 15 min followed by 0.2% (w/v) HgCl2 solution for 25 min and several changes of sterile distilled water.
Contamination-free in vitro culture of ginger can also be optimally achieved by sterilizing the rhi...
