On-Farm Crop Diversity for Advancing Food Security and Nutrition

In 2019, nearly 690 million people were hungry, indicating that the achievement of Zero Hunger by 2030 is not on-track. The enhanced conservation and use of crop diversity, which demonstrably improves farm productivity and hence food security and nutrition, could be one of the solutions to this problem. The broadening of the inter- and intra-specific diversity of crops contributes to dietary diversification and nutrition and improves the resilience of production systems to shocks, especially the biotic and abiotic stresses attributed to climate change. Examples of successful interventions that resulted in enhanced on-farm crop diversity are provided. Relevant tools and guidelines to strengthen national capacities for the enhanced on-farm management of plant genetic resources for food and agriculture are also highlighted. Guidance, based primarily on the Second Global Plan of Action for Plant Genetic Resources for Food and Agriculture , is presented to enable the conservation of farmers ’ varieties/landraces, their genetic improvement and seed delivery systems; promote their cultivation, consumption and marketing; develop and implement policies; foster partnerships and strengthen requisite institutional and human capacities. Finally, the case is made for research and development, including using modern techniques, to achieve these aims.


Introduction
The most recent edition of the report on the State of Food Security and Nutrition in the World [1] contains very worrying statistics: nearly 690 million people are hungry, i.e. 8.9 percent of the world's population! This represents an increase of 10 million people in a single year and nearly 60 million in five years. In fact, in 2019, close to 750 millionabout one in ten people in the worldwere exposed to severe levels of food insecurity. Conversely, the incidence of overweight children and adult obesity continues to rise [1]. Thus, the world is not on track to achieve Sustainable Development Goal (SDG) 2: Zero Hunger by 2030 [2]. Should recent trends continue, the number of people affected by hunger will surpass 840 million by 2030. It is crucial, therefore, to find effective, sustainable solutions to address hunger. As implicit in the Agenda 2030 [2], the eradication of hunger and malnutrition must be achieved through sustainable means, especially those that preclude further damage to the environment.
The conservation and use of crop genetic diversity is a key component of sustainable solutions to hunger and malnutrition as well as improving livelihoods. Unfortunately, this crop diversity is threatened by such factors as urban encroachment on farmland, unsustainable use of natural resources, the promotion of genetically uniform varieties in replacement of local varieties, introduction of alien invasive species, changing patterns of human consumption, absence of, or inappropriate, legislation and policy, as well as climate changes [3]. The loss of this genetic diversity reduces the options for sustainably managing resilient agriculture [4] in the face of adverse environments and rapidly fluctuating meteorological conditions. As such, it is essential to strengthen their improvement and management on-farm and to enhance their documentation and complementary conservation ex situ to safeguard these valuable resources [5].
The Second Global Plan of Action for Plant Genetic Resources for Food and Agriculture (Second GPA) [5] is the internationally agreed framework for the conservation and sustainable use of the full range of plant genetic resources used for food and agriculture, including farmers' varieties/landraces managed on-farm. The actions which countries commit to take in order to achieve these aims are enunciated in the Second GPA in 18 thematic Priority Activities, several of which are specific to crop diversity managed on-farm. Developed as the global policy response to the gaps and needs identified in the Second Report on the State of the World's Plant Genetic Resources for Food and Agriculture [6], the Second GPA provides guidance on: • promoting farmers' varieties/landraces, which is used as an indication of overall crop diversity in this chapter, through developing and strengthening national programmes; • increasing regional and international cooperation, including research, education and training and enhanced institutional capacity for the conservation and use of plant genetic resources for food and agriculture (PGRFA); and, • developing and implementing evidence-based policies to promote and improve the effectiveness of on-farm conservation, management, improvement and use.
This chapter highlights the importance of inter-and intra-specific crop diversity managed on-farm as a mechanism to address malnutrition and food insecurity, especially under worsening climate change scenarios. To promote the cultivation and use of the widest possible crop diversity, guidance, based on the relevant Priority Activities of the Second GPA, is provided. These encompass the actions necessary for the conservation and on-farm management of PGRFA; enhanced access to, and use of, local crop diversityincluding through responsive seed systems; and genetic improvement as means to the sustainable use of crop diversity. Relevant enabling policy instruments and initiatives for the conservation and sustainable use of crop diversity, developed over the last 50 years, are also described.

Important elements of crop diversity conservation and use
With about 80% of all foods being plant-based, any effective solutions for the current trend of worsening food insecurity and malnutrition must address the shortcomings of crop production systems. Crop genetic diversity not only represents the basis of food and agricultural systems, it is also an enormous reservoir of useful genes and gene complexes that endow plants with coping mechanisms for evolution and habitat changes [7,8]. The inter-and intra-specific variation of crops provides the basis for more productive and resilient production systems that are better able to cope with stresses such as drought or overgrazing [9]. This diversity also enhances the nutritional status of people [10][11][12]. Changes in land use, together with high rates of urbanization and emigration, displacement of traditional crops in favor of a few starchy staples, and abandonment of marginal agricultural areas, are posing an unprecedented threat to this diversity. Exacerbating this are the threats posed by climate change manifested through the increasing frequencies, distribution and intensities of extreme weather events.

The narrow genetic base of crop production systems
There are approximately 380,000 vascular plant species [13,14], of which less than 30,000 (or barely 7%) have been consumed as food by humans [15]. Of these, some 6 000 (or 22% of edible plants) have been actively cultivated for human consumption [16,17]. Despite this diversity, agricultural production systems depend on a narrow list of crop species. This is illustrated by the fact that less than 200 plants were the sources of global food production in 2019, with only nine of them (sugar cane, maize, rice, wheat, potatoes, soybeans, oil palm fruit, sugar beet and cassava) accounting for over 66 percent of all crop production and 53 percent of global average daily calories [3,18] (See Figure 1).
Agricultural production systems, based on just a few crops, are more vulnerable to biotic and abiotic stresses, including incidences of extreme weather events which, in the past, have resulted in crop failures. Compounding this, many local crops and varieties are cultivated as small and isolated populations and thus tend to lose genetic diversity [19]. These small populations undergo limited geneflow and are subject to genetic drift, founder effects and inbreeding. This, seen ever more frequently due to progressive introduction of commercial varieties, changing climatic conditions, migration to urban areas and expansion of land use for infrastructure and social development, represents an unprecedented threat to local crop diversity [20].
In order to address the impact of the above on changes on diversity, it is essential to monitor farmers' varieties/landraces on-farm [3]. Understanding changes in genetic diversity over time entails the assessment of: • species richness and evenness and associated environmental variables; • population size and genetic structure of farmers' varieties/landraces; and, • the impact of management or farming practices on populations. Further, at the genetic level, diversity can be assessed using a range of modern genomics-based approaches, such as molecular markers to determine changes over time as well as phylogenetic analyses. An overview of these approaches can be found in Bruford et al. [21] and Dulloo et al. [22]. The Second GPA [5] provides guidance on developing and strengthening systems for monitoring and safeguarding genetic diversity and minimizing genetic erosion of plant genetic resources for food and agriculture. Priority Activity 16 of the framework highlights the importance of establishing and implementing monitoring mechanisms for the regular assessments of genetic erosion. Information from extension services, local non-governmental organizations, seed sector and farming communities can be linked to early warning systems at the national and higher levels. This Priority Activity also underscores the need to enhance the use of advanced methods, such as those based on information and communication technologies and molecular and spatial analytical tools, for monitoring the status of the most threatened diversity in crops.

Challenge of climate change
Crop production is affected by the consequences of climate change [23], such as increasing temperatures, changing precipitation patterns, higher concentration of carbon dioxide (CO 2 ) in the atmosphere and the occurrence of extreme weather events such as floods and drought conditions. Climate change is also affecting biotic factors such as emergence of new pests and diseases and change in the virulence of existing ones. While specific impacts in crop production vary by crop and the climate in which they are grown, there is a growing scientific consensus that increasing temperatures will be detrimental, especially in many developing tropical countries where food insecurity and malnutrition remain pervasive.
Temperature increase and prolonged drought affect a range of biological processes. For example, the physiological responses of plants to high temperature and/or drought conditions are translated into negative effects on growth rates, and therefore on yield. Substantial declines in yields of important crops have already been reported and are predicted to particularly affect those regions where food security is already a major concern [24, 25]. Fruit and vegetable crops are highly vulnerable to climate change during their reproductive stages and to more disease prevalence, and thus production of these crops is also expected to be affected [26]. A detailed study on data from 23 countries in different regions undertaken by Iizumi and Ramankuttym [27] identified temperature variation as a key constraint to maize, soybean, rice and wheat yields. The study showed that the year-to-year variations in yields of these crops from 1981 to 2010 significantly decreased by 19% to 33%.
Climate change also alters the quality of plant nutrients by affecting soil biology, physics and chemistry, and therefore impacts the availability of nutrients [28]. Food quality might similarly be negatively impacted. For example, temperature increases over the past decades in Japan have led to earlier blooming of apples, which in turn has impacted acidity, firmness and water content, and thereby reducing quality [29].
Climate change is expected to alter the range and severity of pest and disease incidence [25]. Predictive models forecast that there will be either increases or decreases of incidence, depending on the region and its climatic conditions; however, the mean probability of pest and disease incidence is expected to rise globally [30]. Quiroz et al. [31] report that climatic changes in the Andean region have led to an increase in pest and disease occurrence in potato cropping, which is driving farmers to shift their production to higher altitudes.
The effects of climate change on major crops are well studied, particularly at species level (i.e. [32][33][34][35]). The majority of studies focus mainly on the yield of a specific crop under climate change, yet there are fewer studies comparing the effects on climate change on different varieties of the same species. The use of interand intra-specific crop diversity is central to traditional risk management practices in many farming communities (e.g., [36][37][38]). Such practices will be even more essential as the effects of climate change become more frequent and profound. Many farmers' varieties/landraces are suited to local ecosystems, climatic conditions and farming practices, and have been shown to be more resilient to unpredictable and hardy conditions [8,[39][40][41][42].
The Second GPA [5] addresses climate change in most of its Priority Activities, which responds to concerns about the impact of climate change on agriculture. As mentioned above, climate change impacts farmers' varieties/landraces cultivated, with the result that farmers will need to have access to new germplasm. Priority Activity 2 of the Second GPA draws attention to the need for adapted crop varieties to cope with future environmental conditions. It recommends that a range of initiatives and practices should be employed to help farming communities benefit from local crop genetic diversity in their production systems.

Diversified diets and nutritional components
Plants are the basis of nutritionwhether directly or indirectlyproviding key elements in the human diet. While it is clear that malnutrition overall is a major concern, the impact of malnutrition is disproportionately higher on women and children [1]. This can be addressed both through increasing the dietary diversity of the food consumed as well as increasing the quality of produce through breeding initiatives, such as biofortification, to develop nutrient-dense crop varieties.
In the last century, there have been major advances in food production, improving yields in many staple crops [43]. However, the focus of production has been on calorific intakeoften negatively correlated to nutritional value in terms of protein content and quality [44][45][46].
In response to the above, systemic approaches to agriculture now include nutrition as a key component. This is essential for ensuring not only that sufficient calories are produced but that other key health requirements are addressed [43,47]. In particular, there is a renewed interest in nutrient-rich neglected and underutilized species (NUS) [48][49][50][51][52][53]. While many of these species are environmentally resilient and cultivated in marginal areas as well as being rich in nutrients, bottlenecks for their increased production and consumption are common [16,43,54]. These include low yields, access to quality seeds and planting materials, low market demand and a lack of knowledge in their consumption. These issues, which occur along entire value chains, can be addressed through research and development (R&D) and coherent policy frameworks. In many cases however, financial resources are required to generate innovative solutions and build capacities for their implementation.
The Second GPA [5] provides guidance on promoting diversification of crop production; broadening crop diversity and promoting development and commercialization of all varieties, primarily farmers' varieties/landraces and underutilized species. Its Priority Activities 10 and 11 require that countries promote both the diversity of crops on-farm and the development and commercialization of the widest range of crops and their varieties, in particular farmers' varieties/landraces and NUS, respectively. Additionally, Priority Activity 11 highlights the need to develop and implement policies and incentives to create demands and the matching markets for the products of these crops.
Boxes 1 and 2 illustrate how local crops can be mainstreamed successfully, resulting in increased quality, availability and demand for these fruits and vegetables. The two examples presented, one in Micronesia and the other in Kenya, highlight the need for multisectoral approaches and strategies.
Vitamin A deficiency is one of the key causes of blindness in children [55]. This public health problem is prevalent in many countries, especially in Africa and South-East Asia [56]. One of the approaches for addressing the prevalence of Vitamin A deficiency has been to increase the nutritional diversity of local fruits and vegetables consumed.
Bananas are a key staple in many countries and one of the world's most popular fruits. Studies of different banana cultivars have revealed great differences in carotenoid content, from 5945 mgb-carotene/ 100 g in the some of the yellow/orange-fleshed Fe'i cultivars to 58mgb-carotene/100 g in the white-fleshed cultivar of the Cavendish subgroup [57,58]. Fe'i banana (Musa troglodytarum) is indigenous to the islands of the Pacific (Figure 2) and is known to be rich in Vitamin A.

Figure 2. Fe'i banana, showing the rich orange color of the fruit, an indicator of its high carotenoid content.
During the 1970s in the Federated States of Micronesia, diets based on non-local foods, together with an increase in consumption of refined white rice, flour, sugar, fatty meats and other processed foods [59], caused a serious Vitamin A deficiency [60]. In response, international agencies and local governments teamed up to promote the production and consumption of local banana cultivars, especially those identified as containing significant amounts of bio-available Vitamin A. The approaches included the development of policies promoting local cultivation, guidance on agronomic techniques, youth clubs, school activities and farmers' fairs. As a result of the various initiatives, the local production and consumption of the yellow/orange-fleshed banana variety, Karat, containing 2 230 μg/100 g of the provitamin A (50 times that found in white-fleshed bananas), was effectively promoted and these local nutritious bananas are now available in most markets. The success of this multisectoral approachhealth, agriculture and educationis regarded as a model, linking dietary and agricultural diversity for healthy diets, to be replicated with other locally available, nutrient-dense crops in vulnerable populations.

Management of on-farm diversity
Enhanced crop diversity, including farmers' varieties/landraces, confers resilience on crop production and reduces vulnerability to shocks and are potential sources of traits for crop improvement, especially for developing varieties tolerant to biotic and abiotic stresses [3]. A significant amount of crop diversity, including farmers' varieties/landraces, is only maintained in farmer's fields, orchards or home gardens. Many farmers choose to cultivate farmers' varieties/landraces due to agronomic, culinary, or quality preferences [3,40]. Much of this crop diversity also has locally important cultural values. The dynamic on-farm management of this diversity contributes to their continual evolution and adaptation due to farmers' selection and seed exchange systems [67].
In order to support countries in enhancing the diversity of crops and varieties which are cultivated by farmers, the Voluntary Guidelines for the Conservation and Sustainable Use of Farmers' Varieties/Landraces [3], were developed. They serve as reference material for preparing a National Plan for the Conservation and Sustainable Use of Farmers' Varieties/Landraces and are a useful tool for development practitioners, researchers, students and policymakers who work on the conservation and sustainable use of these valuable resources.

Germplasm conservation and on-farm management
The diversity of crops and varieties maintained on farmers' fields must also be backed up ex situ, to ensure their conservation in an effective, integrated and rational manner in case of loss on-farm. Conserving this diversity ex situ is additionally advantageous in that it can be assessed and made more readily available to There are many diverse species and varieties of indigenous leafy vegetables consumed locally in tropical sub-Saharan Africa. These include African nightshades (Solanum scabrum), leafy amaranth (Amaranthus spp.), spider plant (Cleome gynandra), cowpea (Vigna unguiculata), Ethiopian kale (Brassica carinata), mitoo (Crotalaria ochroleuca and C. brevidens), kahuhura (Cucurbita ficifolia), jute plant (Corchorus olitorius) and pumpkin leaves (Cucurbita maxima and C. moschata) [61]. The nutritional importance of African leafy vegetables (ALV) has been recognized by various experts over recent decades [62][63][64][65]. Yet, despite their nutritional advantage over many imported vegetables, levels of consumption had been decreasing in many countries, including Kenya [66].
One of the key reasons for the decline in the consumption of ALV includes migration to cities, causing a shift in production. With these changes, knowledge of the cultivation of ALV was also being lost, including, very importantly, methods of the production of quality seeds. Increasing the quality of seeds can increase yields. For instance, selecting those seeds with lower rates of dormancy results in higher germinability and hence, improved yields ultimately.
In this respect, African nightshades, for example, require the removal of the wet pulp that contains growth inhibitors, which affect germination rates [61]. Initiatives to improve ALV cultivation by disseminating this information, along with other techniques that enhance seed germination, to farmers through participatory methods were implemented successfully. The resulting uptake in the cultivation of quality ALV by smallholder farmers increased the production and quality of African nightshades in Kenya. Extension workers collaborated closely with researchers and international organizations to reconstruct a knowledge base, combining traditional and more technical information on these species.
Although these crops used to be considered a "poor man's food" until 15 years ago, due to, inter alia, improvements in seed quality, awareness raising and value chain interventions, ALV are now commonly found in Kenyan supermarkets [61,63]. ALV, now gaining in popularity, as evidenced by seed companies' interest and the increase in area cultivated, are contributing to addressing malnutrition as well as to improving livelihoods [65].

Box 2.
Enhancing the quality of seeds to boost production: Seed dormancy in African leafy vegetables in Kenya. researchers and plant breeders. Crop germplasm, a significant proportion of which are farmers' varieties/landraces, is conserved in more than 650 genebanks worldwide [68]. Complementary ex situ conservation of crop diversity is essential for safeguarding global food security for the present and future. The application of standards and procedures that ensure their continued survival and availability is therefore essential. The Genebank Standards for Plant Genetic Resources for Food and Agriculture [69] set the benchmark for current scientific and technical best practices, and support key international policy instruments for conserving crop germplasm in genebanks.
Ex situ conservation of plant genetic resources in genebanks and other facilities safeguards a large and important amount of resources that are vital to global food security [6]. Genebank conservation entails acquisition, storage, characterization, evaluation, regeneration, safety duplication and documentation of germplasm accessions [69,70]. The methods used include the storage of orthodox seeds in seed genebanks and safeguarding species that produce nonorthodox seeds or are propagated vegetatively as live plants in field genebanks or as plantlets through in vitro culture or cryopreservation [69]. Genebanks serve the dual aims of the conservation of PGRFA and the provision of these genetic resources to plant breeders, researchers and other users.
Many collections, especially at the national level, remain vulnerable as they are exposed to natural disasters, including those caused by climate change, and manmade calamities such as civil unrest. These collections are similarly at risk due to avoidable adversities resulting from lack of funding and/or poor management. Well-managed genebanks both safeguard genetic diversity and make it available to breeders. As such, genebanks require adequate and continuous levels of sustainable funding.
In this context, Priority Activity 2 of the Second GPA [5] underscores the need for improved on-farm conservation and the management and use of farmers' varieties/ landraces and underutilized crops. It also highlights the need to foster linkages between these activities and the conserving this diversity in genebanks. The Second GPA also recommends that governments consider how production, research, economic incentives and other policies impact the on-farm management and improvement of PGRFA. The actions that should be taken to enhance the ex situ conservation of germplasm are provided in the following Priority Activities of the Second GPA: • Priority Activity 5 on the targeted collecting of germplasm; • Priority Activity 6 on sustaining and expanding effective ex situ conservation of diverse germplasm; and • Priority Activity 7 on regeneration and multiplication of ex situ accessions, including for distribution and safety duplication.

Enhancing access to, and use of, local crop diversity
The development of farmers' varieties/landraces is commonly undertaken through participatory plant breeding (PPB), which aims to bridge the formal and informal seed systems by supporting smallholder farmers and their collective efforts [71,72]. PPB often uses demonstration plots in Farmers Field Schools [73] to increase farmers' awareness of the quality of varieties and seed produced, and to support adoption. Vernooy et al. [74] reported that PPB resulted in both the conservation of farmer-preferred landraces and the development of new PPBdeveloped varieties, as well as farmer-managed seed production and distribution (e.g., in China and Mexico). Community seedbanks played a crucial role in these activities through seed collection and distribution; seed production of improved local varieties; and education and awareness activities. Community seedbanks are informal, locally governed institutions whose core function is to preserve seeds for local use. They play an important role in increasing access to diverse and locally adapted crops and varieties [74,75], especially farmers' varieties/landraces. These community-based endeavors also enhance related local knowledge and skills in the workflow for seed delivery, i.e. selection, treatment, storage, multiplication and distribution [3].
Community seedbanks can be an effective part of a comprehensive strategy for the conservation and sustainable use of crop diversity. Community-based smallscale seed initiatives, often linked to community seed banks, will play a vital role in the improvement of, and access to, quality declared seeds and planting materials, maintenance of crop diversity for food security, and positively contribute to the national breeding programs. For example, the formation of seed clubs in Vietnam enabled working with farmers to promote varietal selection through participatory plant breeding and the national varietal registration of these local varieties. This has enhanced farmers' access to the quality seeds and planting materials of preferred varieties [76]

(see Box 3).
Enhanced farmers' access to quality seeds and planting materials of well-adapted crops and varieties is realized through the strengthening of community-level seed production with suitable quality assurance regimes, including protocols for quality declared seeds and quality declared planting materials. The Quality Declared Seed System [78] consists of guidelines and protocols that aim at assisting small-scale farmers, specialists in seed production, field agronomists and agricultural extension services in the production of quality seed. This system provides an alternative for seed quality assurance and is particularly useful for countries with limited resources In Vietnam, the Southeast Asia Regional Initiatives for Community Empowerment (SEARICE) and the Mekong Delta Development Research Centre of Can Tho University (MDI-CTU) have been collaborating with communities on the formation of seed clubs to drive community-based conservation and sustainable use of plant genetic resources. These clubs enable local seed supply systems through seed conservation, exchange, and crop improvement activities. In particular, they facilitated: • participatory variety rehabilitation, i.e. whereby the original characteristics of the farmers' variety/ landrace is restored through selection; • participatory plant breeding, where farmers collaborate in the process of crop varietal development and have opportunities to make decisions throughout; and • participatory variety selection, which involves farmers growing and selecting varieties in their own fields, providing a way for breeders to learn which varieties perform well on-farm and are preferred by farmers.
These activities, which bridged the formal and informal seed systems [77], have resulted in the development of 360 farmers' varieties, five of which are nationally certified [76]. The formal registration of farmers' varieties, made possible by the policy and technical assistance provided by MDI-CTU and funding provided by SEARICE, paved the way for the eventual production of quality declared seedsthereby enhancing the confidence of the farmers in the seeds. This approach to community empowerment has been fundamentally important in the improvement of access to and availability of seeds, maintenance of crop diversity for food security, and positively contribute to the national breeding program through the linkages established between the formal and informal seed sectors.

Box 3.
Seed clubs in Vietnam. [79]. The system is less demanding than full seed quality control systems yet guarantees a satisfactory level of seed quality. Its partner publication, Quality Declared Planting Material [80], was prepared in collaboration with the International Potato Centre and follows the principles and approach of FAO's Quality Declared Seed System.
It is necessary to develop and implement national seed regulatory frameworks and to enable the participation of multiple actors, including farmers. This can be undertaken through cooperatives and small-and medium-scale seed enterprises, and the private sector, while supporting institutional and human capacities along the entire seed value chain. Areas of intervention typically include strengthening capacities for the production and processing of seeds and their quality assurance, packaging, storage and marketing. Priority Activity 11 of the Second GPA recommends that countries promote the "development and commercialization of all varieties, primarily farmers' varieties/landraces and underutilized species" [5]. Linked to this, Priority Activity 12 of the Second GPA focuses on supporting seed production and distribution. It underscores the importance of developing/reviewing seed regulatory frameworks that facilitate the development of seed systems and their harmonization at regional levels, taking into account the specificities of different seed systems [5].
To support practitioners along the entire seed value chain, the six-module Seeds Toolkit [81][82][83][84][85][86] is a resource to enhance knowledge and skills for delivering quality seeds and planting materials of well-adapted crop varieties to farmers. The modules are designed as practical guidance to assist in the implementation of the national seed strategies and capacity building activities, especially for small-scale farmers and small-and medium-scale entrepreneurs.
For policy specific guidance, stakeholders may refer to the Voluntary Guide for National Seed Policy Formulation [87]. This explains seed policies and how they differ from seed laws; describes the participatory process of seed policy formulation, the nature and layout of seed policy documents and their key elements; and addresses issues involved in their implementation.

Genetic improvement as means to sustainable use of on-farm crop diversity
A continuous stream of improved crop varieties that are adapted to particular agro-ecosystems and production systems is required for meeting the challenges posed by food insecurity and malnutrition, especially in the face of climate change. In this regard, Priority Activity 9 of the Second GPA recommends countries to support "plant breeding, genetic enhancement and base-broadening efforts" [5].
Crop breeders must aim to develop varieties that are productive, nutritious, resistant to biotic and abiotic stresses, and are well-adapted to target agroecologies and meet consumer preferences and market demands. Genetic diversity is an essential resource for breeders to improve new cultivars with desirable characteristics [88]. For crop diversity to be useful in addressing malnutrition and climate change through breeding, their characteristics need to be measured, evaluated and recorded in information systems that are available to all relevant stakeholders. The process of characterization entails the description of a minimum set of standard phenotypic, physiological and seed qualitative traits. The evaluation of PGRFA requires an analysis of agronomic data obtained through appropriately designed experimental trials. Both characterization and evaluation use crop descriptor lists that are available for a large number of crop species [89][90][91]. Additionally, to support standardizing the information, FAO and Bioversity International published passport descriptors that are widely used for the documentation and exchange of germplasm [92]. The FAO World Information and Early Warning System on PGRFA (WIEWS) [68] provides access to passport data of materials held in genebanks worldwide. Other global germplasm management systems, such as GRIN-Global [93] and GENESYS [94], document not only passport but also characterization and evaluation data in genebanks. GENESYS also includes information on the climate at the origin of accessions, and provide the option to search for accessions originating from similar climates. These systems provide plant breeders with a catalog of traits and germplasm for crop improvement.
Conventional plant breeding procedures can be time-consuming and expensive [95]. For example, the breeding, delivery and adoption of new maize varieties has taken up to 30 years [96]. Advances in biotechnology have substantially increased the efficiency for the identification of desirable traits for crop improvement and the knowledge of the genetic mechanisms that control the expression of traits of interest [97]. More targeted breeding can be undertaken as the links between traits and genes are better understood. This is especially important for those traits under polygenic control such as yield and those conveying heat, drought and other stress tolerances [98].
Crossing high-yielding varieties with lower-yielding but resilient local germplasm such as landraces can reduce genetic vulnerability [99] through the broadened genetic base of the improved varieties. This is achieved most effectively through pre-breeding, i.e. the generation of intermediate materials by crossing nonadapted germplasm that possess novel traits with standard breeding lines [5,100]. A detailed step-by-step overview of pre-breeding procedures is provided in an e-learning course [101], developed under the auspices of the Global Partnership Initiative on Plant Breeding Capacity Building (GIPB). This course is made up of five modules covering the introduction to pre-breeding; genebank management relevant to pre-breeding; pre-breeding project management; creating and managing variation; and the distribution and use of the pre-bred materials and associated regulatory considerations.
In situations where sourcing heritable variations from existing germplasm is not possible or otherwise impractical, the induction of allelic variations through mutagenesis is a viable option [102]. Mutations can be induced by physical (i.e., gamma and x-ray technology) or chemical means [103] for a comprehensive review on this topic). DNA mutations tend to be chance events and therefore require that scientists generate massive numbers of putative mutants that are then subsequently screened for particular traits, a lengthy and costly process. However, advances in high throughput molecular genetics, cell biology and phenotyping techniques mitigate these constraints and facilitate the integration of induced mutations into improved crop varieties [103].
Morphological assessments using traditional phenotyping methods can be labor intensive, time consuming, subjective, and frequently destructive to plants. In fact, the access to large-scale phenotypic data has been one of the major bottlenecks hindering crop breeding [104]. High-throughput phenotyping (HTP) is a recently developed method that has potential to overcome this bottleneck and offers large-scale, accurate, rapid, and automatic data acquisition for crop improvement [105,106]. A large number of advanced technologies [107, 108], including sensors, information technology and data extraction, combined with systems integration and reduced costs, means that morphology and physiology can be assessed non-destructively and repeatedly across entire populations throughout their development [104,109]. Novel HTP approaches are necessary to advance the understanding of genotype-to-phenotype cause and effect relationships and therefore accelerate plant breeding [110,111]. This can be of great importance for assessing the production and resilience traits of farmers' varieties/landraces. Many traits have been mapped to specific genes and as a result, more analyses are being conducted per unit of time that allow for more specific mapping of traits. Quantitative trait loci (QTL) mapping results provide useful information to understand the genetic mechanisms of important traits and improve the efficiency of marker-assisted selection and genomics-assisted breeding [112,113]. Taken together, existing genomics knowledge and tools may be used to overcome the constraints to the development of adapted varieties that combat malnutrition and climate change [114,115].
Advances in phenotyping technology and methodologies for multi-population data analysis have made possible the mapping of QTL [116,117]. In addition, DNA sequencing has become more rapid, more precise and less expensive [104,110]; the genomes of most staple crops, and some minor ones, have been sequenced [118]. A recent initiative driven through the African Orphan Crops Consortium (AOCC) is applying genome-enabled methods to improve the production of 101 underresearched ('orphan') crops on the continent [119]. To date, eight genomes have been sequenced and published and another 26 are underway [120]. The ultimate goal of this initiative is to develop resilient, palatable and nutritious varieties of local crops for local peoples to consume and sellthereby enhancing their nutritional status and livelihoods.

Existing policy frameworks
As means to enhance intra-and inter-specific on-farm crop diversity, diverse initiatives, policies and global frameworks have been developed and implemented. In recent years, focus has been on areas of synergies and streamlining efforts among the health, environmental and agricultural sectors (Figure 3). The number of policy and legal frameworks targeting crop diversity, reflects the growing global interest and concern and the commitment of countries for their conservation and sustainable use [51,121]. While crop diversity has been a key focus of many policy discussions since 1950 onwards [7], the International Undertaking on Plant Genetic Resources which was adopted by resolution 8/83 of the FAO Conference in 1983 was a watershed moment. The objective of this Undertaking was "to ensure that plant genetic resources of economic and/or social interest, particularly for agriculture, will be explored, preserved, evaluated and made available for plant breeding and scientific purposes" [123].
This laid the groundwork for the development of cornerstone frameworks for crop diversity, especially: • the Global Plan of Action (GPA) for the Conservation and Sustainable Use of Plant Genetic Resources for Food and Agriculture (PGRFA) adopted by 150 countries in 1996 [124]; • the International Treaty on Plant Genetic Resources for Food and Agriculture (the Treaty) that entered into force in 2004, providing a legal framework whereby governments, farmers, research institutes and agro-industries can share and exchange PGRFA and benefits derived from their use [125]; • the Global Crop Diversity Trust, established in 2004 by FAO and Bioversity International on behalf of the CGIAR, to support the efficient and effective ex situ conservation of crop diversity over the long term [126]; • the Second GPA in 2011 [5]; • the Cordoba Declaration [127], which emphasized the importance of underutilized and promising crops at the international level; • the Second International Conference on Nutrition (ICN2) held in Rome in 2014 [128], which showcased the profile of NUS and adopted the Rome Declaration on Nutrition after which 2015-2025 was declared the UN Decade of Action on Nutrition [129]; and • adoption of the 2030 Agenda for Sustainable Development by 193 Member States of the United Nations [130].

Looking forward
Addressing livelihood options for smallholder farmers requires that the focus of R&D be broadened to include a much wider range of crop species and cropping systems. This diversity is essential for breeding new plant varieties that confer the ability to adapt to changing environments, including new pests and diseases and adverse climatic conditions, on cropping systems. Thousands of years of farming and targeted selection have resulted in an invaluable heritage of locally adapted varieties of major and minor crops [16,127]. The greater the diversity, the greater the chance that at least some of the individuals will possess an allelic variant suited to changing environments, and will produce offspring with that variant [7].

Bridging conservation, sustainable use and the seed sectors
To achieve the most benefits from PGRFA while at the same time safeguarding them, activities that address conservation must be linked to those concerned with plant breeding which in turn must feed into seed delivery systems. In many countries and regions, there is a lack of these linkages between these three modules of the PGRFA management continuum [131] (Figure 4).
This continuum approach is also relevant for the efforts to leverage farmers' varieties/landraces to enhance on-farm crop diversity and will require the concerted actions of extension workers, researchers, breeders, seed enterprises and farmers. Similarly, greater cooperation at different stages in the production chain, from the development and testing of new varieties, through value-adding activities, to the opening up of new markets is essential.

The enabling environment
In order to have long-term impact on the ground, clear and non-conflictual policies are needed, together with effective delivery systems. The policies must be evidence-based and offer relevant interventions that can rapidly be deployed on the ground. Often policies can be at variance with one another, with a resulting negative impact on crop diversity, livelihoods and/or diets. For example, subsidies for promoting staple crops may have a negative impact on the cultivation of minor, but highly nutritious and resilient crops and varieties [16]. Addressing this, FAO developed Guidelines for Developing a National Strategy for Plant Genetic Resources for Food and Agriculture [132]. These guidelines support countries in developing national strategies for PGRFA, which include identifying a national vision, goals and objectives, and the corresponding plan of action, including responsibilities, resources, and timeframes for activities. They take into account each country's needs, capacities and constraints.
Efforts must continue to target the development of appropriate national strategies and policies to promote the diversification of cropping systems, including the on-farm conservation and use of underutilized species, enable R&D and the uptake of their outputs. The Second GPA [5] highlights the importance of conservation and sustainable use of crop diversity in terms of policy and capacity development. National policies should aim to strengthen capacities in crop improvement in order to produce varieties that are specifically adapted to local environments. These policies may include appropriate for the protection of new varietiesas applicable, varietal release and seed certificationor other appropriate quality assurance regimes. These would promote and strengthen their use and ensure that they are included in national agricultural development strategies. Building national programmes and institutional capacities is critically important as a means to promote public awareness on the importance of the diversity of PGRFA [5,131]. The support to policy-makers as well as training and capacity building for scientists, breeders, extension specialists, seed producers, farmers, indigenous peoples and local communities on themes that enable the promotion of the development and commercialization of all crop varieties, primarily farmers' varieties, landraces and underutilized species, is recognized as a fundamental necessity [3]. Relevant topics for such training and capacity building activities include activities that promote the increased on-farm management of crop diversity such as the identification of all suitable materials and the development and implementation of sustainable management practices, postharvest processing and marketing methods and the documentation of relevant local and traditional knowledge. Additional activities include those that promote establishing, running and advising local small-scale seed enterprises.
The Second GPA [5] provides guidance on the human and institutional capabilities that should be strengthened for the conservation and sustainable use of PGRFA, including farmers' varieties/landraces. These are summarized below: • Priority Activity 13 focuses on developing national programmes, recognizing that efforts to coordinate national planning, priority setting and fundraising are needed. Emphasis is placed on enhancing collaboration between the public and private sectors, national and international cooperation, strengthening links between PGRFA conservation and use, developing information systems and publicly accessible databases, identifying gaps in the conservation and use of PGRFA, increasing public awareness and implementing national policies and legislation and international treaties and conventions.
• Promoting and strengthening networks for PGRFA, as described in Priority Activity 14, are crucial for improved coordination, communication and organizational skills. Resources and capacity should be available for activities such as planning, communications, travel, meetings, network publications such as newsletters and meeting reports, and network strengthening, including the preparation of successful proposals for submission to donors.
• Information systems for PGRFA facilitates evidence-based decision making for their effective conservation and use. Priority Activity 15 provides guidance for national and regional programmes, including for strengthening and harmonizing documentation, characterization and evaluation of germplasm.
• In order to monitor and safeguard genetic diversity and minimize genetic erosion of crop diversity, capacities must to be strengthened for gathering and interpreting information in conducting inventories and surveys (Priority activity 16). Training on monitoring should be provided to breeders, farmers and indigenous and local communities. It is important to develop training materials, including self-teaching tools, in local languages as needed.
• As described in Priority activity 17, the long-term availability of adequate human resources capacity in all areas of PGRFA conservation and use, including management, legal and policy aspects, must be developed and strengthened. This includes support for enabling national and regional organizations and programmes to update curricula, provide advanced education and strengthen research and technical capacities in all relevant areas.
• Communicating effectively about the many benefits of crop diversity to food security and sustainable livelihoods is critical to the success of any intervention. Priority Activity 18 highlights the importance of national public awareness programmes and the development of international links and collaborative mechanisms such as networks, involving different sectors, agencies and stakeholders. The aim is to increase the value of crop diversity by bringing this information to the attention of policy-makers and the general public.

Conclusions
Five years after the world committed through the SDG to end hunger, food insecurity and all forms of malnutrition, we are not on track to achieve these objectives by 2030. The sense of urgency is even more pressing due to the looming 2030 deadline of the SDGs, which underscores the need to 'think outside of the box'. Options for addressing food insecurity and malnutrition should include increasing the diversity of crops and varieties cultivated. This chapter highlighted the danger of the continued overreliance on a few crops and their varieties. It prescribed the means for incorporating a wider diversity of farmers' varieties/landraces into crop production systems. These local crop genetic resources tend to be adapted to low input production systems, which is prevalent in many food insecure countries of the world. The underlying premise is that improving agricultural production while using the diverse plant genetic resources available can benefit directly the livelihoods of smallholder farmers and farming communities. The ensuing result is a positive impact on food security and nutrition, environmental resilience and effective management of crop diversity.
The Priority Activities of the Second GPA provide guidance for the enhanced integration of farmers' varieties/landraces into cropping systems. These include recommendations for promoting on-farm crop diversity directly and the conservation of these critical resources in genebanks. The Second GPA also addresses continued genetic improvement of germplasm and suitable seed delivery systems, especially those that are community-based and are tailored to low input production systems. Advances in molecular genetics, phenotyping and computing capacities enhance the prospects of generating compelling R&D outputs. In the same vein, policies and strategic partnershipsat local, national, regional and global levelsthat facilitate the participation of a multiplicity of stakeholders are also critically important.