CHAPTER ONE

INTRODUCTION

1.1 Background to the Study

The term vegetable is used to describe the tender edible shoot, leaves, fruits and root ofplants and spices that are consumed whole or in part, raw or cooked as a supplement tostarchy foods and meat and they may be aromatic, bitter or tasteless ((Dian and Ryler 2011; Pennington and Fisher, 2009). Vegetable definitions are rather arbitrary by nature and commonly based on usage rather than plant morphology. For example, one widely useddefinition of a vegetable is: a herbaceous plant or portion of a plant that is eaten whole or inpart, raw or cooked, generally with an entree or in a salad but not as a dessert.

Vegetables produced in the South west region of Cameroon are supplied to the local markets and to the neighboring countries. Solanum spp, Vernonia amydalina, Telfera occidentalis, Okra, Potato and onion/shallot are commonly marketed vegetables accounting for a substantial volume of the marketed products (Ndam, 2005). The marketing of vegetables in Fako division is characterized by seasonal gluts and shortages which in turn affect the marketing behavior of producers, traders and consumers.

The diet of most Cameroonians and other Africans is dominated by starch rich staple foods such as; rice, cassava, yam, and plantain, which in the absence of cooked vegetables is considered incomplete. Their production is in the hands of resource-poor farmers, mostly women, in urban and peri-urban settings where they constitute a major source of income and consumption thus serves as cheap and important source of protein, minerals, vitamins, amino acid, antioxidants and metabolites (Mih and Ndam, 2007; Mih et al., 2008).

Increase in awareness of the health benefits of vegetables has resulted in an increase in consumption. Many vegetables are consumed cooked or raw to retain the natural taste and heat liable nutrients (WHO, 2016). However, the intake of heavy metal (such as Cu, Zn, Cd, Pb, Cr, As and Hg) contaminated vegetables may pose a risk to human health; hence the heavy metal contamination of food is one of the most important aspects of food quality assurance (Thompson et al., 1990). Plants take up heavy metals by absorbing them from airborne deposits on the parts of the plants exposed to the air from the polluted environments as well as from contaminated soils through root systems. Also, the heavy metal contamination of fruits and vegetables may occur due to their irrigation with contaminated water (Jackson et al, 1992; Lokeshawari et al, 2006; Pawan et al; 2013). Additionally, vegetable contamination is also due to application of manure and chemical fertilizers which are reported to be a source of metals in food (Wilson and Pyatt, 2007).

Cd and Pb are most toxic elements for man (Volpe et al, 2009). Some heavy metals such as Cr, Mn, Zn, Cu, Ni and Fe are considered essential components for biological activities in the body, however, their presence in elevated levels is reported to cause problems to human (Jamp, 2003).

Therefore, the safety of raw vegetables is a great concern as they have shown to harbor pathogenic bacteria. Consumption of vegetable is commonly viewed as a potential risk factor for infection with enteropathogens such as Salmonella and Escherichia coli with recent outbreaks linked to lettuce, spinach and tomatoes (Takeuchi and Frank 2000). Routes of contamination are varied and include application of organic wastes to agricultural land as fertilizer, contamination of waters usedfor irrigation with faecal material, direct contamination by livestock, wild animals and birds and postharvest issues such as worker hygiene (Seymour and Appleton 2001). The ability of pathogens to survive in the field environment has been well studied, leading to the implementation of guidelines such as the Safe Sludge Matrix, which aim to limit the likelihood of viable pathogens remaining at point-of-sale. The behaviour of enteropathogens in the phyllosphere is a growing field of research, and it is suggested that inclusion in phyllosphere biofilms or internalization within the plant augments the survival (Fett, 2000). Improved knowledge of plant—microbe interactions and the interaction between epiphytic and micro¬organisms on the leaf surface will lead to novel methods to limit enteropathogen survival in the phyllosphere. It is therefore necessary to investigate microbial contamination in vegetables.

Plant interact with each other. some plant may suppress the growth of others become dominant exerting the allelopathic effect. Allelopathy is an age-old concept and the term was first used by Molisch in 1937 to mean all biochemical interactions, stimulatory or inhibitory among plants including microorganisms (Weston;2005). Today the term denotes both harmful and beneficial biochemical interactions between all types of plants including microorganisms, through the release of chemicals from plant parts by leaching, root exudation, volatilization, residue decomposition and other processes in both natural and agricultural systems (Ndam et al., 2013). Allelopathy can affect many aspects of plant ecology including occurrence, growth, and plant succession, the structure of plant communities, dominance, diversity and plant productivity (Rice, 1984). Allelopathy is a universal phenomenon and is not the domain of few selected plants. In multi-storey cropping systems, where numerous crops and trees are grown together, vegetables are essential components and allelopathic interactions arise. Several vegetables possess antimicrobial principles and hence, allelopathically inhibit phytopathogenic fungi and bacteria. Certain vegetables possess nematicidal principles and therefore, offer immense potential for nematode control in their cultivation. Several studies have been done to elucidate the role of allelochemicals in vegetables across the world. Some vegetables with strong allelopathic effects have been investigated (Yu, 1999). The allelopathic interactions between vegetables and other crops/weeds/trees and the potential of vegetables for weed, pathogen and nematode control is a vital component of sustainable agriculture. Allelopathic interactions are complex and can involve differentclasses of chemicals. Allelochemicals are present in all plant tissues, several chemicals can be released together and may exert additive or synergistic effects (Putnam and Duke; 1978). The vegetables production faces many problems such as; Yield decline due to auto toxicity and soil sickness (when vegetables are grown continuously for several years). Low yield of vegetables in monoculture and cropping systems due to allelopathic effects of crops, weeds and trees Huang et al., 2006; Jacob et al.,2007; Qasem, 1995; Yu, 1999). Researchers study only the competition for nutrients and moisture and not the allelopathic effects (Putnam and Duke, 1978). The allelochemical interactions and their effects on vegetables and other plants areimportant aspects to be considered in vegetables production. Hence, studies on allelopathy in vegetables production is vital as it deals with;Problems of soil sickness and auto toxicity, Use of vegetables allelopathy for weed management, pest management and Identify ideal combinations (companion, rotations and agroforestry systems) of vegetable crops

1.2 Statement of the Problem

Food safety is a major public concern worldwide and food consumption has been identified as the major pathway for human exposure to certain environmental contaminants, accounting for > 90% of intake compared to inhalation or dermal routes of exposure (Rattan et al., 2005). About 30% of human cancers are caused by low exposure to initiating carcinogenic contaminants in the diet. During the last decades, the increasing demand of food safety has stimulated research regarding the risk associated with consumption of foods contaminated by pesticides, heavy metals and pathogens (Kannan et al., 2003).

Due to the heightened health benefits of vegetables, WHO and FAO in 2003 launched a global initiative to promote the consumption of vegetables. Excessive accumulation of heavy metals in agricultural soils may not only result in soil contamination, but also lead to elevated heavy metal uptake by vegetables, and thus affect food safety and quality (Muchuwetii et al., 2006). Heavy metal accumulation in vegetables and soils is often increasing concern because of the potential human health risk. This food chain contamination is one of the important pathways for the entry of these toxic pollutants into the human body.

The use of contaminated water for vegetable irrigation is very dangerous to human health (Asongwei et al 2014). During vegetable cultivation, fertilizers and chemicals are used to meet higher demands of food production leading to heavy pollution (Gholizadeh et al 2009). In Africa it is reported that excessive use of fertilizer particularly poultry manure at a rate of 20 — 50t/ha and mineral fertilizers such as urea and NPK ( (20 — 10 — 10) at rate of 1.2 — 2t/ha is a key source of vegetable pollution. Potential runoff associated with application of these fertilizers to soil leads to environmental risk to soil water crop and hence human health (Midmore and Jasen 2003; Brooks et al 2012; Atidegla, 2011; De Donetal, 2003). It is of paramount important to evaluate

Accumulation of heavy metals such as Fe, Cd, Pd, and Ni that are risk factors for cardiovascular and nervous problems (WHO, 1992). This a sure means of working towards food safety and security.

Recent studies have demonstrated the potential of raw vegetables to transmit pathogens (Cardamone et al, 2015; Nwanko et al 2015). Burgeoning global trade has increased the spread of Bacterial and related diseases y with ravaging heath implications (Al-Kharouni et al 2016). Studies carried out on vegetables sold in markets of Fako division show they are contaminated by bacteria (Akoachere et al, 2018). However, an assessment of antibacterial agents from the prominent and highly consumed vegetables in Fako Division in terms of market sales is yet to be done. Increasing trends of bacteria resistance to antibiotics has triggered the search for bioactive compounds from plants with alternative mechanisms of action to counteract pathogenic bacteria (Buffet-Bataillon et al. 2012). Vegetables harbors enormous amounts of therapeutic agents that have diverse applications in the pharmaceutical, nutraceutical and agrochemical industries. The active principles responsible for the therapeutic effects of vegetables are phytochemicals, usually secondary metabolites, including but not limited to alkaloids, steroids, flavonoids, terpenoids and tannins (NONITA et al. 2010). The tedious process of plant—based discovery of antibacterial is often based on knowledge of ethnophannacopoeia of the plant. Such vegetables are screened in vitro for bioactivity using several techniques. Further identification of the lead compounds is based on the diversity of secondary metabolites or phytochemicals produced by the vegetables.

Vegetable plants are becoming integral components of many subsistent fanning systems in most developing nations of Africa and Asia due to the increasing awareness of human needs for food and herbal remedies. Contrary to the acclaimed curative effects of several groups of plant’s secondary metabolites, accumulation of these organic components in the soil negatively affect seed germination and seedling growth of other vegetation through a phenomenon called allelopathy (Omezzine et al., 2009; Umer et al., 2010). Allelopathy is an ecological and chemical interaction characterized by stimulatory and inhibitory effects among different plant families. Worldwide, the inhibitory properties of the extracts and residues of many vegetables co-habiting with desired crops on the same field have been a major source of concern (Hussain et al., 2007; Nazir et al., 2007). In many parts of the world, where allelopathy is viewed as a biological agent of weed control in agricultural production different methodological research have examined the allelopathic interactions that exist between several plant species and crop cultivars (Khan et al., 2009). For example, Fischer (1986) and Einhellig and Leather (1988) indicated that monoterpenes, sesquiterpenes and alpha-pinenes from the essential oils of certain plants and crop cultivars inhibit seed germination and cause anatomical and physiological changes in the seedling growth of many targets. Mungole et al. (2010) confirmed the adverse effects of one or combined influence of phytotoxic chemicals in the extracts of plants on crops and weeds.

Hassan and Ghareib (2009) found that several organic constituents released into the rhizosphere via mulching with straw and residue incorporation negatively affect the growth and yields of domesticated crops. On the contrary, in this part of Africa, where farmers combine vegetables with the existing farming operations, there is dearth of information on this topic. Therefore, an understanding of the allelopathic interactions of the extracts or residues of vegetables and crops growing together on the same field is an important research base with economic implications.

1.3 Justification of Study

Heavy metal load and microbial contamination are detrimental to human health. With the increasing demand and consumption of vegetables around the world and in Cameroon precisely the Fako division it is essential to safe lives. Diseases such as diarrhea, dysentery and cholera are caused by heavy metal load and microbial contamination. A study of the chemical interactions among the vegetables and other plants is vital for food security, safety and ecosystem management. To this premise, the dynamics of vegetables based on the volume of sales, seasonal availability, market price, culinary changes medicinal value and consumption preference pattern will be evaluated in Fako Division.

Fako division is chosen as the study site because it is one of the highest consumers of vegetables in Cameroon. Secondly, observations made during market survey, call for concern on hygienic practices of the vendors as vegetables were displaced on ground and contaminated water used in watering. Thus, there is necessity in assessing the heavy metal and microbial contamination of vegetables sold and their antibacterial and allelopathic potentials also assessed.

Screening the varieties of vegetables for heavy metal load, antimicrobial and allelopathic potential and later on do genetic manipulations to breed new varieties is a big step in allopathic medicine and crop protection. Exploiting vegetables allelopathy for weed control and identify the compatible and beneficial associations of vegetables with other crops, vegetables and tree is an essential ingredient in sustainable agriculture and ecosystem functioning and management. Hence, it is hoped that this study will lead to the documentation of vegetables consumed in Buea and the exploitation of the biological potential in the pharmaceutical and agricultural industries.

1.4 Research Questions

1. How diverse are vegetables sold in Fako division?
2. What is the level of heavy metal contamination in some selected highly consumed vegetables in Fako Division?
3. What biological activities are exhibited by vegetables consumed in Fako Division and what secondary metabolites are responsible for these?

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