**3. Biosynthesis and accumulation of phytochemicals in plants**

**1. Introduction**

**2. Plant metabolites**

The purpose of this chapter is to highlight the potential adverse effects of phytochemical accumulation alterations in fruits and vegetables on the well-being of people. The chapter deals with the introduction of plant metabolites with the distinguishing between primary and secondary metabolites based on their roles in plants. The chapter also deals with the biosynthesis and accumulation of phytochemicals in plants that are based on the effects of different environmental factors on phytochemical compositions in plants. This includes the review of the effect of pesticides on phytochemicals in agricultural products through comparison on products produced by organic farming and conventional farming. The potential adverse effect of phytochemical accumulation limitation in fruits and vegetables on the well-being of people is also highlighted through the discussion of the health benefits of the actions of phy-

In addition to nutritional value, consumption of fruits and vegetables also provide health benefits to humans due to the nature of their secondary metabolites [1]. Plants of all kinds, including fruits and vegetables, possess two types of metabolites with distinct roles known as primary and secondary metabolites as shown in **Figure 1**. Primary metabolites, generally possessed by all plants, are metabolites that contribute to the plants' growth [2]. Primary plant metabolites include carbohydrates, lipids, proteins, and nucleic acids. Secondary metabolites, sometimes found in specific plants or plants parts, are for the performance of protection

tochemicals in prevention and treatment of diabetes and cancer.

180 Phytochemicals - Source of Antioxidants and Role in Disease Prevention

**Figure 1.** Plant metabolites and their different roles in plants.

The biosynthesis of phytochemicals in plants is mainly achieved through the Shikimate and the Acetate-Mevalonate pathways [4], as shown in **Figure 2**. Most end-products of the catabolic metabolism of primary metabolites serve as precursors for the biosynthesis of a range of secondary metabolites that are commonly referred to as phytochemicals. Aromatic amino acids enter secondary metabolite biosynthesis via the Shikimate pathway. The Shikimate pathway leads to the production of simple compounds such as gallic acid and *p*-coumaric acid that are precursors to complex products like tannins and nitrogen-containing phytochemicals [5]. Acetyl-CoA, an end-product of carbohydrate metabolism, leads to the biosynthesis of terpenes and steroids through the Acetate-Mevalonate pathway [6].

**Figure 2.** Biosynthesis of phytochemicals.

The biosynthesis of phytochemicals in plants, as described above, is reported to be also induced by external stimuli [7]. As such, phytochemical compositions of plants are mostly found to differ both qualitatively and quantitatively depending on the exposure or susceptibility to the stimuli [8]. The external stimuli that induce phytochemical biosynthesis in plants may be summed up through environmental factors under which plants grow. Environmental factors under, which plants grow and affect their phytochemical compositions may include geographical location parameters such as altitudes and soil types, seasonal variations, and exposure to pollution [9]. In the case of fruits and vegetables, such environmental factors may also include agricultural practices such as the use of insecticides and herbicides. Some findings on the effects of certain agricultural practices on the presence and accumulation of phytochemicals in fruits and vegetables, as well as their implications on people's health are discussed below.

environment compared to conventional farming, which relies heavily on inputs such as pesticides, synthetic fertilizers, and excessive irrigation. The growing environment in organic farming exposes plants and crops to biotic stress due to pests and diseases [14]. Herbivore and pathogen attack lead to enhanced biosynthesis and accumulation of defense-related phytochemicals in fruits and vegetables [15]. Deficiency of nitrogen, phosphate, and iron that emanate from non-application of synthetic fertilizers in organic farming contribute to the accumulation of phenolic compounds in fruits and vegetables [15]. Drought or less irrigation associated with organic farming also induce accumulation of polyphenols and total tannins in crops through the activation of the phenylalanine ammonia-lyase enzyme that is involved

Potential Adverse Effects of Alteration of Phytochemical Accumulation in Fruits and Vegetables

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**3.2. Potential adverse effects of phytochemical accumulation alterations in plant** 

In Africa, people have relied on consumption of fruits and vegetables for many years with no profound problems of life style diseases. Fruits and vegetables are known to possess phytochemical compounds with biological activities that have the ability to prevent and reverse the development of chronic diseases such as diabetes and cancer [17, 18]. Therefore, a question arises as to how come the world, Africa in particular, is today burdened with the advent of life style diseases such as diabetes and cancer in the midst of massive agricultural production of fruits and vegetables. The answer to this imminent question may lie in the understanding of the possession or the presence of plant metabolites in fruits and vegetables, more especially the accumulation of secondary metabolites that are commonly referred to as phytochemicals. The secondary metabolites found in different plants parts; including fruits and vegetables, possess many biological activities. Amongst the biological activities exerted by different phytochemicals are the enzyme inhibitory properties. There are two enzymes that are reported to contribute to the fast postprandial release of glucose from a carbohydrate-rich meal, namely the alpha-amylase and alpha-glucosidase. Inhibitory actions of these enzymes that are inherent in some phytochemicals present in plant products such as fruits and leafy vegetables contribute to the regulation of blood glucose levels as the slow release of glucose from diet sources may afford its proper metabolism, which mitigate against the development of diabetes mellitus [19]. Carbohydrates are of course important dietary requirements to supply living organisms with the necessary energy for growth. However, malfunctioning or poorly regulated carbohydrate metabolism may lead to the rapid postprandial release of glucose into the bloodstream with potential discrepancies in its further breakdown. The advent of discrepancies in the breakdown of glucose may result in its accumulation in blood, which gives rise

In addition, the enzyme inhibitory properties inherent in some phytochemicals may affect kinases that are involved in cell cycle progression, which may contribute to the mitigation against the development of cancer. Cancer is a disease of uncontrolled cell growth, which is propelled through up-normal continuous cell division cycle [21]. Cell division cycle progression is regulated through the activity of phosphorylation enzymes known as kinases [22]. Cell division cycle progression is also depended on the intactness of the DNA that is safeguarded

in the biosynthesis of phytochemicals in plants [16].

to the potential development of diabetes mellitus [20].

**products**
