Handbook of Vegetables and Vegetable Processing

Part I

Biology, Biochemistry, Nutrition, Microbiology, and Genetics

Chapter 1

Biology and Classification of Vegetables

Theodore J. K. Radovich

Introduction

Vegetables enrich and diversify the human diet. They are the primary source of mineral nutrients, vitamins, secondary plant metabolites, and other compounds that support human health and nutrition. Vegetables, especially roots and tubers, can also possess significant caloric value, serving as staple crops in many parts of the world, particularly in the tropics. Although vegetables account for less than 1% of the world's plants, the genetic, anatomical, and morphological diversity of vegetables as a group is astounding (Graham et al. 2006; Maynard and Hochmuth 2007). Hundreds of vegetable taxa are grown for food in subsistence and commercial agricultural systems worldwide. This chapter reviews and explains the biology and classification of vegetables.

Biology and Classification of Vegetables

A primary reason for the diversity among vegetable crops is the broad definition of the word vegetable itself. Any plant part consumed for food that is not a mature fruit or seed is by definition a vegetable. These include petioles (e.g., celery, Apium graveolens Dulce group), entire leaves (e.g., lettuce, Lactuca sativa), immature fruits (e.g., cucumber, Cucumis sativus), roots (e.g., carrot, Dacus carota), and specialized structures such as bulbs (e.g., onion, Allium cepa Cepa group) and tubers (e.g., white potato, Solanum tuberosum).

Further expanding this already generous definition is the inclusion of mature fruits that are consumed as part of a main meal rather than dessert (e.g., tomato, Solanum lycopersicum). This culinary exception to the anatomical rule was given legal precedence in the US Supreme Court decision Nix v. Hedden (1893) that confirmed common usage of vegetable in reference to tomato. This has since been extended to beans and other fruits. Even dessert melons (e.g., cantaloupe, Cucumis melo Cantalupensis group), which are fruits by every botanical, legal, and culinary definition, are frequently lumped in with vegetables because of similarities in biology and culture that they share with their more vegetal cousins in the Cucurbitaceae (Iltis and Doebley 1980) (Table 1.1).

Table 1.1 Botanical names, common names, and edible parts of select vegetables by family. Families in the Monocotyledons are listed first (shaded) followed by families in Dicotyledons

Source: Abridged and modified from Maynard and Hochmuth (2007).

The biological diversity among vegetables necessitates a systematic method for grouping vegetables in order to efficiently access information and make management decisions. Understanding the biology of vegetable crops will aid decision making associated with production, postharvest handling, and marketing. Ultimately, vegetable classification is inextricably linked with crop biology. Three basic approaches toward classification of vegetables that are based on commonalities among groups are as follows:

1. Tissues and organs consumed

2. Ecological adaptation

3. Taxonomy

All three of the above approaches toward classification are based on some level of commonality in crop biology, with the precision of classification varying from relatively low (plant part consumed) to very high (taxonomic). Table 1.2 gives definitions of selected terms related to vegetable anatomy, biology and classification.

Table 1.2 Definitions of selected terms relating to vegetable anatomy, biology, and classification

Vegetable Tissues and Organs

The phenotypic diversity among vegetables is actually based on relatively few types of specialized cells and tissues. Dermal, ground, and vascular tissue make up the three basic tissue systems. Ultimately, the structure of these cells and tissues determine their function.

Dermal Tissues

Epidermal cells, together with cutin and cuticular waxes, make up the outer layers of leaves, fruit, and other above-ground structures and protect against water loss and other adverse abiotic and biotic factors. The periderm (cork) layer of mature roots and stems is analogous to the epidermis, but consists of nonliving cells supplemented with suberin. Stomatal guard cells are epidermal cells specialized in regulating gas exchange, and are especially dense on the abaxial surface of leaves. Lenticels are specialized, unsuberized dermal structures (appearing as raised dots or bumps) that regulate gas exchange on roots, stems, and fruits. Trichomes and root hairs are dermal cells with excretory, absorptive, and other functions critical to the ecology of vegetables.

Ground Tissues

Ground tissues are comprised of the parenchyma, collenchyma, and sclerenchyma. Parenchyma cells are thin-walled cells that make up much of the ground tissues of vegetables. Parenchyma cells often serve to store starch and other compounds. The cortex and pith of white potato are examples of ground tissues dominated by parenchyma. Collenchyma cells have alternating thin and thick cell walls that provide flexible support for stems, as in the strings of celery (Apium graveolens). Sclerenchyma tissues include sclerids and fibers with tough cell walls. Sclerenchyma cells are typically scarce in edible vegetable organs, but are important components of seed coats, nut shells, and the stony endocarps of peaches (Prunus persica) and related fruits.

Conducting Tissues

Vascular tissues conduct water, minerals, photosynthates, and other compounds throughout the plant. The xylem is part of the apoplast and consists primarily of nonliving tracheids and vessel elements. The xylem transports water, mineral nutrients, and some organic compounds, generally from the roots to leaves. The phloem is part of the symplast, consists primarily of sieve cells and companion cells, and is important in conducting sugars, amino acids, and other compounds from source (usually leaves) to sink (actively growing meristems, roots, developing fruits, and seeds). Both xylem and phloem are supported by parenchyma cells and fiber. Some xylem cells (i.e., tracheids) have thickened cell walls that contribute significantly to the structural support of tissues.

The differentiation and variable structure of plant tissues result in diverse functions among the plant organs (stems, roots, and leaves) and organ systems (e.g., fruits, flowers, buds, and bulbs) consumed as vegetables. The classification of vegetables by edible parts has been termed Supermarket Botany (Graham et al. 2006). Although broad and not always anatomically correct, the grouping of commodities as leafy, fruit, and root vegetables has value to growers, distributors, and others in the market chain because of similarities in cultural and postharvest requirements within groups. In addition to being practical, the division of vegetables by anatomical structure highlights the impressive crop improvement accomplishments of the early agriculturalists, which both exploited and expanded the structural diversity inherent in the plant kingdom.

Leafy Vegetables

Leaves are the primary site of photosynthesis in plants and are generally the most nutrient dense and most perishable of the vegetables. Leaves, particularly dark green leaves, contain relatively high levels of minerals (e.g., Fe, Mg, Ca), enzymes (protein), and secondary metabolites (e.g., carotenes and xanthophylls). These compounds, important to human nutrition, are required by the plant for light collection, electron transport, photoprotection, carbon fixation, and many other biochemical processes abundant in leaves. Stomata are especially dense on the abaxial surface of leaves and are the terminal point of transpiration, which is the primary mechanism for dissipating heat accumulated from intercepting solar radiation. High stomatal density combined with the high surface area make leafy vegetables more susceptible to postharvest water loss than other vegetables. Subsequently, rapid cooling after harvest and storage under high humidity are particularly important postharvest procedures for leafy vegetables (Kader 2002).

Leafy vegetables are concentrated in the Asteraceae (Compositae), Brassicaceae (Crucifereae), and Chenopodiaceae. Culinary herbs, dominated by the Lamiaceae (Labiatae), are also categorized as leafy vegetables. Other vegetables consumed primarily for leaf structures include Impomea aquatica (Convolvulaceae), celery (Apiaceae), and Amaranthus spp. (Amaranthaceae). The leaves of many plants grown primarily for other organs (fruits, roots, specialized structures) are often utilized to supplement the diet. The leaves of taro (Colocasia esculenta) and cassava (Manihot esculenta), as well as the young leaves and shoots of sweet potato (Ipomea batatas) and many cucurbits (Cucurbitaceae) are typical examples of vegetables in this category.

Leafy vegetables that are generally cooked before consumption to soften texture and improve flavor (e.g., mature leaves of many Brassica spp. and Chenopodiaceae) are sometimes classified as greens to differentiate them from leafy vegetables that that are consumed raw, often as salad (e.g., most Compositae and the very young leaves of many Brassica). Potherb is used to describe greens used in small quantity for flavoring in cooking.

While generally softer and lighter in flavor than cooking greens, salad crops vary in their texture and flavor, and these differences are important in differentiating among leafy vegetables consumed raw. Examples include textural differences among lettuce (crisphead vs butterhead types) and variable levels of texture and pungency in species used in mesclun mixes. Textural and flavor differences are caused by variability in leaf structure (cuticle thickness), cell type, succulence, as well as type and quantity of phyochemicals (e.g., glucosinolates) present (Figure 1.1).

Figure 1.1 Anatomy of select leafy vegetables.

Root Vegetables

Root vegetables include true roots (carrot, sweet potato and cassava) as well as specialized structures such as tubers, bulbs, corms (e.g., taro), and hypocotyls (e.g., radish, Raphanus sativus). These specialized structures are classified as root vegetables because of their full or partial subterranean habit, their physical proximity to true roots, and their function as storage organs for starch and other compounds. Most of these specialized structures consist primarily of stem tissue, with bulbs being a notable exception. Although significant variability in caloric value and shelf life exists within the roots crops, they are typically higher in calories and less perishable than other vegetables due to their storage function, suberized periderm or protective skin, and high dry matter content (Figure 1.2).

Figure 1.2 Anatomy of select vegetables classified as root crops.

True Roots

The biology and anatomy of true root vegetables are exemplified by a comparison of three important crops: carrot, sweet potato, and cassava. All true roots consist of secondary vascular tissue arising from a cambial layer, with phloem (cortical) tissue extending outward and xylem tissue inward. Secondary plant products are found throughout root tissues, but many are particularly abundant in the pericycle, which is closely associated with the periderm and is removed upon peeling.

In carrot (a primary tap root), the majority of the edible portion is comprised of sugar-storing parenchyma associated with secondary phloem tissue. Sucrose is the dominant sugar in mature roots, and roots contain little starch. The tissue associated with the secondary xylem in the center of roots (pith) is of coarser texture and small pith is desirable in commercial carrots (Rubatzky and Yamaguchi 1997). In contrast, the majority of the edible portions of sweet potato and cassava are internal to the vascular ring of enlarged secondary roots and consist of starch-containing storage parenchyma, which surround a matrix of xylem vessels. In cassava, all cortical tissue is removed along with the periderm (collectively, the peel) prior to cooking, and a dense bundle of fibrous vascular tissue in the center of roots is also removed before consumption. Although the majority of sweet potato and cassava starch is amylopectin, variation in the minority quantity of amylose affects texture of the cooked product. Glutinous texture, stickiness, or waxiness of the product increases with a decreasing ratio of amylose to amylopectin.

Modified Stems

Tubers are enlarged, fleshy underground stems that share some of the characteristics of true roots, including development underground, a suberized periderm, and starch-storing parenchyma. The best-known vegetable examples of tubers are the white potato and the yams (Dioscorea spp.).

Potato tubers form at the end of rhizomes originating from the main stem. Recessed buds (eyes) and leaf scars (eyebrows) on the skin surface are conspicuous indicators that the potato is derived from stem rather than root tissue (Figure 1.2). In the absence of dormancy or chemical inhibition, these buds will sprout and allow for the vegetative reproduction of potato from seed pieces or small whole potatoes. In contrast to potato, yam tubers lack conspicuous buds, leaf scars, and other outward signs of being derived from stem tissue. Sprouts will form from yam tubers and tuber pieces, but generate most readily from the proximal end of tubers. As with true roots, cooking quality of tubers is influenced by starch type, dry matter content, and cell size.

The swollen hypocotyl tissues of table beet (Beta vulgaris group Crassa) and radish (Rhaphanus sativus) are closely associated with the taproot, and the edible portion is described as the hypocotyl-root axis. The multiple cambia and differentially pigmented vascular tissues in beet result in the characteristic banding observed in cross sections of the vegetable (Figure 1.2).

Corms are a third type of modified stem grouped with the root vegetables and are exemplified by taro (Colocasia esculenta) and other members of the Araceae. Corms are vertically oriented, apically dominant, compressed starchy stem bases that initiate underground, but continue to grow partially above ground. Adventitious shoots eventually arise from the parent corm to form secondary corms or cormels.

Bulb vegetables, mainly in the Alliaceae, are comprised primarily of swollen, fleshy leaves (scales) specialized for storage of carbohydrates and other compounds (Figure 1.2). These leaves arise in a whorl from a compressed conical stem called a basal plate. Dry, papery scales of the bulb exterior protect the bulb.

Fruit Vegetables

Fruit vegetables are concentrated in the Solanaceae, Cucurbitaceae, and Fabaceae, but occur in other families as well. Large fruited annual vegetables of the Cucurbitaceae and Solanaceae are generally warm- and hot-season crops because their wild progenitors evolved in tropical and subtropical latitudes where growing seasons are long enough to produce enough vegetative growth to support large fruits in a single year (see ecological adaptation below). Other vegetables in this group are okra (Abelmochus esculentus) and Phaseolus spp. Intensive selection has since resulted in early cultivars of most fruiting vegetables that will produce fruit in the short growing periods of northern latitudes.

Among the commercial vegetables, simple fruits dominate. Berry, pepo, and legume are the characteristic fruit types of the Solanaceae, Cucurbitaceae, and Fabaceae, respectively. Specialized pods produced by okra (capsule) and the Brassicaceae and Morigaceae (silique) are dry and at least partially dehiscent at maturity but are consumed immature green, while still succulent. Each kernel on an ear of corn is a simple indehiscent fruit (caryopsis) (Figure 1.3).

Figure 1.3 Anatomy of select vegetables composed of fruits and fruiting bodies (mushroom).

In many fruit vegetables, the whole fruit is edible although not necessarily consumed. In tomato, eggplant (Solanum melogena), cucumber, and other vegetables, the entire pericarp along with placenta and other tissue is consumed. These vegetables may be peeled to soften texture and lighten flavor by removing toughened dermal cells as well as cutin, waxes, and other secondary metabolites that are associated with organ protection, and which are concentrated in the epidermis and outer pericarp (exocarp). Immature fruit of bittermelon (Momordica charantia) may also be peeled to reduce bitterness caused by momordicosides and other compounds concentrated in the outer pericarp, while the tough endocarp and spongy placenta of bittermelon are discarded along with the seeds. The edible portion of mature Cucurbita fruit is pericarp tissue. In Cucumis melo (e.g., cantaloupe and muskmelon) the most internal portions of the pericarp (endocarp and mesocarp) are eaten, with the leathery rind (exocarp and some mesocarp) discarded. In watermelon (Citrullus lanatnus) the rind includes much of the pericarp, with placental tissue making up a substantial portion of what is consumed, although succulent parts of the rind can be pickled and otherwise prepared.

Other Vegetables

Other vegetables that are comprised primarily of stem material include stem lettuce (Lactuca sativa), kohlrabi (Brassica oleracea Gongyloides group), asparagus (Asparagus officinalis), bamboo shoot (Poaceae), and heart-of-palm (Araceae). Also, flowers of many plant taxa are consumed either raw or cooked. Important vegetables comprised of floral structures include broccoli and globe artichoke (Cynara scolymus) (Figure 1.4).

Figure 1.4 Anatomy of select vegetables composed of flowers and associated structures. Asterisk (*) indicates floret used as an industry designation for individual branches of inflorescence in broccoli.

Ecological Adaptation of Vegetables

The environmental optima (e.g., temperature, light, and soil moisture) of vegetable crops will depend greatly on the center of origin of their wild progenitors. For example, vegetables whose center of origin lies in the tropics are often generally classified as warm-season, short-day plants. In contrast, crops with temperate origins are often considered cool-season, long-day plants. Our need for food and fiber has resulted in strong, artificial selection pressure for broad adaptability in many vegetables crops (Wien 1997; Sung et al. 2008). Nevertheless, many vegetables can be grouped with regard to their environmental requirements, and knowledge of these requirements is critical for crop managers to make effective decisions (Table 1.3).

Table 1.3 Classification of vegetables based on lifecycle, temperature growth requirements, and photoperiodicity

Source: After Pierce (1987).

Temperature

Classification of vegetable crops by temperature is based on three sets of values, or cardinal temperatures, that describe the minimum, maximum, and optimum temperature ranges for crop growth. Minimum and maximum temperatures represent the limits at which growth and development are thought to stop or at least slow to a negligible rate, while plant growth and normal development are most rapid within the optimum temperature range. Krug (1997) stratifies the simple classification of warm and cool season crops to account for subtle but significant differences in cardinal temperatures. For example, the effective growth range for hot-season crops does not include temperatures as low as the minima for warm-season crops, while heat-tolerant cool crops have temperature maxima that exceed those of other cool-season crops (Figure 1.4).

A practical application resulting from the dominant influence of temperature on vegetable crop biology is the use of a heat unit system (or temperature sum concept) to predict plant growth. The most simple and oft-cited example is that used to predict harvest dates for corn. Daily heat units (HU) accumulated are often calculated using the equation HU = Σ (Tavg − Tbase), where Tavg is the average daily temperature and Tbase is the minimum temperature for the crop, below which no growth is expected. Cool-season crops grown during the summer in temperate zones will frequently be exposed to supraoptimal temperatures, and HU calculations must account for the negative effect of high temperatures on crop growth. In head cabbage, HU calculations using upper and lower threshold temperatures of 21 and 0°C have been used effectively to explain seasonal variability in head size and weight (Radovich et al. 2004; Figure 1.5). If the daily maximum temperature (Tmax) falls below the upper threshold, then HU are calculated as described above for corn. If Tmax exceeds 21°C, then an intermediate cutoff method is employed, where HU = [(Tmin + 21)/2)] − [(Tmax − 21) * 2]. Using this cutoff method, HU = 0 when Tmax ≥ 30°C.

Figure 1.5 Relationship between growing degree-days and head traits of cabbage (Brassica oleracea Capitata group) grown in 2001 (full symbols) and 2002 (open symbols) at the Ohio Agricultural Research and Development Center. Treatment means of cultivars Bravo, Bronco, and Transam, are represented by circles, squares, and triangles, respectively (from Radovich et al. 2004).

Unfortunately, single factor models such as HU are not adequate to predict all developmental events. In the cabbage example above, variation in HU fails to explain year-to-year variability in head density. Similarly, while estimation of head density changes in lettuce is improved by the inclusion of light intensity into the HU equation (i.e., photothermal units), the inclusion of an additional factor is not adequate to satisfactorily predict density changes (Jenni and Bourgeois 2008). This highlights the potentially complex relationship between ontogeny and environmental factors.

While heat drives vegetative growth in most vegetables, a certain number of cold units (time of exposure to temperatures below some critical minimum) are required to initiate flowering in many temperate biennial vegetables. This phenomenon, termed vernalization, is exhibited by Brassica, beets, and other vegetables. In crops that are insensitive to photoperiod, cold units may be calculated similarly as described above, while photothermal units are employed for photoperiodic crops.

Light

All plants require light for photosynthesis. While a degree of shading will improve the growth of some vegetables, this is often a temperature response to cooling resulting from reduced solar radiation. Similarly, while the quality (i.e., wavelength) of light significantly affects crop phenology, light quantity (intensity and daylength) generally impacts vegetable crops in a similar manner. However, crops often differ substantially in their response to photoperiod.

As a rule, plants exhibit some sensitivity to photoperiod in their development, particularly with regard to flowering and storage organ development (Waycott 1995; Martinez-Garcia et al. 2002). As mentioned previously, tropical and temperate crops are frequently considered short- and long-day plants respectively, although the actual stimulus is the duration of the dark period and day neutral cultivars have been developed for many crops. Short-day crops include yam bean, cowpea, sweet potato, and potato. Onion, lettuce, and spinach are examples of long-day vegetables (Mettananda and Fordham 1997).

Taxonomy of Vegetables

Botanical classification is the most precise and ultimately most useful method of organizing plants by biological commonality. The vast majority of vegetables are Angiosperms (subclass Monocotyledons and Dicotyledons) in the division Spermophyta. The Tallophyta (algae and fungi) are also important.

The broadest taxonomic grouping relevant to vegetable production and management is the Family. Similarities in structure and adaptation among plants within Families are generally conspicuous enough to be useful in olericulture. For example, ecological and physiological differences among Families are often adequate enough to be resistant to many of each other's specific pathogens. A practical application of this by crop managers is to avoid successive planting of crops from the same Family when designing vegetable rotations in production.

Subordinate to the Family is Genus, followed by the species designation. Members of a species are usually genetically isolated from those of other species, and can freely interbreed with individuals from the same species. Biological differences tend to be minor below the species level, but infraspecific variability in vegetable morphology and ecological adaptation is relevant enough to warrant further classification.

Significant confusion and a lack of consistency in vegetable nomenclature at the subspecific level centers around three terms: subspecies, varietas, and group. All are categories of vegetables sharing distinct features of functional relevance and have been used interchangeably. Subspecies and varietas are botanical terms, while group is used exclusively by horticulturalists. The differences between subspecies (ssp.) and varietas (also variety, var.) have been recognized as subtle but distinct, with the latter subordinate to the former (Kapadia 1963). However, by current convention, the terms are used interchangeably, with ssp. more frequently used in Europe and var. more common in the United States (Hamilton and Reichard 1992). Characteristics that distinguish ssp. and var. are expected to go beyond the morphological and have geographic, ecologic, or evolutionary integrity (Hamilton and Reichard 1992; Peralta and Spooner 2001). In contrast, horticultural groups may be defined exclusively by functional similarities in morphology, as governed by the International Code of Nomenclature for Cultivated Plants (ICNCP or Cultivated Plant Code) (Brickell et al. 2004).

Botanical precedence has been cited for preferential use of variety over group in infraspecific classification (Kays and Silva Dias 1996). However, botanical classification is dynamic and botanical variety status may change. Also, while botanical varieties of cultivated plants by definition qualify for status as horticultural groups, the reverse is not true. Consequently, variety is used for one species and group for another in some texts, and important authors differ in their use of variety and group for the same vegetables (Rubatzky and Yamaguchi 1997; Maynard and Hochmuth 2007). This inconsistent usage can easily lead to confusion. Therefore, this author proposes that group be used in lieu of variety (if not subspecies) as a consistent, inclusive, and uniquely horticultural term to describe subspecific categories of vegetables sharing distinct features of functional relevance. The vegetables of Brassica oleracea, including broccoli (Italica group), kohlrabi (Gongylodes group), Brussels sprouts (Gemmifera group), head cabbage (Capitata group), and collards (Acephala group) are well-known examples.

The cultivated variety (cultivar, cv.) is subordinate to the group classification, and is used to distinguish plants with one or more defining characteristics. Although the term variety is sometimes used in lieu of cultivar, cultivar should not be confused with the botanical variety (varietas, var.) as described above. To qualify for cultivar status, distinguishing characteristics must be preserved when plants are reproduced.

Although not preferred, the term strain is sometimes used for vegetables derived from a well-known cultivar, but with minor differences in form. Clone is used to describe genetically uniform plants vegetatively propagated from a single individual. The term line generally refers to inbred, sexually propagated individuals.

Writing Nomenclature

As with other organisms, the Latin binomial of vegetables is written in italics, with the first letter of the generic name capitalized and the specific name in lowercase letters. Current convention is to use single quotation marks to indicate cultivar status, e.g., Phaseolus vulgaris ‘Manoa Wonder’, while use of cv. preceding the cultivar name is considered obsolete (Brickell et al. 2004). As a designation, the word group may either precede or follow the group name, and is listed in parentheses prior to the cultivar name, e.g., Brassica oleracea (Capitata group) Bravo. The name of the person (authority) who first described the taxon may also be included in the complete name. For example, Cucurbita moschata Duchesne indicates that the species was named by Duchesne, while Cucurbita moschata (Duchesne) Poir indicates that credit for the naming is given to Duchesne in Poir (Paris 2000).

Acknowledgements

We thank Dr. Arthur D. Wall for his review of this chapter, and Jessica W. Radovich for assistance with graphic design of figures. Christina Theocharis is also gratefully acknowledged for her technical assistance.

References

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Chapter 2

Biochemistry of Vegetables: Major Classes of Primary (Carbohydrates, Amino Acids, Fatty Acids, Vitamins, and Organic Acids) and Secondary Metabolites (Terpenoids, Phenolics, Alkaloids, and Sulfur-Containing Compounds) in Vegetables

N. Hounsome and B. Hounsome

Introduction

Historically, major plant constituents were divided as primary and secondary metabolites. Kössel (1891) defined primary metabolites as present in every plant cell that is capable of reproduction, while secondary metabolites are present only accidentally. Plant metabolites determine the food's nutritional quality, color, taste, and smell, and its antioxidative, anticarcinogenic, antihypertension, anti-inflammatory, antimicrobial, immunostimulating, and cholesterol-lowering properties. Primary metabolites are found across all species within broad phylogenetic groups, and are produced using the same (or nearly the same) biochemical pathways. Primary metabolites, such as carbohydrates, amino acids, fatty acids, and organic acids, are involved in growth and development, respiration and photosynthesis, and the synthesis of hormones and proteins. A general scheme of major primary metabolic pathways in plants is shown in Figure 2.1.

Figure 2.1 General scheme of primary metabolic pathways in plants.

Secondary metabolites include terpenoids, phenolics, alkaloids, and sulfur-containing compounds such as glucosinolates. They determine the color of vegetables, protect plants against herbivores and microorganisms, attract pollinators and seed-dispersing animals, and act as signal molecules under stress conditions (Seiger 1998; Crozier et al. 2006). Secondary metabolism is characterized by the high degree of chemical freedom, which is thought to evolve under the selection pressure of a competitive environment (Hartmann 1996).

Primary and secondary metabolites cannot readily be distinguished on the basis of precursor molecules, chemical structures, or biosynthetic origins. For example, terpenoids include primary as well as secondary metabolites (e.g., phytol and gibberellins are primary metabolites, while limonene and menthol are secondary metabolites). A compound such as phylloquinone (vitamin K1) is usually classified as terpenoid quinone, rather than phenolic, while other quinones, such as benzoquinones and anthraquinones, are regarded as phenolic compounds. Nonprotein amino acids (e.g., canavanine and citrulline) are sometimes discussed as primary metabolites since they act as intermediates in the synthesis of the protein amino acids (Morot-Gaudry et al. 2001). At the same time, they can be regarded as secondary metabolites due to their involvement in plant defense mechanisms (Rosenthal 2001; Besson-Bard et al. 2008).

In this chapter, we provide an overview of the major classes of plant metabolites found in vegetables, emphasizing their roles in human health and nutrition. The chapter also contains information about plant metabolites with reported antioxidant properties. The content of selected primary and secondary metabolites in vegetables is presented in Tables 2.1 and 2.2.

Table 2.1 Content of selected primary metabolites in vegetables

Table 2.2 Content of selected secondary metabolites in vegetables

Primary Metabolites

Carbohydrates

Carbohydrates are a class of organic compounds originating from the Calvin Cycle and consisting of carbon, hydrogen, and oxygen (CH2O)n. In plants, carbohydrates occur as monosaccharides (e.g., arabinose, glucose, fructose, galactose, and rhamnose), disaccharides (e.g., sucrose, maltose, and trehalose), sugar alcohols (e.g., sorbitol, mannitol, and xylitol), oligosaccharides (e.g., raffinose, stachyose, and fructooligosaccharides), and polysaccharides (e.g., starch, cellulose, hemicellulose, and pectins). The chemical structure of selected carbohydrates is shown in Figure 2.2.

Figure 2.2 Chemical structure of selected primary metabolites.

Monosaccharides, sucrose, and polysaccharides are present in all vegetables. Raffinose and stachyose have been found in beetroot, broccoli, lentil, pea, onion, and soybean (Obendorf et al. 1998; Frias et al. 1999; Peterbauer et al. 2001; Muir et al. 2009). Fructooligosaccharides (e.g., kestose and nystose) are accumulated in artichoke, broccoli, garlic, leek, and onion (Shiomi 1992; Benkeblia and Shiomi 2006; Muir et al. 2007; Muir et al. 2009). Sorbitol was found in broccoli, cabbage, cauliflower, kale, maize corn, and tomato; mannitol in broccoli, cauliflower, celery, and fennel (Cataldi et al. 1998; Nilsson et al. 2006; Muir et al. 2009).

In terms of their physiological or nutritional role, carbohydrates are often classified as available and unavailable. Available carbohydrates are those that are hydrolyzed by enzymes of the human gastrointestinal system to monosaccharides, such as sucrose and digestible starch. Monosaccharides require no digestion and can be absorbed directly into the blood stream. Unavailable carbohydrates (sugar alcohols, many oligosaccharides, and nonstarch polysaccharides) are not hydrolyzed by endogenous human enzymes. They can be fermented by microorganisms in the large intestine to varying extents and then absorbed (Asp 1996). Fructooligosaccharides and nonstarch polysaccharides are important components of dietary fiber. Sugars are involved in the control of blood glucose and insulin metabolism, intestinal microflora activity, and food fermentation. Monosaccharides bound to protein and lipid molecules (glycoproteins and glycolipids) are involved in cell signaling. The nonenzymatic binding of sugars to proteins produces advanced glycation end products implicated in many age-related chronic diseases, such as type 2 diabetes, cardiovascular diseases, Alzheimer's disease, cancer, and peripheral neuropathy (Jenkins et al. 2002).

Plant components described as dietary fiber typically include nonstarch polysaccharides, resistant oligosaccharides, lignin, and associated substances such as resistant starch, waxes, cutin, and suberin (De Vries 2003). All these materials pass through the gastrointestinal tract as bulk fiber, undergoing modification and digestion by microorganisms in the colon (Blaut 2002). Substances produced by intestinal bacteria may be absorbed into the body. Some products, such as vitamin K, biotin, and fatty acids, may be beneficial. Other substances, such as alcohols, lactate, and formate, as well as hydrogen gas produced by colon fermentation, may be undesirable. The consumption of high dietary fiber foods has been found to reduce symptoms of chronic constipation, diverticular disease, and some types of colitis (Stollman and Raskin 2004). It has been suggested that diets with low fiber may increase the risk of developing colon cancer, cardiovascular diseases, and obesity (Marlett 2001; McGarr et al. 2005; Slavin 2005). Some researchers believe that dietary fiber improves the ability of diabetics to process blood sugar (Willett et al. 2002). Increasing fiber consumption is a challenge as high-fiber containing vegetables do not always have appealing tastes (Tungland and Meyer 2002).

Amino Acids

Amino acids represent a class of organic compounds containing a basic amino (NH2) group, an acidic carboxyl (COOH) group, and a side chain attached to an alpha carbon atom (Figure 2.2). In plants, amino acids are produced via the glycolysis pathway, the pentose phosphate pathway, and the citric acid cycle. Amino acids play a role as intermediates in plant and animal metabolism, and join together to form proteins. Proteins provide structural material for the human body and function as enzymes, hormones, and antibodies. Dietary proteins are the major source of amino acids. Most proteins are broken down by enzymes into amino acids and absorbed from the small intestine. Humans can synthesize a range of amino acids, including alanine, asparagine, aspartic acid, cysteine, cystine, glutamic acid, glutamine, glycine, proline, serine, and tyrosine. Nine amino acids, called essential, must come from the diet, including arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, tryptophan, and valine. The amino acids arginine, methionine, and phenylalanine are considered essential for reasons not directly related to lack of metabolic pathway, but because the rate of their synthesis is insufficient to meet the needs of the body (Spallholz et al. 1999). Histidine is considered an essential amino acid in children. Vegetables contain all essential amino acids, but some may be in lower proportions than are required for humans (Young and Pellett 1994). High levels of arginine have been found in asparagus, Brussels sprout, potato, and watercress; histidine in broccoli, Brussels sprout, and cauliflower; phenylalanine in beet, carrot, parsley, spinach, and tomato; methionine in cabbage, cauliflower, kale, radish, and watercress (FAO 1981). Over 250 nonprotein amino acids (e.g., homoarginine, carnitine, citrulline, taurine, α-aminobutyric acid, and γ-aminobutyric acid) have been identified in plants. γ-Aminobutyric acid (GABA), present in bean, kale, potato, and spinach (Oh et al. 2003), is an inhibitory neurotransmitter in the human central nervous system and in the retina. Carnitine, found in pea, potato, and zucchini, is involved in lipid metabolism in heart and skeletal muscles (Demarquoy and others 2004). Taurine, identified in lentil, maize, and peanuts, is involved in detoxification and membrane stabilization in human cells (Stapleton et al. 1997). Some nonprotein amino acids (e.g., β-cyanoalanine and canavanine found in bean) are reported to be toxic for humans due to inhibition of protein synthesis and immune system (Bell 2003). Besides their importance to the human metabolism, free amino acids contribute to the taste of vegetables. Glycine and alanine are sweet, valine and leucine are bitter, aspartic acid and glutamate have sour and savory tastes (Solms 1969).

Amines and polyamines are synthesized in plants by decarboxylation of amino acids. Amines, such as tyramine, have been found in broad bean, carrot, lettuce, spinach, onion, pepper, potato, and Savoy cabbage; tryptamine in broccoli and carrot; histamine in broad bean, broccoli, spinach, and tomato; 2-phenylethylamine in broccoli, parsley, pepper, onion, and zucchini (Moret et al. 2005). Polyamines putrescine, spermidine, and spermine are ubiquitous in plants, since they play an important role in stress response (Groppa and Benavides 2008). High levels of putrescine were reported in broad bean, broccoli, cucumber, tomato, and zucchini; spermidine in broccoli, cauliflower, parsley, and spinach; spermine in cabbage, cauliflower, and potato (Moret et al. 2005). Spoilage of vegetables is usually associated with the accumulation of histamine, tyramine, agmatine, putrescine, cadaverine, spermine, and spermidine, which can be used as indicators of the degree of freshness or spoilage of food (Halász et al. 1994). In humans, amines are essential for maintaining the metabolic activity of cells and the control of blood pressure and allergic responses (Santos 1996; Kalač and Krausová 2005). Polyamines have been shown to protect cells from oxidative damage and reduce lipid peroxidation and inflammation (L⊘vaas 1997; Farriol et al. 2003). It has been reported that nitrosable secondary amines (agmatine, spermine, and spermidine) can react with nitrite, forming carcinogenic compounds (e.g., nitrosamines, nitrosopyrrolidines, and nitrosopiperidines). High nitrosamine intake has been linked to gastric cancer (Jakszyn and Gonzalez 2006).

Fatty Acids

Fatty acids are predominantly straight-chain organic molecules, consisting of a hydrophilic carboxylic group attached to a hydrophobic hydrocarbon chain (Figure 2.2). In plants, fatty acids are synthesized from acetyl-COA, which is produced from pyruvate by the glycolysis cycle. Fatty acids are major components of fats, oils, and waxes. They provide the human body with energy and structural material for cell membranes and organ padding. Fatty acids are involved in the absorption of vitamins A and D, blood clotting, and the immune response. Some of them are chemical precursors of prostaglandins and leukotrienes (Nettleton 1995; Yaqoob 2004; Chow 2007). Fatty acid classification is based on the number of double bonds. Saturated fatty acids (such as capric, myristic, palmitic, stearic acids) do not contain double bonds. Unsaturated acids with one double bond are called monounsaturated (oleic, palmitoleic); those with two or more double bonds are known as polyunsaturated (docosahexaenoic acid, eicosapentaenoic acid, α-linolenic acid, arachidonic acid) (Gurr et al. 2002). Two unsaturated fatty acids that cannot be made in the body (linoleic acid and α-linolenic acid) must be provided by diet, and are known as essential fatty acids (Innis 1991). α-Linolenic acid can be found in soybean and in most vegetable oils including corn, sunflower, and safflower oil (Connor 1999). Linolenic acid is present in soybean, wheat germ, and pumpkin seeds (Jakab et al. 2002). Green vegetables, such as Brussels sprout, Chinese cabbage, parsley, and watercress, are known to contain a relatively high proportion of polyunsaturated fatty acids, primarily in the form of α-linolenic acid (Pereira et al. 2001). The consumption of monounsaturated and polyunsaturated fatty acids has been shown to regulate plasma cholesterol levels (Fernandez and West 2005) and reduce risk factors associated with cardiovascular disease, cancer, and type 2 diabetes (Kris-Etherton et al. 2003; Larsson et al. 2004; Nettleton and Katz 2005).

Vitamin B Complex

The vitamin B complex of vegetables includes the following water-soluble vitamins: B1 (thiamine and its phosphates); B2 (riboflavin and riboflavin 5-phosphate); B3 (nicotinic acid and nicotinamide); B5 (pantothenic acid); B6 (pyridoxal, pyridoxamine, pyridoxine, and their 5′-phosphates); B7 (biotin); and B9 (folic acid) (Bender 2003). The chemical structure of some B vitamins is shown in Figure 2.2. In plants, these compounds are vital cofactors for enzymes involved in photosynthesis (riboflavin), respiration (thiamine, riboflavin, biotin), synthesis of organic and amino acids (thiamine, folic acid), and regulation of cell division and flowering (niacin). Vitamin B compounds are produced by different metabolic pathways, including the pentose phosphate pathway, glycolysis, and amino acid metabolism (Heldt 2005; Roje 2007). In humans, B vitamins are involved in tissue respiration and carbohydrate, fatty acid, and amino acid metabolism. Deficiency of vitamin B1 can cause polyneuritis; deficiency of vitamin B2 can lead to cheilosis, angular stomatitis, and dermatitis; deficiency of vitamin B3 can result in pellagra, diarrhoea, dermatitis, and dementia; that of vitamin B6 can cause seborrhoea, glossitis, peripheral neuropathy, and microcytic anaemia; deficiency of vitamin B7 can lead to nausea and dermatitis; and that of vitamin B9 can result in anaemia (Combs 1998). Green leafy vegetables, such as asparagus, Brussels sprout, cauliflower, lettuce, spinach, and turnip, are good sources of B vitamins (USDA 2005). Vitamins B1, B2, and B3 have been found in cabbage, carrot, cauliflower, lettuce, potato, spinach, and tomato (Hanif et al. 2006); vitamin B5 in avocado, carrot, French bean, lentil, pea, and spinach (Pakin et al. 2004); vitamin B6 in broccoli, Brussels sprout, cabbage, leek, onion, and potato (Kall 2003); vitamin B7 in bean, broccoli, carrot, cauliflower, spinach, and potato (Staggs et al. 2004); and vitamin B9 in asparagus, broccoli, leek, and potato (Phillips et al. 2008).

Organic Acids and Vitamin C

Organic acids are a group of organic compounds containing carboxylic groups. In solution, organic acids release protons, which determine their acidic taste. Plants contain acetic, aconitic, ascorbic, citric, fumaric, malic, malonic, oxalic, quinic, shikimic, succinic, tartaric, and other organic acids (Heldt 2005). Malic and citric acids are predominant in plants, while succinic, fumaric, and quinic acids are usually present in lower concentrations. Tartaric acid was found in carrot, celery, chicory, endive, and lettuce (Ruhl and Herrmann 1985); oxalic acid in broccoli, Brussels sprout, cabbage, lettuce, and onion (USDA 2005). Organic acids have important functions as flavor enhancers and natural antimicrobial agents. Organic acids give the vegetables tartness and affect flavor by acting on the perception of sweetness (Kader 2008). The sugar/acid ratio is often used to characterize vegetable ripeness (Sims and Golaszewski 2002). For example, the value of about 7.5 is usually accepted as a beneficial sugar/acid ratio in tomato, although values in the range 3.3–21.7 have been reported (Kmiecik and Lisiewska 2000). Organic acids influence the color of vegetables since many plant pigments are natural pH indicators (Davies 2004). For example, some anthocyanins, found in red cabbages and lettuce, change color from red to blue as pH increases.

Ascorbic and dehydroascorbic acids, known as vitamin C, are organic acids with antioxidant properties. Ascorbic acid is the major form of vitamin C in vegetables, while dehydroascorbic acid represents less than 10% of total vitamin C content (Wills et al. 1984). Vegetables rich in ascorbic acid include bean, broccoli, cabbage, cauliflower, cress, pea, spinach, spring onion, and sweet peppers (USDA 2005). Vitamin C is involved in the synthesis of neurotransmitters, steroid hormones, and collagen; in the conversion of cholesterol to bile acids; and in the absorption of iron and calcium. It assists in the healing of wounds and burns, in preventing blood clotting and bruising, and in strengthening the walls of the capillaries (Combs 1998). Because vitamin C is a biological antioxidant, it is also linked to the prevention of cataract, certain cancers, and cardiovascular disorders (Carr and Frei 1999). The content of ascorbic acid in vegetables is strongly affected by growth conditions, application of nitrogen fertilizers, as well as by storage conditions and processing (Mozafar 1993; Rickman et al. 2007; Miglio et al. 2008).

Secondary Metabolites

Terpenoids, including Carotenoids

Plant terpenoids include around 25,000 metabolites (Goldberg 2003) derived by repetitive fusion of branched five-carbon isoprene units. Terpenoids can be classified with respect to the number of isoprene units present in the molecule as hemiterpenes (C5), monoterpenes (C10), sesquiterpenes (C15), diterpenes (C20), sesterterpenes (C25), triterpenes (C30), tetraterpenes (C40), steroids, polyprenols, and polyterpenes. In plants, terpenoids are represented by volatile oils (monoterpenes), gibberellins (diterpenes), limonoids (triterpenes), carotenoids (tetraterpenes), sterols, sapogenins and steroid hormones (steroids), phytol (polyprenols), rubber, gutta, and chicle (polyterpenes) (Goodwin and Mercer 1983; Gershenzon and Kreis 1999). Examples of terpenoids found in vegetables are shown in Figure 2.3.

Figure 2.3 Chemical structure of selected terpenoids.

Terpenoids are synthesized by the polymerization of isopentenyl pyrophosphate (IPP) and its isomer dimethylallyl diphosphate (DMAPP). In plant cells, IPP (DMAPP) are produced via two different pathways, located in separate intracellular compartments. The methylerythritolphosphate pathway takes place in the chloroplasts and forms IPP (DMAPP) for mono- and diterpenoids. The mevalonic acid pathway, found in the cytosol, produces IPP (DMAPP) for sesquiterpenoids. In the second phase of terpene biosynthesis, IPP and DMAPP undergo head-to-tail condensation to produce C10-compound geranyl diphosphate (GDP), C15-compound farnesyl diphosphate (FDP), and C20-compound geranylgeranyl diphosphate (GGDP). In the third phase, GDP, FDP, and GGDP are used to form monoterpenes, sesquiterpenes, and diterpenes, respectively. GGDP generally undergoes further multiple chain extensions to form polyprenylphosphates, which give rise to polyprenols and polyterpenes. Biosynthesis of terpenoids takes place in chloroplasts, mitochondria, and endoplasmic reticulum. For example, carotenoids, phylloquinone, and chlorophylls are synthesized in chloroplasts; ubiquinone in the mitochondria; and sterols in the endoplasmic reticulum. Terpenoids have diverse functional roles in plants as structural components of membranes (sterols), photosynthetic pigments (phytol, carotenoids), electron carriers (ubiquinone, plastoquinone), and hormones (gibberelins, abscisic acid) (Goodwin and Mercer 1983; Seiger 1998; Gershenzon and Kreis 1999). Major groups of terpenoids found in vegetables include carotenoids, tocopherols and tocotrienols, quinones, sterols, sapogenins, and volatile oils.

Carotenoids, such as α-carotene, β-carotene, lycopene, and xanthophylls (e.g., lutein, neoxanthin, violaxanthin, and zeaxanthin) are orange, yellow, and red lipid-soluble pigments. They are found in all green leafy vegetables such as celery, endive, lettuce, rocket, and watercress (where their color is masked by chlorophyll), as well as in carrot, pumpkin, tomato, and sweet potato (Granado et al. 1992; Crozier et al. 2006; Aizawa and Inakuma 2007). In plants carotenoids protect photosynthetic tissues against photooxidative damage and are precursors of phytohormone abscisic acid, which modulates developmental and stress processes (Demmig-Adams and Adams 1996; Taylor et al. 2000). Carotenoids with pro-vitamin A activity are essential components of the human diet. Vitamin A is involved in hormone synthesis, regulation of cell growth and differentiation, and immune responses (Combs 1998; Bender 2003). It can be produced within the body from certain carotenoids, notably β-carotene (present in carrot, spinach, and sweet potato) and α-carotene (found in carrot, pumpkins, and red and yellow peppers) (Bureau and Bushway 1986). Lack of carotenoids in the human diet can lead to xerophthalmia (night blindness) and fetal death. Carotenoid-rich diets are correlated with a significant reduction in the risk for certain forms of cancer, coronary heart disease, and some degenerative diseases, such as cataract (Johnson 2002a). Carotenoids act as biological antioxidants, protecting cells and tissues from oxidative damage (Edge et al. 1997).

Tocopherols and tocotrienols such as α-tocopherol, β-tocopherol, α-T3, and β-T3 are known as vitamin E. These compounds are found in asparagus, broccoli, Brussels sprout, cabbage, carrot, cauliflower, kale, lettuce, spinach, sweet potato, tomato, and turnip (Piironen et al. 1986; Eitenmiller and Lee 2004; Chun et al. 2006). In plants, tocopherols protect chloroplast membranes from photooxidation to provide an optimal environment for the photosynthetic machinery (Munné-Bosch and Alegre 2002). In humans, vitamin E is present in all cell membranes and plasma lipoproteins (especially in red blood cells). As the major lipid-soluble chain-breaking antioxidants in humans, tocopherols and tocotrienols protect DNA, low-density lipoproteins, and polyunsaturated fatty acids from free radical-induced oxidation. They also play a role in stabilizing the structure of membranes, haemoglobin biosynthesis, and modulation of immune response (Brigelius-Flohe and Traber 1999).

Quinones possess aromatic rings with two ketone substitutions. Phylloquinone, known as vitamin K1, is found in asparagus, broccoli, cabbage, cauliflower, cucumber, celery, kale, lettuce, and spinach (Bolton-Smith et al. 2000; Damon et al. 2005). In plants, phylloquinone is involved in electron transport during photosynthesis and in the generation of the active oxygen species observed as a reaction to pathogen attack or stress (Lochner et al. 2003). In humans vitamin K1 plays a role in the control of blood clotting, bone formation, and repair. A deficiency of this vitamin results in hemorrhagic disease in newborn babies as well as postoperative bleeding, hematuria, muscle hematomas, and intercranial hemorrhages in adults (Combs 1998; Bender 2003). Menadione, known as vitamin K3, has been shown to possess cytotoxic activity and inhibit growth of tumors (Taper et al. 2004). Quinones are highly reactive and are responsible for the browning reaction in cut or injured vegetables (Cowan 1999).

Plant sterols, such as sitosterol, sitostanol, campesterol, brassicasterol, and stigmasterol, are found in broccoli, Brussels sprout, carrot, cauliflower, celery, tomato, soy, and spinach (Normén et al. 1999; Piironen et al. 2003). In plant membranes, sterols regulate the fluidity and the permeability of phospholipid bilayers (Hartmann 1998). Sterols are precursors of plant hormones brassinosteroids, involved in embryonic development, cell division, plant growth, and fertility (Clouse and Sasse 1998). Upon UV irradiation of human skin, these sterols give rise to calciferol (vitamin D2), which is involved in the absorption of calcium and bone growth. Sterols are essential for the synthesis of prostaglandins and leukotrienes, important components of the immune system. Due to their structural similarity to cholesterol, plant sterols inhibit cholesterol absorption. In addition to their cholesterol-lowering effect, plant sterols may possess anticancer, antiatherosclerosis, anti-inflammation, and antioxidant properties (Awad and Fink 2000; Ostlund 2002; Dutta 2003).

Volatile oils include monoterpenes (e.g., linalool, thymol, and fenchol) and sesquiterpenes (e.g., farnesol and carotol). Volatile oils determine the odor of vegetables, since they vaporize at room temperature. Terpenes geraniol and farnesol have been found in tomato, fenchol in fennel, linalool in celery, thymol in thyme, and carotol in carrot (Enfissi et al. 2005; Sowbhagya et al. 2007; Bakkali et al. 2008). Volatile oils possess cytotoxic, antiproliferation, and antimutagenic activities. They inhibit the growth of tumor cells by interacting with the mitochondrial function and inducing oxidative stress (Bakkali et al. 2008). Volatile oils have a pronounced antimicrobial action against food-borne pathogens and spoilage bacteria (Gutierrez et al. 2009), and act as insecticides against mosquitoes and pest insects (Tapondjou et al. 2005; Silva et al. 2008). Although most volatile oils have been found cytotoxic without being mutagenic, some of them (e.g., safrole and estragole) may have secondary carcinogenic effects in animals (Bakkali et al. 2008).

Phenolics

Plant phenolic compounds include around 8,000 metabolites (Goldberg 2003), which contain one or more phenolic residues. Phenolics can be classified by the number of carbon atoms in their skeleton as phenols (C6), phenolic acids (C6–C1), phenylacetic acids (C6–C2), hydrohycinnamic acids, coumarins and chromones (C6–C3), naphthoquinones (C6–C4), xanthones (C6–C1–C6), stilbenes and anthraquinones (C6–C2–C6), flavonoids (C6–C3–C6), lignans and neolignans ([C6–C3]2), biflavonoids ([C6–C3–C6]2), lignins ([C6–C3]n), melanins ([C6]n), and condensed tannins ([C6–C3–C6]n) (Goodwin and Mercer 1983). Plant phenolics are synthesized through two major pathways: the shikimic acid pathway and the malonic acid pathway. The shikimic acid pathway is involved in the biosynthesis of most plant phenolics, while the malonic acid pathway (typical for fungi and bacteria) is of less significance in higher plants. The shikimic acid pathway converts intermediates from glycolysis (phosphoenolpyruvate and erythrose 4-phosphate) to chorismate, the precursor of aromatic amino acids and many phenolic compounds (Herrmann and Weaver 1999). The general scheme of phenolic biosynthesis via the shikimate pathway is shown in Figure 2.4.

Figure 2.4 General scheme of synthesis of plant phenolic compounds via the shikimic acid pathway.

The major groups of phenolic compounds found in vegetables include phenolic, hydroxycinnamic, and phenylacetic acids; coumarins; flavonoids; lignans; lignins; and tannins. Hydroxycinnamic and phenylacetic acids are often referred to as phenolic acids. Flavonoids, coumarins, lignans, lignins, and hydroxycinnamic acids are also classified as phenylpropanoids, since they originate from phenylalanine. The chemical structure of representative phenolic compounds is shown in Figure 2.5.

Figure 2.5 Chemical structure of selected phenolic compounds.

Phenolic compounds are feeding deterrents for many insects and other plant-eating animals. High concentrations of phenolic compounds are often associated with increased resistance to fungal pathogens (Nicholson and Hammerschmidt 1992). Some phenolics determine the color and smell of plants, attracting pollinators. Phenolics are involved in cold acclimation and protection against UV radiation. In plant cells most phenolic compounds are coupled to sugars to reduce their endogenous toxicity. External stimuli such as microbial infection, ultraviolet radiation, temperature, and chemical stressors induce their synthesis (Parr and Bolwell 2000).

Phenolic acids are chemical compounds with at least one aromatic ring bearing one or more hydroxyl groups. These compounds include phenolic acids (e.g., hydroxybenzoic, gallic, and vanillic), hydroxycinnamic acids (e.g., ferulic, coumaric, and caffeic), and phenylacetic acids (e.g., phenylacetic and hydroxyphenylacetic). Chlorogenic acid has been found in bean, carrot, cauliflower, and lettuce; coumaric in cabbage and cauliflower; protocatechuic in bean and carrot; and sinapic in cauliflower and turnip (Mattila and Hellström 2007). In plants, these compounds fulfill antipathogen, antiherbivore, and allelopathic roles (Nicholson and Hammerschmidt 1992; Chou 1999). Salycilic acid plays an important role in cell signaling under stress conditions (Klessig and Malamy 1994). Dietary phenolic acids, such as benzoic, hydrobenzoic, vanillic, and caffeic, were reported to have antimicrobial and antifungal action, probably due to enzyme inhibition by the oxidized compounds (Cowan 1999). Hydroxycinnamic acid derivatives, such as caffeic, chlorogenic, sinapic, ferulic, and p-coumaric acid, possess strong antioxidant activity due to the inhibition of lipid oxidation and scavenging reactive oxygen species (Sroka and Cisowski 2003; Cheng et al. 2007). Some phenolics such as syringic acid may contribute to the bitter and astringent taste of vegetables (Drewnowski and Gomez-Carneros 2000).

Flavonoids represent the most numerous (∼4,000 compounds) group of plant phenolics (Harborne 1993). Flavonoids are classified as flavones (e.g., apigenin, luteolin, and chrysoeriol), flavonols (e.g., quercetin, kaempferol, and isorhamnetin), flavanones (e.g., naringenin and hesperetin), catechins (e.g., catechin and epigallocatechin), anthocyanidins (e.g., pelargonidin, cyanidin, delphinidin, and malvidin), isoflavones (e.g., genistein and daidzein), and chalcones (e.g., butein and phloretin). Flavonols quercetin, kaempferol, and isorhamnetin have been found in bean, broccoli, endive, leek, onion, and tomato; flavones apigenin and luteolin in bean, red peppers, parsley, and thyme; anthocyanidins cyanidin, delphinidin, and malvidin in onion, radish, red cabbage, and red lettuce; and isoflavones in soy (Hertog et al. 1992; Song et al. 1998; Yao et al. 2004; Horbowicz et al. 2008). Most of the flavonoids present in plants are attached to sugars (glycosides) (Ross and Kasum 2002). Many flavonoids, such as anthocyanidins, chalcones, and flavones, are plant pigments, which determine the color of vegetables. Dietary flavonoids possess antiviral, anti-inflammatory, antihistamine, and antioxidant properties. They have been reported to inhibit lipid peroxidation, to scavenge free radicals, to chelate iron and copper ions (which can catalyze production of free radicals), and to modulate cell signaling pathways (Heim et al. 2002; Rice-Evans and Packer 2003). Flavonoids protect low-density lipoprotein cholesterol from being oxidized, preventing the formation of atherosclerotic plaques in the arterial wall. They stimulate enzymes involved in detoxification of cancerogenic substances and inhibit inflammation associated with local production of free radicals (Hollman and Katan 1999). A number of biflavonoids have been shown to possess antituberculosis activity (Lin et al. 2001). Most