The Flavonoids Advances in Research Since 1986

J.B. Harborne

• Publisher: Routledge
• Year: 2017
• eBook ISBN: 9781351410441
• Topic: Sciences physiques
• Subtopic: Chimie

Flavonoids are a group of natural products isolated from a wide variety of plants, and are responsible for much of the coloring found in vascular plants. They exhibit a wide range of biological activities and are of particular interest as potential anti-cancer agents, as insect antifeedants, and as natural insecticides. The Flavonoids: Advances in Research Since 1986 is a self-contained account of this important group of plant products.

1

The anthocyanins

DIETER STRACK and VICTOR WRAY

1.1 Introduction

1.2 Analytical procedures

1.3 Chemistry

1.4 Distribution

Acknowledgements

References

1.1 INTRODUCTION

1.1.1 Occurrence

Anthocyanins are the most important group of water-soluble plant pigments visible to the human eye. With a few exceptions, e.g. the betalains, they are universal plant colorants and largely responsible for the cyanic colours of flower petals and fruits. They may also occur in roots, stems, leaves and bracts, accumulating in the vacuoles (Wagner, 1982) of epidermal or subepidermal cells. They are also found infrequently in the petal mesophyll, e.g. in most members of the Boraginaceae (Harborne, 1988b). In a recent study on the histological distribution of anthocyanins in spathes of various Anthurium species, specific distributions of anthocyanin-containing cells of the ad- and abaxial epidermis, hypodermis and mesophyll were observed (Wannakrairoj and Kamemoto, 1990). The authors suggest that these specific distributions might even be of value in establishing intrasectional species relationships. An interesting tissue compartmentation has been reported by Miyazaki et al. (1991), who identified different patterns of hydroxybenzoyl and hydroxy-cinnamoyl acylated anthocyanins in the tuber periderm and flesh, respectively, of two cultivars of Ipomoea batatas.

The anthocyanins are usually in solution within the vacuole, although they may sometimes be located in spherical vesicles, called ‘anthocyanoplasts’ (Pecket and Small, 1980), in which structural modifications of the C₁₅ nucleus may occur (Merlin et al., 1985). Development of these vesicles, involving light-dependent anthocyanin accumulation, has been observed in cells of Ipomoea batatas suspension cultures (Nozue and Yasuda, 1985). They were also detected in vacuoles from Vitis vinifera suspension cultures (Cormier et al., 1990). Anthocyanoplasts are usually detected in vacuoles, although they may also appear as membranous vesicles in the cytoplasm (Nozzolillo and Ishikura, 1988). The presence or absence of anthocyanoplasts in vacuoles can provide additional data useful to taxonomic studies (Nozzolillo and McNeill, 1985), and has been exemplified with seedlings from 31 taxa of Phaseolus and Vigna. Anthocyanoplasts were found in all tested members of the Vigna subgenus Ceratotropis.

1.1.2 Biosynthesis

The enzymology of the late steps in anthocyanin formation is still incomplete. The compounds involved are collectively named ‘flavonoids’. More than 4000 known flavonoid structures are divided into 12 classes on the basis of the oxidation level of the central pyran ring. In the pivotal step of flavonoid biosynthesis, ordinarily 4-coumaroyl-coenzyme A, derived from L-phenylalanine in the ‘general phenylpropanoid metabolism’ (Hahlbrock and Grisebach, 1975), enters a stepwise condensation reaction with three molecules of malonyl-coenzyme A to form the C₁₅ chalcone intermediate, the tetrahydroxychalcone (naringenin chalcone). In the following well known ‘flavonoidal metabolism’, the actual precursor for anthocyanin formation, the flavan-3,4-cis-diol (leucoanthocyanidin), is formed, which then appears to be converted to the anthocyanidin flavylium cation by a hydroxylation at C-2 followed by two dehydrations. The enzymic conversion of leucoantho-cyanidins to anthocyanidins, however, has not yet been demonstrated, and nor has it been for the analogous reaction with the flavan-4-ol leading to 3-desoxy-anthocyanidins (Forkmann, 1991, this volume).

1.1.3 Function

Anthocyanins play a definite role in the attraction of animals as pollination and seed dispersal factors, and hence they are of considerable value in the coevolution of these plant-animal interactions. Thus, anthocyanin patterns in a given plant family might be more closely correlated with the pollinator class than with taxonomy, as has recently been discussed in the case of floral anthocyanins among 87 Penstemon species (Scrophulariaceae) (Scogin and Freeman, 1987). In particular, the aglycones may be related to pollination ecology, since they constitute the chemical basis of flower colour in angiosperms along with other colour-modifying factors (Harborne, 1988a; Brouillard and Dangles, this volume). Scogin (1988) found, in a survey of anthocyanidins of 146 species of bird-visited flowers, that a ‘bird-visitation pigment syndrome’ was generally uniform across wide geographical distances. The pigment syndromes of perching and hovering birds were distinct. There was no evidence for pelargonidin enrichment in tropical floras, as suggested by Harborne (1976), at least for hummingbird-visited flowers. The author discusses, from advancement indices and pigment frequencies, that floral adaptation for bird visitation may not be accompanied by great evolutionary advancement in pigment composition. In another survey on anthocyanins of the genus Erythrina (Fabaceae), no correlation between floral pigments and class of avian pollinators could be detected (Scogin, 1991).

In contrast to flower pigmentation, the transient appearance of anthocyanins in certain seedcoats, seedlings, leaves, stems and roots does not find an easy explanation. Several speculations on the perception or filtration of light and response to stress factors, including microbial attack, await further thorough studies. Nicholson and coworkers (Nicholson et al., 1987, 1988; Hipskind et al., 1990) found the 3-desoxyantho-cyanidins apigeninidin and luteolinidin, as well as the caffeic acid ester of apigeninidin 5-arabinoside, as phytoalexins produced in response to microbial infection. Anthocy...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• List of contributors
• Preface
• 1 THE ANTHOCYANINS
• 2 FLA VANS AND PROANTHOCYANIDINS
• 3 C-GLYCOSYLFLAVONOIDS
• 4 BIFLAVONOIDS AND TRIFLAVONOIDS
• 5 ISOFLAVONOIDS
• 6 NEOFLAVONOIDS
• 7 FLAVONES AND FLAVONOLS
• 8 FLAVONE AND FLAVONOL GLYCOSIDES
• 9 THE MINOR FLAVONOIDS
• 10 1H NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY OF FLAVONOIDS AND THEIR GLYCOSIDES IN HEXADEUTERO-DIMETHYLSULFOXIDE
• 11 BIOSYNTHESIS OF FLAVONOIDS
• 12 GENETICS OF FLAVONOIDS
• 13 FLAVONOIDS AND FLOWER COLOUR
• 14 FLAVONOIDS AND INSECTS
• 15 THE IMPACT OF PLANT FLAVONOIDS ON MAMMALIAN BIOLOGY: IMPLICATIONS FOR IMMUNITY, INFLAMMATION AND CANCER
• PLANT SPECIES INDEX
• SUBJECT INDEX