Flavonoids and flower colour

In this review covering the years since the previous edition of The Flavonoids, it is intended to deal only with those flavonoids which have been discovered to be directly involved in flower pigmentation. This chapter has been written in a way that, it is hoped, avoids as much as possible overlap either with recent publications in the same field or with other contributions to this volume. The same topic has been addressed by one of us (Brouillard, 1988) in the previous edition, and it is assumed that the contents of that work constitute the basic knowledge for the comprehension of the present chapter. Analytical methods, especially when they are applied to the anthocyanins, are considered elsewhere in this book, particularly in Chapters 1 and 10, but not here. Moreover, previous good reviews on similar matters are those by Harborne and Grayer (1988) (analysis, chemistry, distribution and applications of anthocyanins), by Strack and Wray (1989) (structures, properties, extraction, isolation and analytical methods of anthocyanins) and by Brouillard and Cheminat (1988) (chemical structures of flavonoids in relation to plant colour). An entire volume has been devoted to the biochemistry and chemistry of fruit phenolics, including the fruit anthocyanins (Macheix et al., 1990), and the function of anthocyanins as fruit colorants has been extensively reviewed (Mazza and Brouillard, 1987a; Francis, 1989; Macheix et al., 1990). Volume 1 in the series Methods in Plant Biochemistry (Harborne, 1989) encompasses all plant phenolics known today. Genetic engineering is now able to provide or suppress pathways for colour in flowers of some species, especially Petunia hybrida (Meyer et al., 1987; Gerats et al., 1990). This exciting field is reviewed in Chapter 12. That flower and fruit colours are important for humans and animals will be emphasized in the last two chapters of this volume. Anthocyanin biosynthesis is treated in Chapter 11, and here we essentially focus our attention on the production of colour by flavonoids. To provide colour to flowers, coloured as well as colourless flavonoids must accumulate in vacuoles and remain in a stable chemical state for periods of time ranging from hours to weeks. It is this stability that is presently important, and there is no doubt that it is largely dependent upon the physicochemical conditions prevailing in those flavonoid- containing vacuoles. Part of the results and conclusions reached in this review may be extended to other plant organs with coloured flavonoids like, for instance, fruits and stems. New structures of the coloured flavonoids and older structures that have been reinvestigated, especially in the case of anthocyanins, will only be given if found in flowers. At this stage, it is worth pointing out that some of the recently discovered polyacylated anthocyanins are probably the largest flavonoids as well as the least easy to handle. For a complete survey of such astonishing structures, see the recent reviews by Goto (1987) and by Goto and Kondo (1991). Although analytical methods useful in the investigation of anthocyanin structures will not be dealt with here, it is recognized that the best analytical tool for the structural elucidation of the complex anthocyanins is ¹H nuclear magnetic resonance (NMR) spectroscopy, with its multiple facets. As usual, it is more difficult to apply 566a new technique to the anthocyanins than to any of the remaining flavonoid families, and therefore ¹³C NMR spectroscopy has until now rarely been used with the anthocyanins (Agrawal, 1989; Terahara et al., 1990a).