Dahlia pinnata Cav.

Bidens variabilis (Desf.) Baill., Coreopsis crassifolia Sessé & Moc. (Illeg.), Coreopsis georgina Cass., Dahlia astrantiaeflora (Sweet) G. Don, Dahlia × hortensis Guillaumin, Dahlia hybrid, Dahlia nana Andrews, Dahlia pinnata var. nana B.D. Jacks., Dahlia pinnata var. variabilis (Willd.) Voss, Dahlia purpurea (Willd.) Poir., Dahlia purpurea var. flavescens (DC.) Poir., Dahlia purpurea var. lilacina (Willd.) Poir., Dahlia purpurea var. pallida (Willd.) Poir., Dahlia purpurea var. rubra (DC.) Poir., Dahlia pusilla Zucc. ex DC., Dahlia rosea Cav., Dahlia royleana Knowles & Westc., Dahlia sambucifolia Salisb., Dahlia sphondyliifolia Salisb. (Illeg.), Dahlia superflua (DC.) W.T. Aiton, Dahlia variabilis (Willd.) Desf., Georgia superflua DC., Georgia superflua var. flavescens DC., Georgia superflua var. lilacina (Willd.) DC., Georgia superflua var. pallida (Willd.) DC., Georgia superflua var. purpurea DC., Georgia superflua var. rubra DC., Georgia variabilis (Willd.) Spreng., Georgina astrantiaeflota Sweet, Georgina purpurea Willd., Georgina rosea (Cav.) Willd., Georgina variabilis Willd.

Asteraceae

Aztec Dahlia, Dahlia, Garden Dahlia, Pinnate Dahlia

The species is native to Mexico and Central America to Columbia.

Dahlia grows best in full sun in friable, moderately moist, well-drained, fertile soils with pH 6–6.5. It thrives best in volcanic soils as found in its natural habitat in Mexico. It is frost tender and drought intolerant.

The flower petals are eaten in salads; the tubers are eaten as vegetables in some parts of Mexico (Uphof 1968; Hedrick 1972; Facciola 1990; Roberts 2000). A sweet extract of the tuber, called ‘dacopa’, is used as a beverage or as a flavouring, mixed with hot or cold water or milk or sprinkled on ice cream. Its naturally sweet mellow taste is said to combine the characteristics of coffee, tea and chocolate. Some culinary recipes of Dahlia flowers include Mexican mealie and chili dish, cream cheese and Dahlia dip and sun-dried tomato and Dahlia bread (Roberts 2000).

The tubers are rich in reserve carbohydrate, inulin, a fructan, which is composed of β(2 → 1) linked d-fructose residues and provides a good natural source for the production of fructose syrups that are widely used in the food industry (Mino et al. 1985; Mangunwidjaja et al. 2002). Fructose is an interesting sweetener because of its high sweetening power. Fructose is claimed to be less cariogenic than other sugars and to be more suitable for diabetics since it is insulin nondependent (Barker and Petch 1985). Purified inulin from Dahlia tubers was partially hydrolyzed to form fructooligosaccharides by using citric or phosphoric acids (pH, 2.0–2.5) as mild acid catalysts (Fontana et al. 2011). These whole hydrolysates can be advantageously added as nutraceuticals to carbonated beverages and acidic foods, such as soft drinks and yogurts.

The common garden variety Dahlia was once an important root crop and medicinal plant among the pre-Columbian Indians of Central Mexico, Yucatan and Guatemala. Its roots were valued both for the nutritious inulin stored inside them and for the antibiotic compounds concentrated in the skin of the tubers (Whitley 1985).

A deciduous, branched, perennial herb, to 1.8 m high with large subterraneous tuberous roots. Internode of stems hollow, leaves opposite or whorled, simple to 2-pinnatisect with 3–5 elliptic, serrated leaflets, glabrous and glaucous beneath (Plates 2 and 3). Inflorescence of solitary or few, long-pedunculate, involucrate, radiate capitulum, 10–20 cm across. Involucral bracts in 2 series, the outer series rather fleshy and foliaceous, inner series membranous and shortly united at the base, receptacle scaly. Disc florets bisexual, actinomorphic, tubular, 5-lobed sometimes mostly or all replaced by ray florets as in cultivated hybrid progeny. Ray florets, several marginal rows, zygomorphic, female and fertile, or neuter, the ligules patent with incurved or recurved margins, purple in wild plants and white, yellow, pink, red or purple in horticultural improved plants (Plates 1, 2 and 3). Achenes dorsally compressed, pappus absent or shortly bidentate.

Pigments responsible for the diverse range of floret colours ivory, yellow, pink, red, purple and red-black in Dahlias had been reported to include flavonoids, mainly anthocyanins, butein, and flavones and their derivatives (Bate-Smith and Swain 1953; Nordstrom and Swain 1953, 1956, 1958; Bate-Smith et al. 1955; Harborne et al. 1990; Yamaguchi et al. 1999). Pigments were first studied in Dahlia in the early 1950s when the presence of anthocyanins, flavones, flavonols, chalcones and aurones were elaborately described in Dahlia species (Bate-Smith and Swain 1953; Nordstrom and Swain 1953, 1956, 1958; Bate-Smith et al. 1955). Bate-Smith and Swain (1953) isolated 2,4,4′-trihydroxychalcone from yellow varieties of Dahlia variabilis. Apart from anthocyanins and 6′-deoxychalcones, flavones and flavonols could accumulate in Dahlia flowers (Nordstrom and Swain 1953, 1956, 1958). Blue-coloured flower of D. variabilis var. Dandy was found to contain apigenin, its 4- and 7-monoglucosides and 7-rhamnoglucoside, luteolin 5-mono-glucoside and 7-diglucoside, cyanidin arabinoglucoside, and a few minor compounds (Nordstrom and Swain 1953). Flavonoid glycosides extracted from the yellow petals of two yellow Dahlias included 3′,4′,6-trihydroxyaurone in var. ‘Pius IX’. The 6-mono- and diglucosides of this compound were found in var. ‘Coton’ together with the 7-mono- and diglucosides of liquiritigenin, the 4-monoglucosides of both butein and 2,4,4′-trihydroxychalcone, and the 4-diglucoside of the last named compound (Nordström and Swain 1956). The ivory Dahlia ‘Helly Boudewyn’ contained the same flavone glycosides as those which had previously been found in the blue Dahlia ‘Dandy’, and naringenin and eriodictyol were found in a white Dahlia ‘Clare White’ (Nordström and Swain 1958). Polyacetylene compounds with an ene-diynediene chromophore were found abundantly in D. pinnata, whereas most traces of compounds with this chromophore were detected in D. coccinea (Bendixen et al. 1969). Three Dahlia pinnata varieties were found to contain 1-phenylhept-5-ene-1,3-diyne and 1-phenyl-hepta-1,3,5-triyne and derivatives which had previously been found in one Dahlia coccinea variety and in a horticultural form (Lam 1973).

Kaufmann and El Baya (1970) and Harborne et al. (1990) found 6′-deoxychalcones (derivatives of butein and isoliquiritigenin) and the corresponding 4-deoxyaurones (derivatives of sulfuretin) to be the chemical base of yellow flower colour in D. variabilis; these were mixed with anthocyanins (derivatives of pelargonidin and cyanidin) in orange and red forms. The yellow pigments, the 4′-malonylsophoroside and 4′-malonylglucoside of butein, were found in the petals of Dahlia variabilis and D. coccinea, co-occurring with malonylated anthocyanins in these flowers (Harborne et al. 1990). A screening of more than 200 Dahlia cultivars revealed that the different red tones were based on the same set of anthocyanins and that variation in the anthocyanin concentration, the modification pattern of the core structures, and probably also pH were responsible for the formation of different hues (Halbwirth et al. 2008). Orange, rose and lilac cultivars frequently showed lower anthocyanin contents than red and magenta cultivars. Rose and lilac cultivars appeared to be primarily based on a lower chalcone synthase activity (Halbwirth et al. 2008). Thus, Dahlia flower colour appeared to be exclusively based on the accumulation of flavonoids and biochemically related anthochlors (chalcones, aurones) (Harborne 1967; Giannasi 1975; Halbwirth et al. 2008). Recently, Thill et al. (2012) reported that Dahlia resulted from the accumulation of red anthocyanins, yellow anthochlors (6′-deoxychalcones and 4-deoxyaurones) and colourless flavones and flavonols, acting as copigments.