Survey of Phenolic Compounds Produced in Citrus

Flavonoid Composition of Citrus

Table 3 and table 4 summarize our flavonoid analysis of Citrus species and their relatives. Additional information on flavonoid composition summarized from the literature can be found in appendix 2. Table 3 lists the 12 flavanones, and table 4 lists the 4 flavones quantitated in our survey. The cultivars of the genus Citrus and its related species accumulate flavanone glycosides, along with some flavone and flavonol glycosides. The basic structure of these compounds in shown in figure 4. Most cultivars can be classified by the predominant glycosylation pattern on the flavonoid skeleton (Kumamoto et al. 1986, Robak and Gryglewski 1988). This glycosylation occurs at the "7" position on all compounds tested except rutin, which is glycosylated at the "3" position. Two main flavonoid glycsosylation patterns can be used to categorize Citrus species: the 7–O–neohesperidoside (2–beta–l–rhamnosyl–d–glucose) pattern, which is found only in species related to the pummelo, and the 7–O–rutinoside (6–beta–l–rhamnosyl–d–glucose) pattern, which is found in all species of citrus. The structures of these sugar groups are also shown in figure 4.

The dominant neohesperidosyl flavanones were naringin (found at high percentages in C. rugulosa [attani], citrangedin, grapefruit, kumquat, and pummelo), neoeriocitrin (one type of bergamot lemon and some sour orange tissues), poncerin (somePoncirus tissues), and neohesperidin (in C. rugulosa and tangelo). The dominant rutinosyl flavanones were hesperidin (C. macrophylla [alemow], citrangor, most citrons, C. sulcata, C. ujukitsu, lemon, lime, megacarpa papeda, mandarin, rough lemon, and sweet orange), eriocitrin ('Spadifora' citron, 'Seedless Lisbon' lemon), and narirutin (citrangor and some papedas).

In addition, the presence of rutin and diosmin is characteristic of cultivars related to the citron, while somewhat simple (two to five peaks) flavonoid patterns with hesperidin as a major flavonoid seem to be characteristic of the cultivars most closely related to citron and mandarin, two of the primary citrus species. Pummelo and its close hybrids, as noted above, have a more complex flavonoid pattern dominated by the flavonoid–7–O– neohesperidosides.

Chromatograms obtained from extracts of mature fruits and leaves reveal that there generally are more flavanones occurring for each cultivar than has been previously reported in the literature (Hattori et al. 1952, Mizelle et al. 1965, Hagen et al. 1966, Maier and Metzler 1967, Fisher 1968, Albach and Redman 1969, Albach et al. 1969, Coffin 1971, Brunet and Ibrahim 1973, Dreyer and Huey 1974, Tomas et al. 1978, Kamiya et al. 1979, Tatum and Berry 1979, Robbins 1980, Lawrence 1982, Park et al. 1983, Matsubara et al. 1985, Kumamoto et al. 1986, Gaydou et al. 1987, Mizuno et al. 1987, Albach and Wutscher 1988, Jacobs and Rubery 1988, Robak and Gryglewski 1988, Stolk and Siddiqui 1988, Vandercook and Tisserat 1989). In general the previous publications on this subject either report only the presence or absence of flavonoids (but do not quantify them) or quantify a limited number of flavonoids (one to six) in juice or fruit. Often, only commercially important cultivars were examined, such as lemon, orange, and grapefruit. Table 3 and table 4 quantify 16 major flavonoids in fruit and leaf tissue from 107 citrus cultivars and near citrus relatives.

In addition we have found a number of cultivars that may contain malonyl esters of the flavanone glycosides. The first such compound, naringin–6"–malonate was identified in grapefruit at the Pasadena laboratory (Berhow et al. 1991). It is possible that some of the compounds we have labeled as naringin–6"–malonate may be other flavanone malonyl esters, although they appear to elute at reproducible retention times for naringin–6"–malonate. Further investigation is required on these esters.

The genetics of flavanone enzyme pattern inheritance is unknown. Also, which flavanone glycosylation pattern is dominant or recessive is not known. Although we did not analyze all cultivars available in the Citrus Variety Collection at the University of California, Riverside, our results indicate that the glycosidyl flavanone pattern is inheritable, since homozygous or heterozygous conditions are observed.

The flavone rutinosides diosmin and isorhoifolin with the flavonol rutinoside rutin are common constituents in Citrus species (Robak and Gryglewski 1988, Vandercook and Tisserat 1989). Figure 2 shows the spectra used in our analysis for this flavone class as well as for the neohesperidosyl flavone rhoifolin in Citrus. Flavones differ from flavanones by having a "2,3" double bond in the C–ring; rhoifolin corresponds to naringin, isorhoifolin corresponds to narirutin, and diosmin corresponds to hesperidin. Rutin, a prominent flavonol, is unique from the other Citrus flavonoids—it is the 3–O–rutinoside of the flavonol quercetin, and seems to be accumulated in species related to the citron.

Higher concentrations of flavones and flavonols occur in the leaves than in the flavedo, albedo, and juice vesicles. Attani, pummelo, sour orange, lemelo, Natsudaidai orange—species with mainly neohesperidosyl flavanones—also contain the neohesperidosyl flavone rhoifolin. Grapefruit, a pummelo × orange cross (Barrett and Rhodes 1976), contains rhoifolin and small amounts of isorhoifolin. Citrumelo, a P. trifoliata × C. paradisi hybrid (Hodgson 1967), contains rutin, isorhoifolin, and rhoifolin.

For discussion of individual citrus types, click below:

Citron	Natsudaidai orange
Citrumelo	Papeda
Grapefruit	Pummelo
Lemon	Rough lemon
Lime	Sour orange
Mandarin	Sweet orange

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United States Department of Agriculture
Agricultural Research Service

The material on this page is in the public domain.

Original posting: April 1, 1999.