Survey of Phenolic Compounds Produced in Citrus
Flavonoid Composition of Citrus

Grapefruit (C. paradisi) is a pummelo × sweet orange hybrid (Swingle and Reece 1967, Scora 1975a). Older surveys, reviews and analyses of grapefruit mention naringin (Maier and Metzler 1967, Swingle and Reece 1967, Nishiura et al. 1969, Dreyer and Huey 1974, Metzler 1977, Kamiya et al. 1979, Anis and Aminuddin 1981, Park et al. 1983, Jourdan et al. 1985, Albach and Wutscher 1988) and poncerin (Anis and Aminuddin 1981) as well as narirutin, neohesperidin, hesperidin, didymin (Hagen et al. 1965, Mizelle et al. 1965, Hagen et al. 1966, Albach and Redman 1969, Albach et al. 1969, Coffin 1971, Horowitz and Gentili 1977, Park et al. 1983), and narirutin–4'–glucoside and naringin–4'–glucoside (Mizelle et al. 1967) with additional traces of a dihydrokaempferol glycoside (Maier and Metzler 1967) in grapefruit. Most recent flavanone research on Citrus, and most quantitative studies, have included different grapefruit cultivars. Nishiura et al. (1971a,b) found mainly naringin in the leaves and ripe peel with naringin and poncerin in the juice of the ripe fruit. In part, the large variation in the results of the above studies may be due to insensitive or selective flavanone isolation or detection techniques. However, some studies of Citrus hybrids, including grapefruit cultivars, have noted that the hybrids can be divided into groups depending on their flavanone content or major flavanone (Tatum et al. 1978, Kamiya et al. 1979, Rousseff et al. 1987).

We can artificially divide the leaf of grapefruits and pummelo hybrid cultivars into the following four main groups depending on their major flavonoids:

• naringin/rhoifolin group ('Camulos', 'Duncan', 'Jochimsen', 'Star Ruby', 'Whitney Old Line Marsh')
• neohesperidin/neoeriocitrin/naringin group (C. rugulosa, C. natsudaidai, lemelo, 'Philippine' pummelo hybrid, shaddock × 'St. Michael' orange, 'Yuma Ponderosa Lemon' pummelo hybrid, 'CRC #343' grapefruit)
• hesperidin group ('Hall' grapefruit, 'Reed Seedling Marsh' grapefruit, 'Yellow Rind Mandarin' grapefruit, C. shunokan, C. sulcata)
• eriocitrin/narirutin/naringin group ('Red Aranyan' pummelo hybrid).

Similarly, Kamiya et al. (1979) has artificially divided pummelo and the Natsudaidai hybrids into five groups based on the flavanones in their leaves:

• naringin group
• naringin and neohesperidin group
• naringin and neoeriocitrin group
• neohesperidin and neoeriocitrin group
• naringin, neohesperidin, and neoeriocitrin group.

Consequently, any flavanone composition results for grapefruit could vary greatly depending on the cultivar chosen for study.

A few detailed quantitative studies on grapefruit flavanones have been compared with results of this survey to demonstrate the sensitivity and nonselectivity of our method (Hagen et al. 1965, 1966; Jourdan et al. 1985, Rousseff et al. 1987, Albach and Wutscher 1988). For example, using HPLC Rousseff et al. (1987) obtained the following values in 'Star Ruby' grapefruit juice: narirutin = 35 mg/L, naringin = 73 mg/L, hesperidin = 4 mg/L, and neohesperidin = 5 mg/L. Our values for juice vesicles in 'Star Ruby' grapefruit were narirutin = 210 mg/L, naringin = 1,040 mg/L, hesperidin = 0 mg/L (consistent with the fact that our method does not detect below 10 mg/L) and neohesperidin = 20 mg/L. Using a radioimmunoassay, Jourdan et al. (1985) obtained an average value of 4.64 mg naringin per gram fresh weight in mature leaves of 'Duncan' grapefruit. However, their assay simultaneously detects (and is incapable of distinguishing between) other flavanone neohesperidosides. Our analysis gives a value of 1.70 mg naringin per gram fresh weight in mature leaves of the same cultivar.

The HPLC method of this survey has been used to survey several different cultivars for four consecutive growing seasons and has always indicated that the percentages of flavanones remain fairly constant with those obtained for the 1990 season presented here, as have studies quantifying naringin concentrations for successive seasons (Albach et al. 1981).

The results of this investigation demonstrate that there is a large variety of flavanones present in grapefruit cultivars such as 'Star Ruby', 'Jochimsen', 'Camulos', 'CRC #343', and 'Duncan', all of which contain large percentages of naringin, neohesperidin, and neoeriocitrin. 'Whitney Old Line Marsh', 'Jochimsen', 'Star Ruby', and 'Duncan' resemble each other in flavanone composition and seem more typical of other grapefruit cultivars since they contain high concentrations of naringin and little hesperidin or eriocitrin. 'Camulos' and 'CRC #343' contain other flavanones not found in the more widely consumed 'Star Ruby' and 'Duncan' grapefruits.

Oddly, 'Hall', 'Reed Seedling Marsh', and 'Yellow Rind Mandarin' grapefruit contain only flavonoid rutinosides and do not accumulate naringin, naringin–6"–malonate, naringin–4'–glucoside, or poncerin. 'Reed Seedling Marsh' should be very similar to the 'Redblush', 'Star Ruby', and other "Marsh" cultivars (Hodgson 1967) in that it should contain mainly neohesperidosyl glycosides. It is possible that the 'Reed Seedling Marsh' is not typical of the Marsh line.

Rhoifolin has been identified in grapefruit (Horowitz and Gentili 1977), and large quantities have been isolated from this species (Rowell and Beisel 1963). Some of the grapefruit cultivars examined in this study have large percentages and concentrations of rhoifolin, particularly 'Star Ruby' and 'Duncan'. However, large concentrations of diosmin and moderate concentrations of isorhoifolin were also detected in other cultivars. ('Camulos' and 'Star Ruby', for example). Those grapefruit cultivars containing mostly neohesperidosyl flavanones also have high concentrations of rhoifolin and may have moderate concentrations of isorhoifolin. 'Reed Seedling Marsh' and 'Yellow Rind Mandarin' grapefruit with high concentrations of rutinosyl flavanones also contain high concentrations of diosmin. Tangelo, a mandarin × grapefruit cross (Swingle and Reece 1967, Barrett and Rhodes 1976), contains mainly rhoifolin.

Analysis of both diploid and tetraploid specimens of several grapefruit cultivars showed that flavor-related characteristics differ between fruit from the diploid plants and tetraploid plants. Other than this study, the influence on polyploidy on the chemical composition of leaves and fruits of citrus species is unknown.

Generally, tetraploid leaves are broader in proportion to their length than diploids and considerably thicker. Leaf color tends to be darker. The wings of the petioles are broader in some cultivars, and they often fuse with the leaf blade (Cameron and Soost 1968). Thorniness is more pronounced in tetraploids, vigorous shoots are less common, growth is slower, and the tree is smaller, less erect, and more compact. Tetraploids are slower in fruiting and produce less fruit. Juice vesicles are tougher and the yield of juice in proportion to the whole fruit weight is much lower.

The diploid and tetraploid forms of several cultivars of grapefruit were examined for flavonoid composition. These cultivars included examples from both predominantly neohesperidin-dominant or naringin-dominant leaf flavonoid pattern types. The naringin-dominant type, which contains naringin as the major flavanone, is characteristic of most grapefruit cultivars and was represented here by the 'Seedy Marsh' and the 'Hall'. The cultivars represented by the neohesperidin-dominant leaf type have neohesperidin as the major flavanone, with only some naringin present in the 'Royal' and 'Imperial' cultivars.

Diploid cultivars were found to consistently contain generally higher Brix levels, lower acid content (citric acid/mL), and higher acid/Brix ratios in the juice than their tetraploid counterparts (table 7). In contrast, higher levels of narirutin, naringin, and neohesperidin were found in the juice of tetraploid cultivars compared to diploid cultivars. These factors make the juice from diploid fruit taste better than the juice from the tetraploid fruit.

Flavonoid levels in leaves varied among both the diploid and tetraploid cultivars (table 8). In naringin-dominant grapefruit leaf types such as 'Hall' and 'Seedy Marsh', naringin levels were higher in tetraploid leaves than in diploid leaves. 'Hall' leaves had less narirutin but more neohesperidin in tetraploid leaves than in diploid leaves. However in 'Seedy Marsh' leaves the opposite occurred. Neohesperidin-dominant leaf types—'Imperial' and 'Royal'—have higher neohesperidin levels but less narirutin in tetraploid leaves than in diploid leaves.

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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.