Citrus species

Citrus species are well-known for their accumulation of flavone- and flavanone-glycosides, and thus should contain all of the enzyme activities necessary for the synthesis of these compounds. Two tentative consensus sequences for FNS-II have been identified by in silico analysis of the CitEST database, apparently representing the first identification of putative FNS-II genes in this genus [29]. Biochemical determination of function and analysis of the proteins encoded by these genes will be an important step toward elucidating flavone synthesis in Citrus. 

Substantial quantities of luteolin-7-O-glucuronide, luteolin-7-O-glucoside, and luteolin-7-O-rutinoside occur in Red Oak Leaf and Lollo Rosso, two red-leaved varieties of lettuce (Lactuca sativa) [Llorach et al., 2008], Polymethoxylated flavones such as nobiletin, scutellarein, sinensetin, and tangeretin (Fig. 1.8) are found exclusively in citrus species [Crozier et al., 2006c], while diosmetin-7-O-glucuronide has been isolated from the fruits of a Chinese herb, Luffa cylindrical. 

Figure 1.8 Polymethoxylated flavones found in citrus species.

Table I (8) shows the concentrations of limonoids in seeds of several citrus species, as well as the relative proportions of neutral and acidic limonoids in each. Tables II and III (8j show the relative amounts of the major individual neutral and acidic limonoids, respectively, in several species. The cases in which two different samples of the same species were analyzed show the...

The biosynthesis the /3-hydroxylated compound synephrine has been studied in Citrus species (325). An elegant experiment carried out in Cleopatra mandarin seedlings showed that tyramine is rapidly methylated to N-methyltyramine... 

Versteeg et al. (50) have isolated various multiple forms of pectin esterases in oranges and shown that they differ in affinity to pectins and pectates and in heat stability and therefore may play different roles in cloud loss phenomena. One form (about 5% of the total PE activity) was found to be much more heat stable than the other forms it was also active at low pH (2.5) and at low temperatures. Another form did not cause self-clarification for this enzyme a similar mode of attack was proposed as for fungal pectin esterases which produce low-ester pectins less sensitive to calcium. Multiple forms of pectin esterases are present in the fruits of all of the orange varieties and citrus species tested. The two isoenzymes known to be responsible for cloud loss and gelation in citrus products were found to occur in all of the component parts of the orange fruit (51). In the French cider industry the endogenous PE of apple is used for the self-clarification of apple juice (52). 

Limonoids are C2(, nortriterpenoids deriving from a C30 triterpene precursor. The best known limonoids are the Azadirachta indica (neem tree) antifeedant azadirachtin (C50L C40 C6 -C60(epoxide methylene cross-link) furan) and the Citrus species (Rutaceae) bitter antifeedant limonin (G50L G40 G6 G6 C50L(epoxide)-furan). Limonin gives a delayed bitter taste to Citrus fruit. The limonoids are typically bitter compounds with insect antifeedant activity... 

Seeds of various Citrus species, Poncirus trifoliata [D. L. Dreyer, J. Org. Chem. 30, 749 (1965)] Nuphar japonicum D.C. 

Flavonoids are a group of polyphenolic compounds present ubiquitously in plants. Flavanones are a class of flavonoids, present abundantly in Citrus species. 

Figure 9.2 Base structure and commonly present flavanones in Citrus species.

Table 9.1 Selected extraction methods for flavanones from citrus species. 

D-Limonene Fresh, orange peel Citrus (Citrus species)... 

So far only the oils obtained from citms fruits have been taken into account. Additionally, essential oils of the leaves, twigs and blossoms of citrus species are also available. Water vapour distillates of leaves and twigs are called petitgrain oils, while the distillate of orange blossoms is commercially available as neroli oil. Furthermore, also an orange flower absolute is produced from orange blossoms [2]. 

Citrus, Microcitrus, Severlnia and Swlnglea although photosensitizers have been demonstrated in other genera of the family (7). Various furocoumarin derivatives which have been isolated from Citrus species (46-48) are undoubtedly responsible for much of the bioactivity of the species assayed, however the role that other... 

I would like to thank Drs. Eloy Rodriguez and G.H.N. Towers for critical reading of this manuscript, Dr. C. Campbell (Tropical Research and Education Center-IFAS, University of Florida, Homestead, FL) for access to the various Citrus species and related Rutaceae genera used in this study and J.A. Downum for field and technical assistance. The support of NSF (PCM 8209100) and NIH (AI18398) to E. Rodriguez and the FIU Foundation (571204900) is gratefully acknowledged. 

Thirty-six limonoid aglycones have been isolated from Citrus and its hybrids. These aglycones are present in intact fruit tissues as open D-ring forms such as limonoate A-ring lactone (2), while in seeds they are present in both open and closed forms. It appears that these citrus limonoids are biosynthesized through at least four different limonoid pathways the limonin pathway, the ichangensin pathway, the calamin pathway and the 7-acetate limonoid pathway (12). The products of the limonin pathway are found in all citrus species. The products of the other three pathways are only found in certain species of citrus and its close relatives. 

Unlike most other citrus species in which limonin is predominant (over 50% of total limonoids) Citrus ichangensis accumulates ichangensin (11) as the predominant limonoid in its fruit tissues and seeds (26). Fruit tissues contain 50 times as much ichangensin as limonin. They also contain relatively high concentrations of deacetylnomilin (12). 

Tetranortriterpenoids.—The occurrence of limonoids in Citrus species has been reviewed. ... 

In addition to p-carotene, canthaxanthin, p-apo-8 -carotenal and ethyl p-apo-8 -carotenate, the ketone citranaxanthin (C33), which occurs in certain citrus species, has been cited (ref. 57) as a compound produced on a manufacturing scale. It is obtained by the condensation of C30, p-apo-8 -carotenal with propanone in the presence of aqueous potassium hydroxide (Scheme 18). Its use does not appear to be applicable to the UK. 

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