Analyze phenolics produced in citrus.

Phenolic compounds are produced as a result of a plant's interaction with the environment. Some are synthesized and accumulated in plant cells as a defense mechanism after exposure to microorganisms. Some phenolics affect animal and plant health or the flavor and palatability of fruits and juices.

Using high-pressure liquid chromatography, USDA-ARS researchers examined the phenolic compounds in leaf, flavedo, albedo and juice vesicle tissues of 35 species of citrus (Rutaceae). They used two techniques to quantify phenolics in plant tissues: identifying the number of phenolic peaks per chromatogram and the levels of phenolics based on percentage and concentration.

Food processors, chemists, breeders and others can use these characteristic phenolic patterns to aid in the preliminary classification and recognition of unknown cultivars endemic to the complex Aurantioideae subfamily. Further, the knowledge of the association of phenolic patterns and concentrations can be used in biochemical studies to identify sources of tissues that have high concentrations of flavonoids and which might contain high concentrations of flavonoid- and phenolic-modifying enzymes.

Investigators found that the average number of peaks, or phenolic compounds, occurring in citrus leaf, flavedo, albedo and juice vesicles were 21, 17, 15 and 9.3, respectively. The overall composition of four different classes of phenolic compounds absorbing at 285 nm (flavones, flavanones, coumarins/cinnamic acid derivatives and psoralens) for the Rutaceous species and cultivars was determined in leaf, flavedo, albedo and juice vesicle tissues.

Percentages and concentrations of 11 flavanone glycosides, three flavone glycosides and one flavonol glycoside were determined and categorized. Most taxa can be classified by their flavonoid glycosylation pattern as either predominantly neohesperidosyl or predominantly rutinosyl. The dominant neohesperidosyl flavanones were naringin, neoeriocitrin and neohesperidin. The dominant rutinosyl flavanones were hesperidin, eriocitrin and narirutin. Cultivars containing mainly neohesperidosyl flavanones also contained neohesperidosyl flavones. Similarly, species and cultivars containing rutinosyl flavanones contained rutinosyl flavones.

Previous studies have been of limited quantitative value and often targeted only a few key flavonoids in commercially important cultivars. The researchers performed this flavonoid study as an aid in the taxonomic evaluation of the citrus cultivars, not only of the commercial cultivars but of important hybrids and rootstocks as well.

Further information. Mark Berhow, USDA-ARS, National Center for Agricultural Utilization Research, 1815 North University St., Peoria, IL 61604; phone: [phone]; fax: [phone]; email: [e-mail].