Flavonoid occurrence, regulation in plant tissues and dietary contribution to health

Interest in the flavonol content of food products continues to be fuelled by reports of wide ranging health benefits, many dependent on the ability of flavonols to act as powerful antioxidants (Rice-Evans et al, 1997). Recent work has identified tomato fruits as a rich source of flavonols (Hertog et al, 1992, Crozier et al., 1997). In this study the flavonol content of tomato fruits was investigated in relation to variety, size, season and country of origin. The flavonol content of ten commonly consumed tomato based food products was also assessed. Free and conjugated flavonols were identified and quantified using reversed phase HPLC with sensitive detection by UV and fluorescence detection. The total flavonol content of tomato varieties analysed in this study varied from 0.9-22.2 mug/g fresh weight. Smaller cherry tomato fruits grown in warm sunny climates such as Spain and Israel were found to contain far higher concentrations of flavonols than British fruits. The adoption of 'high flavonol' tomato varieties and production methods allowing greater sun exposure of developing tomato fruits may allow for an increase in the flavonol content of British produce. Tomato flavonols were able to survive industrial processing methods and could be detected in a wide range of tomato-based food products. Tomato juice and tomato puree were found to be particularly rich in flavonols, 14-16 mg/L and 70 mug/gfwt respectively. This study has enabled the identification of tomato fruits and processed products rich in flavonols. Identification of flavonol rich foods is clearly important with respect to their potential nutritional value. However, it is also necessary to determine whether these flavonols are absorbed by the human body during digestion. Following consumption of Spanish cherry tomatoes or tomato juice, conjugated quercetin was detected unchanged in plasma and urine. This suggests that tomato flavonols are absorbable and bioavailable. Flavonol synthesis in plants involves complex environmental regulation, the principal components of which include light, nutrition, disease and temperature. Previous studies indicated a link between plant nutrition and flavonoid accumulation but were unable to identify individual flavonoid compounds. In addition, although tomato was frequently used as a test system for the study of nutrient stress on flavonoid accumulation, the effects of nutrition on tomato fruit tissues were not assessed. The effect of reduced nitrogen and phosphorus nutrition on the flavonol content of plant tissues was initially tested on seedlings of Arabidopsis thaliana and tomato. Conjugated quercetin, kaempferol and isorhamnetin were detected in both Arabidopsis and tomato seedling tissues. Exposure to nitrogen or phosphate stress demonstrated a clear inverse relationship between nitrogen and phosphate nutrition and flavonol content. On the basis of this observation, a trial was established under commercial conditions to determine the effect of nutrient stress on the flavonol content of tomato leaf and fruit tissue. In line with previous work (Carpena et al., 1982; Bongue-Bartelsman & Phillips, 1995) reduced nitrogen availability caused an increase in the flavonol content in the leaves of tomato plants, reduced phosphorus nutrition did not elicit this response. Low nitrogen or phosphate availability caused an increase in the flavonol content of tomato fruit skins early in the ripening process. Any effect of nutrient stress on the flavonol content of tomato fruit tissues was lost as ripening progressed. This study provides clear evidence that the flavonol content of plant tissues is influences by their nutritional status. (Abstract shortened by ProQuest.)