E. Everaert

Quantitative analysis of hop acids, essential oils and flavonoids as a clue to the identification of hop varieties

The hop acids, the essential oils and the flavonoids of the hop (Humulus lupulus L.) varieties Saaz, Nugget and Wye Target have been studied for cultivar identification. Optimised reverse phase-high performance liquid chromatographic separations were carried out for the analysis of the hop acids and the flavonoids, whereas capillary gas chromatography was applied in order to characterize the essential oils. Appropriate handling of the quantitative chromatographic data by principal component analysis provided a clue to differentiate hop varieties.

Flavonoid biosynthesis in petals of Rhododendron simsii

Abstract The results of a comparative in vivo study of flavonoid biosynthesis in Rhododendron simsii petals, with [2- 14 C] p -coumaric acid and [2- 14 C]caffeic acid indicate that p -coumaric acid is the true precursor. Moreover, supplementary in vitro experiments with enzyme preparations show that p -coumaroyl-CoA is a much better substrate for chalcone synthase than caffeoyl-CoA. This suggests that in Rhododendron the 3′-hydroxylation of the B-ring occurs at the stage of a C 15 intermediate. In addition, flavonoid 3′-hydroxylase activity was demonstrated by means of comparative in vivo experiments with, respectively, [4a,6,8- 14 C]naringenin and [4a,6,8- 14 C]dihydrokaempferol as C 15 precursors. In both cases, the formation of radioactive 3′,4′-hydroxylated co-pigments and anthocyanins was established. Furthermore, co-pigment biosynthesis may proceed via the sequence: naringenin, eriodictyol, dihydroquercetin (3′-hydroxylation at the flavanone stage), whereas the formation of the anthocyanins may take place via naringenin, dihydrokaempferol and dihydroquercetin (3′-hydroxylation at the dihydroflavonol level).

The Possible Role of Phenolics in Incompatibility Expression in Eucalyptus gunnii Micrografts

As a result of incompatibility, Eucalyptus gunnii showed in the graft area of in vitro micrografts a typical accumulation of gallic, ellagic, gentisic and p-coumaric acid: (+)-catechin also increased in concentration. In compatible grafts, a lower concentration of phenolics accumulated. The possible role of both specific types of phenolic accumulation and the higher activities of soluble peroxidases and polyphenol oxidase in incompatible grafts as opposed to compatible ones is discussed.

Metabolism of Flavonoids by Rhizobia

In common with many other soil microorganisms rhizobia are capable of catabolizing a variety of aromatic compounds. Monocylic aromatics and hydroaromatics such as benzoate and shikimate can be degraded to 3-oxoadipate via catechol or protocatechuate prior to entry into the tricarboxylic acid cycle (Chen et al., 1984; Parke, Ornston, 1984) and protocatechuate itself is a universal growth substrate for rhizobia. The possibility that rhizobia could catabolize polycyclic flavonoids responsible for nod gene induction has been raised as a consequence of experiments which showed that such compounds also induced expression of Rhizobium genes with unknown functions and showing no homologies with nodulation gene promoters (Sadowsky et al., 1988; Perret et al., 1994). The first demonstration of flavonoid catabolism by Rhizobium involved utilization of the flavan-3-ol catechin by an isolate from Leucaena leucocephala with attendant formation of phloroglucinol carboxylic acid and protocatechuate (Gajendiran, Mahadevan, 1988). Rao et al. (1991) subsequently reported that R. loti could cleave the C-ring of the pentahydroxy flavone quercetin by means of a novel mechanism which yielded phloroglucinol and protocatechuate among the degradation products. Further studies with other Rhizobium species/biovars and their respective nod gene inducers confirmed that flavonoid degradation is a common metabolic feature in members of this genus (Rao, Cooper, 1994). In this paper we report on recent developments from our studies of flavonoid metabolism by free-living rhizobia: 1) evidence for C-ring modification and fission during interactions between rhizobia and their flavonoid or isoflavonoid nod gene inducers; 2) the establishment of a contribution from carbon atoms of nod gene-inducing naringenin to the structure of R. leguminosarum bv viciae Nod factor Rlv-IV.