The presence, yield and composition of secondary metabolites in plants, viz. the volatile components and those occurring in essential oils, can be affected in a number of ways, from their formation in the plant to their ¼nal isolation. Several of the factors of in½uence have been studied, in particular for commercially important crops, to optimize the cultivation conditions and time of harvest and to obtain higher yields of high-quality essential oils that ¼t market requirements. In addition to the commercial importance of the variability in yield and composition, the possible changes are also important when the essential oils and volatiles are used as chemotaxonomic tools. Knowledge of the factors that determine the chemical variability and yield for each species are thus very important. These include: (a) physiological variations; (b) environmental conditions; (c) geographic variations; (d) genetic factors and evolution; (e) political/social conditions; and also (f) amount of plant material/space and manual labour needs.

The data given in the literature published during 1976-1986 Table!. concerning antimicrobial activities of essential oils are treated from an experimental point of view and with regard to a possible practical application. Attention is paid to four factors which are important when testing essential oils: the assay technique; the growth medium; the microorganism; the essential oil.

The essential oils from fresh and dried rhizomes of ALPINIA GALANGA showed an antimicrobial activity against gram-positive bacteria, a yeast and some dermatophytes, using the agar overlay technique. The main components of the oils were also tested and terpinen-4-ol was found most active. An N-pentane/diethyl ether extract of dried rhizomes was active against TRICHOPHYTON MENTAGROPHYTES. 1'-Acetoxychavicol acetate, 1'-acetoxyeugenol acetate and 1'-hydroxychavicol acetate identified by MS and NMR were found in the antifungally active fractions obtained by LSC. Acetoxychavicol acetate was active against the seven fungi tested and its MIC value for dermatophytes ranged from 50 to 250 microg/ml. Dried sliced rhizomes contained 1.5% of this compound. The compound was not found in rhizomes of ALPINIA OFFICINARUM, ZINGIBER OFFICINALE and KAEMPFERIA GALANGA.

During the seasons 1989-1993, Valeriana officinalis plants were investigated for their contents of essential oil, valerenic acid and derivatives, and valepotriates. Harvesting of the subterranean parts was started in August of the year in which the seeds were sown, and continued until the last week of April of the subsequent year. Despite marked variations from year to year, the maximum contents of essential oil in the subterranean parts of V. officinalis were found in September, ranging from 1.2% to 2.1% (v/w) based on dry weight (DW). Over the vegetation periods investigated, the composition of the oil remained more or less constant. Valerenic acid and its derivatives, and the valepotriates reached their maxima in February-March, with contents of 0.7-0.9% (DW) and 1.1-1.4% (DW), respectively. During the period 1989 - 1993, five V. officinalis strains were investigated for their contents of essential oil, valerenic acid and derivatives, and valepotriates in order to select plants suitable for phytomedicines. The selection procedures described in this paper finally yielded plant material (in 1993) with a satisfactory content of essential oil (0.9%) combined with a high content of valerenic acid and derivatives (0.5%) which can be harvested in September of the year of sowing.

Phlorisovalerophenone synthase (VPS), a novel aromatic polyketide synthase, was purified to homogeneity from 4.2 mg protein extract obtained from hop (Humulus lupulus L.) cone glandular hairs. The enzyme uses isovaleryl-CoA or isobutyryl-CoA and three molecules of malonyl-CoA to form phlorisovalerophenone or phlorisobutyrophenone, intermediates in the biosynthesis of the hop bitter acids (a-and b-acids). VPS is an homodimeric enzyme, with subunits of 45 kDa. The pI of the enzyme was 6.1. K m values of 4 mm for isovaleryl-CoA, 10 mm for isobutyryl-CoA and 33 mm for malonyl-CoA, were found. The amino-acid sequences of two peptides, obtained by digestion of VPS, showed that the enzyme is highly homologous to plant chalcone synthases. The functional and structural relationship between VPS and other aromatic polyketide synthases is discussed.Keywords: Humulus lupulus; chalcone synthase; polyketide synthase; hop bitter acids; phlorisovalerophenone synthase.The cones of the hop plant (Humulus lupulus L.) have been used for centuries in the beer-brewing process. Their major contribution to beer is the characteristic bitterness that results from the isomerization of the hop a-acids into a more soluble and stable form during the brewing process; isomerized a-acids are the main bitter substances in beer. In the plant, hop bitter acids consist of both a-acids, mainly humulone, cohumulone and adhumulone, and b-acids, mainly lupulone, colupulone and adlupulone [1]. These compounds are synthesized during the development of the H. lupulus female inflorescences into cones and are accumulated in the yellow glands covering the basal part of the bracteoles of the cones [2,3]. In general, glands are a rich source of secondary metabolites. Recently, strategies for bioengineering the development and metabolism in glandular tissues have been reviewed [4].In previous papers [5,6] a novel biosynthetic pathway leading to the bitter acids in H. lupulus was proposed. The suggested intermediates phlorisovalerophenone and phlorisobutyrophenone were also detected in hop cones. Furthermore, protein extracts from the cones were able to synthesize these compounds from malonyl-CoA plus either isovaleryl-CoA or isobutyryl-CoA. Apparently, the catalytic mechanism involved in this biosynthesis is similar to that observed in other plant condensing enzymes, like chalcone synthase and stilbene synthase (Fig. 1). These enzymes catalyze a reaction which proceeds by a sequential condensation of three acetate units to a starter residue to form the tetraketide intermediate that is folded to form a ring [7,8]. This type of reaction, which was first described by Birch and Donovan [9], classifies chalcone synthase and stilbene synthase as polyketide synthases. Chalcone synthase is a key enzyme in the biosynthesis of flavonoids. It catalyzes the formation of naringenin from three molecules of malonyl-CoA and coumaroyl-CoA. Stilbene synthase, an enzyme in the biosynthesis of stilbene phytoalexins, is structurally and functionally related to chalcone synthase. Usi...