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Glandular peltate trichomesThe essential oils in the Lamiaceae are stored in glandular peltate trichomes. These trichomes are situated on the epidermal surface on both sides of the leaves and show a very typical anatomy. They are composed of one basal stalk cell, an ‘endodermal’ cell that prevents backflow of secreted substances through the apoplast, and a gland head formed by 10 – 14 secretory cells. The secretory cells have a relatively large nucleus and a great number of small osmiophilic vacuoles as their prominent cytological characteristics. The essential oils are produced in the secretory cells and then secreted in the sub-cuticular space, where they are stored. |
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Thymus vulgaris L.The common thyme (Thymus vulgaris, Labiatae), is a perennial plant of the northern part of the West Mediterranean region. Like most labiates, thyme is an aromatic plant with glandular trichomes on the leaves and floral parts contain monoterpenoid essential oils. When these glandular hairs are damaged they evaporate the containing volatile essential oils thus producing an intensive fragrance that embraces the plant. |
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Chemotypes and Genotypes in Thymus vulgaris L.In T. vulgaris seven genetically distinct chemotypes are described that can be distinguished on the basis of the dominant monoterpene produced in the glandular trichomes. In southern France, this species has the six chemotypes, geraniol (G), α-terpineol or trans-sabinene hydrate (A), thuyanol-4 (U), linalool (L), carvacrol (C), and thymol (T), which are named after their dominant monoterpene. In Spanish populations 1,8-cineole is present, whereas the geraniol chemotype hasn't been found.
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The presence of the dominant monoterpene in
T. vulgaris is controlled by an epistatic series of five
biosynthetic loci. This epistatic series has the following sequence:
G → A A plant with a dominant G allele will have the G phenotype, regardless of whether it has dominant or recessive alleles at the other loci. There are probably two loci that code for the G phenotype, otherwise a single pair of alleles at each locus codes for the remaining chemotypes.So if a plant is homozygous recessive at the G loci (i.e. gg) and has a dominat A allele (i.e. A-) then it will have the A phenotype. When the plant is homozygous recessive at the G and the A loci but has a dominant U allele then it will have the U phenotype, and so on down the chain. A plant homozygous for recessive alleles at all five loci has the T phenotype. It has to be added that the L genotype shows at the seedling stage (1-3 months old) not the L phenotype but one of the phenolic (C or T) ones. It only develops its "true" phenotype after this very young seedling stage. |
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Ecology of the ChemotypesThe monoterpene variation in T. vulgaris plants may represent an adaptive strategy in relation to environmental variation. The different chemotypes are showing a geographic and localized distribution in southern France. The phenolic chemotypes (C and T) dominate thyme populations in hot dry sites close to the Mediterranean sea, whereas the non-phenolic (G, A, U and L) chemotypes dominate sites further inland, particularly above 400m elevation, i.e. in wetter, cooler climates. Within the non-phenolic chemotypes, it is the α-terpineol chemotype which is most abundant on the wettest soils. For the two phenolic types, carvacrol is limited to the driest conditions, whereas thymol is less specialized and can occur on moist soils. The phenolic types, particularly carvacrol, are absent on sites which experience sub-freezing temperatures. Feeding experiments with a range of herbivores (goat, sheep, grasshoppers, molluscs and beetles) and inhibitory effects on microbial population growth (fungi and bacteria) have shown that different chemotypes vary in their ability to deter herbivore feeding. Every chemotype can be the preferred one depending on the component of the environment studied and all but the |
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