Despite the economic and therapeutic importance of Mexican oregano essential oil, there is no information regarding secretory structures and the effect of environmental factors on their density is unknown. The main goal of this study was to describe the morphology of glandular trichomes in Lippia graveolens H.B.K. (Verbenaceae), and evaluate the hypothesis that glandular trichome density on leaves of L. graveolens would be greatest for populations found in the most arid locations. Morphology and trichome density were investigated using a combination of scanning electron and stereoscopic microscopy in nine wild oregano populations. Emberger's index of aridity was calculated to characterize the populations located in a bio-climatic gradient. Correlation analysis was used to assess the relation between trichome density and aridity. Lippia graveolens presented two types of glandular trichome on both sides of the leaf: peltate and capitate, of which the former presented higher density. Geographic variation in trichome density was observed at three spatial scales: among bio-climatic regions, among populations within regions and among plants within populations. We found that plant populations at the most arid sites had, on average, lower trichome density. Our results showed a trend for a greater trichome density in more humid sites. Nevertheless, the importance of inter-population variation in trichome density suggests that micro-environmental and genetic factors should be taken into account, in order to better explain the observed differences.
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