Variation in aroma level of holy basil (Ocimum tenuiflorum L.) leaves is related to volatile composition, but not trichome characteristics

Abstract

Holy basil or tulsi (Ocimum tenuiflorum L.) is an herb with a unique aroma in culinary uses. The aroma comes from volatile compounds in essential oils produced and stored in glandular trichomes. Different glandular trichome characteristics affect essential oil yield. Essential oil compositions and aroma levels vary among holy basil genotypes. However, relationships between aroma levels and trichome characteristics, as well as between aroma levels and essential oils, in holy basil have not been studied. Trichome characteristics, including types, diameter, volume and density, were not related to aroma levels of the five holy basil accessions studied in the present work. Eugenol, (E)-caryophyllene, and β-elemene were major constituents of the essential oils from leaves of most accessions, whereas methyl eugenol was found only in accessions with weak aroma. Based on the chemical compositions, accessions with low aroma level were clustered separately from those with higher levels. A similar result was also revealed by e-nose analysis. Our results demonstrate that major essential oil constituents appear to be responsible for the difference in aroma level between the holy basil accessions. Accessions with stronger holy basil aroma were rich in eugenol, while lacking in methyl eugenol, and could be distinguished from those with weaker aroma that were methyl eugenol-rich, but deficient in eugenol.