The allelochemical parthenin is sequestered at high level in capitate-sessile trichomes on leaf surfaces of Parthenium hysterophorus.

Allelochemicals that are produced and released by the weed Parthenium hysterophorus L. (common name: parthenium) are responsible for several detrimental effects of the plant, most notably its aggressive displacement of desirable species. Although the allelochemicals involved are well documented, and their biological activity on specific test species fairly well quantified, less is known about their specific location in the plant. Various plant parts were analyzed to determine the concentration of the sesquiterpene lactone, parthenin. Leaves had the highest levels of parthenin. Leaf washing (dipping) with TBME yielded up to 13.4 mg g⁻¹, homogenized leaf material had 14.5 mg g⁻¹ and the aqueous extract of leaves gave only 1.3 mg g⁻¹ parthenin. Five trichome types could be identified. Selective collection of capitate-sessile trichomes from the leaf surface allowed parthenin to be determined at 24.3 mg g⁻¹. Capitate-sessile trichomes contained virtually 100% parthenin. On a per unit basis, capitate-sessile trichomes held 0.3 μg parthenin. The findings suggest that the capitate-sessile trichomes are the main source of parthenin. The achene-complex also contained parthenin in relatively high amounts. Extrapolation of per plant parthenin volumes to field-scale production makes it plausible that the high production and release of parthenin, could to a large extent explain the increased ability of P. hysterophorus to displace other species.