Who's who in the fleabane (Conyza spp.) family?

Flaxleaf fleabane (Conyza bonariensis(L.) Cronquist) has become a problem weedthroughout the entire cropping belt of south-easternAustralia. One of the barriers for effective control ofC. bonariensis is the morphological confusions withother fleabane species. This study used the scanningelectron microscopic technique to investigate the differencesin trichome and stomatal densities betweenC. bonariensis and tall fleabane (C. sumatrensis(Retz.) E.Walker). Results showed that C. bonariensishad significantly higher trichome densities than C.sumatrensis. The trichome densities in C. bonariensisranged from 67.2 to 221.9 trichomes mm-2 on theadaxial leaf surface and from 74.0 to 168.1 trichomesmm-2 on the abaxial leaf surface, while the trichomedensities in C. sumatrensis ranged from 40.3 to 147.9trichomes mm-2 on the adaxial surface and from 53.8to 134.5 trichomes mm-2 on the abaxial surface. Therewere no significant differences in trichome densitiesbetween the adaxial and abaxial surfaces in both species.For the stomatal density, C. bonariensis had anaverage 314.3 ± 12.5 stomata mm-2 on the adaxialsurface, which was significantly higher than thaton the abaxial surface (237.4 ± 9.1 stomata mm-2).However, C. sumatrensis had a lower adaxial stomataldensity (217.7 ± 8.5 stomata mm-2) compared to theabaxial surface (275.1 ± 15.3 stomata mm-2). Boththe trichomes and stomata can influence the herbicidecontrol of this weed, with the high trichome numberspotentially restricting herbicide uptake and the highstomatal numbers facilitating the uptake. The restrictive trichome barrier should be overcome to increase the leaf penetrability and herbicide uptake, thereby improving foliar herbicide efficacy on C. bonariensis.