Carum carvi L. essential oil: A promising candidate for botanical herbicide against Echinochloa crus-galli (L.) P. Beauv. in maize cultivation

Natural, essential oil-based emulsions could be in future an alternative for synthetic herbicides. In this study, the possibility that foliar-applied caraway or peppermint essential oils (EO) can selectively inhibit the growth of Echinochloa crus-galli (L.) P. Beauv. (barnyardgrass, a typical maize weed) but not that of maize plants was tested, in attempt to develop an eco-friendly botanical herbicide. The phytotoxic potential of oil-in-water emulsions of each EO with addition of commercial adjuvant, mainly composed of fatty acids methyl esters, was examined. Their effect on visible plants injuries, biomass accumulation, chlorophyll a fluorescence and changes to biochemical patterns of both the main crop (maize) and barnyardgrass via an untargeted metabolomic approach were studied. It was found that oil-in-water emulsion containing 2.5% of adjuvant and of caraway EO did not affect significantly the growth of maize plants, did not induce foliar symptoms and did not alter the status of the photosynthetic apparatus, as revealed by chlorophyll a fluorescence. On the contrary, this emulsion exerted significantly negative effects against barnyardgrass growth, inducing foliar injuries and reducing the photosynthetic efficiency of photosystem II. It was also found that the studied emulsions caused a series of biochemical changes in the plant tissues, with caraway emulsion being more phytotoxic, as compared to the peppermint EO-emulsion. In conclusion, oil-in-water emulsion containing 2.5% of caraway EO could be used in future as a foliar-applied botanical herbicide against barnyardgrass in maize cultivation.