Under Eucalyptus plantations in the South Brazilian grassland region, few plants establish, and allelopathy may be involved in shaping this pattern. We aimed to assess the phytotoxicity of essential oil and aqueous extract of Eucalyptus saligna Sm. leaf litter on grassland species. We tested the effects of E. saligna pure oil and extract on germination, seedling growth, H2O2 levels and electrolyte leakage of seedling membranes of Paspalum notatum Flüggé, Eragrostis plana Ness (Poaceae), Trifolium repens L. and Lotus corniculatus L. (Fabaceae). Essential oil and aqueous extract of E. saligna affected all recipient species, even at the lowest amounts/concentrations, inhibiting germination and initial growth, and also increasing H2O2 levels and electrolyte leakage of seedling membranes. Essential oil consisted mainly of monoterpenes and presented α-pinene and 1,8-cineole as the major compounds. The extract contained phenolics, and lower levels of these compounds in the extract were associated with decreased phytotoxicity. We conclude that E. saligna contains phytotoxic compounds in leaf litter that generate oxidative stress and lead to membrane damage, affecting seeds and seedling growth. In addition, we relate E. saligna phytotoxicity to monoterpenes that may be released from leaf litter by volatilisation, and phenolics that may be leached by rainfall. Our study indicates that E. saligna has allelopathic potential on the tested grassland species.

We evaluated whether phytotoxic effects of the essential oil from Eucalyptus saligna Sm. leaf litter were related to its major compounds 1,8-cineole (eucalyptol) and α-pinene. Effects of the essential oil and its major compounds, combined and alone, were tested on germination and growth of the weed Eragrostis plana Ness. Volatile organic compounds (VOCs) emitted from the essential oil were determined in the airspace of Petri dishes during the experimental period. These volatile fractions were obtained by solid-phase microextraction (SPME) followed by gas-chromatography coupled to mass spectrometry (GC-MS). Effects of the oil and its major compounds were also assessed in aqueous solution on coleoptile growth of Triticum aestivum L. The essential oil showed the strongest phytotoxic effects on E. plana, followed by eucalyptol, eucalyptol + α-pinene, and α-pinene. A similar order of phytotoxicity was observed on coleoptile growth of T. aestivum. The proportion of oxygenated monoterpenes increased along the experimental period, mainly eucalyptol, and of monoterpene hydrocarbons diminished in the volatile fractions. We conclude that phytotoxicity of E. saligna leaf litter essential oil may be mainly related to eucalyptol and additive or synergistic effects of other minor compounds. Analysing VOCs emission is relevant to elucidate compounds involved in phytotoxicity of a plant species.