Herbicidal activity of Mediterranean essential oils and their effects on soil bioindicators

Abstract

Environmental and health concerns caused for traditional crop protection systems have stimulated interest in alternative weed management strategies. Worldwide, efforts are being made to reduce the heavy reliance on synthetic herbicides that are used to control weeds. Natural herbicides based on allelopathic substances, such as volatile essential oils (EOs) extracted from plants, has been suggested to be one of the possible alternatives for achieving sustainable weed management. From one hand, EOs have shown ability to inhibit weeds seed germination and growth, on the other hand there is a lack of studies about the effects of such substances on soil microorganisms. Therefore, in this thesis the phytotoxic and herbicidal activities of EOs extracted from Mediterranean plants were investigated for their potential use as natural herbicides. The effects of EOs, as well as, of other plant extracts, such as hydrolates, aqueous extracts and fresh leaves obtained from Mediterranean plants, were tested on soil microorganisms. The donor species of EOs were selected based on previous experience of the research group and according to the current literature about the herbicidal activities of the secondary metabolites of these species or from species that are taxonomically closely related: Eucalyptus camaldulensis Dehnh., Eucalyptus occidentalis Endl., Eucalyptus globulus Labill., Eucalyptus torquata Luehm., Eucalyptus lesoufii Maiden, Thymbra capitata (L.) Cav., Mentha × piperita L. and Santolina chamaecyparissus L. The target weeds were two monocotyledons, Avena fatua L. and Echinochloa crus-galli (L.) P. Beauv, and two dicotyledons, Portulaca oleracea L. and Amaranthus retroflexus L. EOs composition was analyzed by means of Gas Chromatography (GC) and Gas Chromatography-Mass Spectrometry (GC-MS). The in vitro assays were performed in germination chambers, to assess the effects of EOs on weed seed germination and seedling growth. The in vivo trials were conducted in greenhouse conditions, where EOs emulsified by Fitoil were applied on weed species by watering. The study of EOs effects on soil microorganisms was carried out in a laboratory pot experiment, where soils were treated with EOs and leaf extracts and then incubated at room temperature (20-23°C). Results obtained from the in vitro bioassays revealed that all used EOs displayed of effectiveness against assayed weeds, controlling completely their germination process or reducing it and significantly inhibiting their seedling growth. Among them, T. capitata was the most effective. At lower doses, it blocked completely the seed germination of A. retroflexus, P. olecerea, A. fatua and E. crus-galli. Greenhouse trials demonstrated herbicidal activity of T. capitata, M. piperita and S. chamaecyparissus, increasing their phytotoxicity with the dose. T. capitata was the most effective against all weeds at the maximum dose and P. oleracea was the most resistant weed. Soil microorganisms, after a transient upheaval period, induced by the addition of EOs, generally recovered their initial function and biomass. Only T. capitata EO at the highest dose did not allow soil microorganisms to completely recover their initial functionality. Results of leaf extracts application on soil provided evidence that Eucalyptus leaves and their extracts (EOs, hydrolates and aqueous extracts), affected soil microbial community in different ways, and those effects were dependent on the Eucalyptus species. So far, the results obtained make feasible to suggest EOs application as bio-herbicides in controlled environments, such as horticulture and in greenhouse conditions. However, the optimum dose of application must be determined, to control weeds and simultaneously, not negatively affect soil microorganisms.