Mentha suaveolens as allelopathic biomass for weed control: phenolics, organic acids, and volatile organic compounds profiles

The need to find alternatives to synthetic herbicides has led to the evaluation of the use of allelopathic biomass from different agroforestry species as soil amendments for weed control. Mentha suaveolens Ehrh. (apple mint) could be a suitable candidate as an ecoherbicide due to its availability in the agroecosystem, well-studied biological activities, and known chemical composition. For the present study, two greenhouse pot experiments were conducted by incorporating flowering aerial biomass of apple mint into the soil. In the first one, the potential phytotoxic effects of M. suaveolens at doses 1 and 2% were evaluated on the germination and growth of maize and its accompanying weeds. In a second temporary assay, the duration of phytotoxicity and the effects of apple mint (0.5% dose) on the physicochemical properties of the soil were elucidated. The soluble (phenolics and organic acids) and volatile compounds potentially releasable from the allelopathic biomass were identified. The apple mint exerted adverse effects on the germination and growth of dicotyledonous weed species, especially Amaranthus retroflexus and Solanum nigrum, with almost 100% inhibition, as well as on the growth of monocotyledonous weeds such as Digitaria sanguinalis, with a reduction of more than 95%. On the contrary, maize yield and soil properties pH, CECe, organic matter, and exchangeable cations were improved by the ecoherbicide. Chemical analyses of apple mint aerial biomass revealed the presence of 7 phenolic compounds, 9 organic acids, and 32 volatiles. For this study, the effects of incorporating M. suaveolens aerial biomass into soil were evaluated for the first time, and it was demonstrated that it has potential as an eco-friendly plant-based tool for Integrated Weed Management.Agencia Estatal de Investigación | Ref. PID2019-111253RB-I00Agencia Estatal de Investigación | Ref. PDC2022-133375-I0

Short-term effect of eucalyptus leachates on green food webs in headwaters

Despite the reported allelopathic nature of the Eucalyptus genus, eucalypt leachates have unknown effects on the trophic base of stream green food webs. Eucalypt plantations have increased worldwide, including riparian ecosystems. We aimed to test whether short-term eucalypt leachates might alter water chemistry, periphytic algal biomass and diatoms, and herbivorous invertebrates’ (i.e., scrapers). We studied two oligotrophic and well-preserved headwaters from NW Spain. The experiment followed a before-after control-impact paired (BACIp) design, with weekly sampling before and after eucalypt leaves addition to streams. In the stream with lower discharge, the eucalypt treatments seemed to affect increases in biomass accrual (Chl-a) and diatom assemblages, disfavouring sensitive species (e.g., Eunotia minor and Achnanthidium pyrenaicum). Therefore, the ecological status was reduced from high to good. In the stream with higher discharge, invertebrate assemblages changed with scrapers having their abundances modified in comparison with the control (e.g., Habrophlebia sp. and Elmidae). Results suggest that eucalypt leaves leachates exert toxic effects on periphyton and on diatom and invertebrate assemblages, and this effect might be mediated by discharge. Alterations at the base of stream food webs may compromise their good ecological status. Further studies are necessary to identify whether this toxicity is due to allelopathic processes

Biomass from allelopathic agroforestry and invasive plant species as soil amendments for weed control-a review

Effective weed management faces increasing legislative restrictions for the use of herbicides due to their toxicity and environmental persistence. In addition, the linear increase in resistant weeds threatens to render authorized herbicides useless. In a post-herbicide era, under the IWM strategy, allelopathy can play a relevant role since many plants can produce a variety of allelochemicals with different structures and modes of action, capable of inhibiting the germination and growth of different weed species. Inspired by green manuring with cover crops, the use of allelopathic biomass from weeds, invasive species, residues of forestry plantations, and other abundant wild plants has some advantages over green manures grown in situ or other alternatives such as applying plant extracts or essential oils. Beyond the ecosystem services provided by green manures, the potential use of allelopathic biomass offers extra opportunities for the science and practice of holistically integrated weed management because (i) the investment of resources and time for producing cover crops would be alleviated, and (ii), new use of agroforestry residues and a sink for harmful weed biomass is provided. In this review, we compile the current knowledge of those allelopathic species whose biomass, used as soil amendment, effectively controlled weeds. In addition, the complex allelopathic processes underlying the effectiveness of cover crops and allelopathic biomass used as green manures for weed control are revisited.Agencia Estatal de Investigación | Ref. PID2019-111253RB-I00Agencia Estatal de Investigación | Ref. PDC2022-133375-I0

Cytisus scoparius and Ulex europaeus produce volatile organic compounds with powerful synergistic herbicidal effects

New herbicides based on natural products are claimed to address weed resistance and environmental concerns related to synthetic herbicides. In our previous studies, certain volatile organic compounds (VOCs) produced by Ulex europaeus and Cytisus scoparius were argued to be responsible for the phytotoxicity of both shrub species. Interactions among VOCs were hypothesized to explain the inconsistency between the effects of the identified pure compounds and those naturally emitted from fresh plant material. In this work, eugenol, verbenone, terpinen-4-ol, α-terpineol, and linalool were assayed as binary mixtures of Amaranthus retroflexus and Digitaria sanguinalis. Powerful synergistic inhibitory effects were revealed for germination and early growth. Only 3.1 ppm of verbenone was enough to inhibit A. retroflexus germination when paired to other VOCs. Eugenol was capable of exacerbating the effects of terpinen-4-ol on A. retroflexus, even though it was innocuous when acting alone at 12.5 ppm. The verbenone and linalool pair produced very significant synergistic effects in terms of D. sanguinalis germination. The synergistic effects were predominantly irreversible for D. sanguinalis, since seeds exposed to paired VOCs were unable to recover their germination capacity after removing the phytotoxins or produced damaged seedlings. Both shrub species have been revealed as sources of natural herbicide molecules, with promising synergistic modes of action that deserve to be studied in depth.Ministerio de Economía y Competitividad | Ref. CGL2016-78660-

Complex synergistic interactions among volatile and phenolic compounds underlie the effectiveness of allelopathic residues added to the soil for weed control

The introduction of allelopathic cover crops for green manuring or mulching is a regular practice in Integrated Weed Management. In this context, the alternative use of the abundant phytotoxic residues of allelopathic plants from the agroecosystem, e.g., the foliage of Eucalyptus, Acacia, or Cytisus species, is promising. Previous studies identified the phytotoxic compounds potentially involved in the effectiveness of some plant residues when added to the soil for weed control. The low quantities of allelochemicals present in the tissues and the weak phytotoxicity of each of them in their natural concentrations did not explain the significant levels of weed control observed at field scale. Here, to study hypothetical synergistic interactions among the volatile (VOCs) and water-soluble compounds released to the soil matrix, complex mixtures of VOCs, phenolics, or both, mimicking the chemical profiles of Cytisus scoparius were prepared and then tested in vitro on the germination and early growth of two weeds. The effects were calibrated against the VOCs naturally emitted by the fresh plant material and aqueous extract, acting together or not, and with or without soil. The presence of the aqueous extract significantly increased the phytotoxicity of VOCs on Amaranthus retroflexus root growth compared to the volatiles emitted alone. In addition, the soil factor enhanced synergistic interactions among VOCs and water-soluble compounds, resulting in a 54% decrease in total germination and an 80% inhibition of root and shoot growth. Multi-level synergistic chemical interactions should explain the bioherbicidal effectiveness of allelopathic residues applied as a soil amendment.Ministerio de Economía y Competitividad | Ref. CGL2016-78660-RMinisterio de Ciencia e Innovación | Ref. PID2019-111253RB-I0

The phytotoxic potential of the flowering foliage of gorse (Ulex europaeus) and Scotch broom (Cytisus scoparius), as pre-emergent weed control in maize in a glasshouse por experiment

In our previous studies, the phytotoxicity of Ulex europaeus (gorse) and Cytisus scoparius (Scotch broom) was demonstrated in vitro and argued to be caused by the release of volatile and water-soluble compounds from fresh plant foliage. In light of these positive results, there was a need to test the effects ex vitro. In this work, gorse and Scotch broom were used as soil amendments in pot experiments in a glasshouse by incorporating slashed plant material into the soil at a ratio of 1% w/w on a dry mass basis. The phytotoxic effects on the emergence and early growth of maize and five accompanying weed species were analyzed, as were the effect on soil fertility and soil community-level physiological profiles. Thirty days after incorporation, significant decreases in weed density of 32.2% and 59.5% were found for gorse and Scotch broom soil amendments, respectively. Gorse soil amendment was notably effective impairing the establishment of Amaranthus retroflexus and diminishing the plant height of Digitaria sanguinalis and Portulaca oleracea. Scotch broom soil amendment was capable of significantly inhibiting the emergence of D. sanguinalis, Convolvulus arvensis, P. oleracea, and A. retroflexus, with a notable reduction of weed biomass. No undesirable side effects on maize crop or soil quality, including microbial activity, were detected. Our results suggest that the incorporation of gorse and Scotch broom foliage is promising for pre-emergent weed control in maize; however, field trials that support and expand these glasshouse results are essential.Ministerio de Economía y Competitividad (España) | Ref. BIOINPUT CGL2016-78660-

On the bioherbicide potential of Ulex europaeus and Cytisus scoparius: profiles of volatile organic compounds and their phytotoxic effects

The phytotoxic potential of the legume shrubs Ulex europaeus L. (gorse) and Cytisus scoparius (L.) Link. (Scotch broom) is studied in this work for the first time. On the basis of their richness in active principles, the previous evidence of biological activity, and the abundance of biomass in their native range and invaded areas, a question arose: can U. europaeus and C. scoparius be considered as potential sources of natural herbicides for sustainable agriculture? By means of volatile bioassays, the flowering fresh plant material of both shrub species was shown to produce and emit volatile organic compounds (VOCs) able to inhibit the germination and/or early growth of two agricultural weeds: Amaranthus retroflexus and Digitaria sanguinalis. Novel complete VOCs profiles from the volatile extracts of the shrub species were obtained by GC and GC/MS. A total of 20 compounds were identified from U. europaeus flowering biomass, theaspirane and eugenol, among others, being described in gorse for the first instance. The chemical profile of C. scoparius yielded 28 compounds and was rich in oxygenated monoterpenes such as terpinen-4-ol, verbenol, α-terpineol, and verbenone, which were also identified in this species for the first time. Using dose-response bioassays with pure compounds, these VOCs were argued to be involved in the phytotoxicity observed for the plant materials, even at very low concentrations. The phytotoxic effects were predominantly irreversible, particularly for D. sanguinalis, since the seeds exposed to the VOCs produced damaged seedlings, were unable to recover germination capacity after removing the phytotoxin or, when recovered, produced unviable seedlings. Our results extend the interest of the abundant U. europaeus and C. scoparius for the obtention of natural products with bioherbicide potential, or to be used as allelopathic biomass in the development of new sustainable agricultural practices.Ministerio de Economía y Compatitividad | Ref. CGL2016-78660-

Unravelling the bioherbicide potential of Eucalyptus globulus Labill: Biochemistry and effects of its aqueous extract

In the worldwide search for new strategies in sustainable weed management, the use of plant species able to produce and release phytotoxic compounds into the environment could be an effective alternative to synthetic herbicides. Eucalyptus globulus Labill. is known to be a source of biologically active compounds responsible for its phytotoxic and allelopathic properties. Our previous results demonstrated the bioherbicide potential of eucalyptus leaves incorporated into the soil as a green manure, probably through the release of phytotoxins into the soil solution. Thus, the aims of this study were to understand the phytotoxicity of the eucalyptus leaves aqueous extract applied in pre- and post-emergence, and to identify and quantify its potentially phytotoxic water-soluble compounds. The effects were tested on the germination and early growth of the model target species Lactuca sativa and Agrostis stolonifera, and on physiological parameters of L. sativa adult plants after watering or spraying application. Dose-response curves and ED50 and ED80 values for eucalyptus aqueous extracts revealed pre-emergence inhibitory effects on both target species, effects being comparable to the herbicide metolachlor. While spraying treatment reduced the aerial and root biomass and increased the dry weight/fresh weight ratio of lettuce adult plants, watering application reduced protein contents and chlorophyll concentrations with respect to control, reflecting different modes of action depending on the site of phytotoxin entry. Via HPLC analyses, a total of 8 phenolic compounds (chlorogenic, two ρ-coumaric derivatives, ellagic, hyperoside, rutin, quercitrin, and kaempferol 3-O-glucoside) and other 5 low weight organic acids (citric, malic, shikimic, succinic and fumaric acids) were obtained from aqueous extract, the latter being identified for the first time in E. globulus. Despite some phytotoxic effects were found on lettuce adult plants, the use of eucalyptus aqueous extract would be discarded in post-emergence, whereas it was promising as a pre-emergence bioherbicide.Ministerio de Economía y Competitividad | Ref. CGL2016-78660-RMinisterio de Educación, Cultura y Deporte | Ref. AP2009-480

Control de malas hierbas en producción ecológica de forrajes: cultivos alelopáticos

Las condiciones edafo-climáticas del Noroeste de España son idóneas para la producción ecológica de forrajes. Aquí, la producción ecológica de leche ofrece buenas perspectivas, existiendo canales de comercialización establecidos, y una buena aceptación y creciente demanda de leche ecológica y sus derivados. Sin embargo, muchos ganaderos, aún dispuestos a la adopción de prácticas agrícolas sostenibles y ambientalmente correctas, son todavía reticentes a la conversión. Al margen de las explotaciones con escasa base territorial, con gran dependencia de forrajes externos, la principal preocupación del ganadero que decide convertirse es el abandono del uso de ftosanitarios de síntesis. Incluso en las granjas ecológicas certifcadas, donde se logra con éxito un manejo adecuado de los pastos y mezclas de pratenses para ensilar, los agricultores asumen que la producción de maíz, el cultivo forrajero por excelencia, es inviable sin el uso de herbicidas. Algunos de nuestros trabajos ya han demostrado que cultivar maíz forrajero ecológico es posible, siempre y cuando el cultivo se enmarque en un plan de rotaciones bien diseñado. La propia rotación previene la proliferación de la flora acompañante introduciendo cultivos multiespecíficos, alternativas diversificadas, sotosiembras, etc., así como cultivos con potencial alelopático, para aprovechamiento del forraje o como abonos verdes. Nuestra investigación pretende dar nuevas respuestas a la demanda de herramientas ecológicas eficientes para el control de la flora arvense, mediante la valoración del potencial bioherbicida de algunos cultivos forrajeros: Brassica rapa L. – nabo–, Lolium perenne L. –raigrás inglés–, Vicia faba L.–haboncillo–, y Zea mays L. –maíz–. Para ello, se llevaron a cabo: (a) bioensayos en laboratorio de sus extractos acuosos sobre la germinación y crecimiento temprano de Lactuca sativa L. como especie diana modelo; y, con los extractos más bioactivos, (b) ensayos de dosis-respuesta en condiciones controladas sobre L. sativa, y (c) ensayos in vitro del efecto del material vegetal de la especie más efectiva incorporado al suelo sobre L. sativa. Finalmente, sobre la base de los resultados obtenidos, se discuten las pautas de manejo como abono verde de los cultivos más fitotóxicos, para obtener un control óptimo del establecimiento de especies arvenses. Vicia faba, utilizado como abono verde en un sistema de rotaciones, se presenta como un cultivo muy prometedor para el control de la flora arvense. Además, para la producción de cultivos forrajeros como el maíz, es preciso introducir leguminosas de grano en las rotaciones, que asegurarán niveles de nitrógeno suficientes para las altas exigencias nutricionales del maíz que no pueden ser cubiertas por el estiércol o el compost producido en la granja ecológica