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.

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 Pot 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.

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.

The Consistency Between Phytotoxic Effects and the Dynamics of Allelochemicals Release from Eucalyptus globulus Leaves Used as Bioherbicide Green Manure.

In the worldwide search for new strategies in sustainable weed management, the use of allelopathic plants incorporated into the soil as green manure can help control weeds by releasing allelochemicals into the environment. In previous experiments, Eucalyptus globulus leaves incorporated into the soil as green manure were shown to have a notable potential for weed control. But, 'what was exactly happening at chemical level?' and 'which were the compounds potentially responsible for the phytotoxic effects observed during those greenhouse assays?' In the present study, in-vitro phytotoxicity bioassays and chemical analysis of eucalyptus leaves were carried out in order to explore the relationship between the temporal phytotoxic effects and the dynamics of chemical composition. For that, eucalyptus leaves were removed from soil at different sampling times during 30 days and analyzed for phenolic and volatile organic compounds (VOCs) by HPLC and HS-SPME/GC-MS, respectively. The phytotoxic potential of the aqueous extract and the volatile fraction was tested on the germination and early growth of Lactuca sativa. Eucalyptus leaves incorporated into the soil as green manure showed a continuous release of different phenolic and volatile compounds during a 30-day period of decomposition. Both fractions had phytotoxic effects during the time assayed; however, the target process of phytotoxicity was different: phenolic compounds being the factor causing germination inhibition and VOCs responsible for growth reduction. The dynamics of release of this cocktail of allelochemicals into the soil environment may be the responsible for the phytotoxicity observed in our previous works.

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.