Cytisus scoparius and Ulex europaeus Produce Volatile Organic Compounds with Powerful Synergistic Herbicidal Effects

Abstract

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.

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

“…Gorse reduced the emergence of the dicot A. retroflexus, although neither its naturally emitted volatiles [21] nor its aqueous extract [22] have shown in vitro effects on germination. This successful control of germination under more realistic conditions could be due to the additive or interactive action of different allelochemicals [23]. It also could be possible that the seeds of A. retroflexus in the pot experiment germinated, but the early growth was impeded, and the seedlings did not manage to emerge.…”

“…Also, the growth of the surviving D. sanguinalis seedlings in pots could be impaired by the joint action of the oxygenated monoterpenes [23] together to water-soluble compounds (phenolic acids and flavonoids) [22].…”

“…The findings from our previous studies led us to test here, for the first time, the abundant foliage of gorse and Scotch broom incorporated into the soil for pre-emergent weed control in maize. Once their phytotoxic nature is unraveled [21][22][23], soil amendment with shrub foliage could be a feasible way of releasing a cocktail of allelochemicals of different chemical classes and modes of action. The cocktail may increase herbicidal efficiency and/or minimize the development of resistance.…”

“…Pardo-Muras et al [21][22][23] have studied the phytotoxicity of gorse and Scotch broom in vitro. The fresh foliage of both species could produce and release volatile organic compounds [21] and water-soluble compounds [22] capable of inhibiting the germination and early growth of the agricultural weeds Amaranthus retroflexus L. and Digitaria sanguinalis (L.) Scop, without affecting maize.…”

“…The fresh foliage of both species could produce and release volatile organic compounds [21] and water-soluble compounds [22] capable of inhibiting the germination and early growth of the agricultural weeds Amaranthus retroflexus L. and Digitaria sanguinalis (L.) Scop, without affecting maize. Powerful synergistic effects among these allelopathic compounds were described [23] underlying the phytotoxicity on both weeds. These authors have shown that the flowers, as well as the vegetative parts, exerted different levels of phytotoxicity on different target weed species and physiological process (germination or early growth).…”