Several cover crops (CCs) exert allelopathic effects that suppress weed growth. The aim of the present study was to evaluate the effects of aqueous extracts containing different concentrations [0, 0.5, 1, 2.5, 5, 7.5 and 10% (w/v)] of Brassicaceae CCs (Sinapis alba, Raphanus sativus, Camellina sativa) and of the CCs Fagopyrum esculentum and Guizotia abyssinica on germination and early growth of Ambrosia artemisiifolia L. The allelopathic effects were species and concentration-dependent. C. sativa, for example, caused the greatest potential to inhibit germination, shoot, radicle length and fresh seedling weight, whereas S. alba and R. sativus inhibited germination and early growth of A. artemisiifolia only at concentrations ≥7.5%. In contrast, no inhibition was observed when aqueous extracts of F. escultneum and G. abyssinica were added at any of tested concentration. Liquid chromatography-tandem mass spectrometry detected 15 phenolic compounds in Brassicaceae CCs with the highest content (µg/g) of vanillin (48.8), chlorogenic acid (1057), vanilic acid (79), caffeic acid (102.5) and syringic acid (27.3) in C. sativa. Our results suggest that C. sativa is the most allelopathic CCs and that the fruits of C. sativa are the plant organs richest in allelochemicals.

Peppermint (Mentha piperita L.) and chamomile (Chamomilla recutita (L.) Rausch.) are aromatic plants with considerable economic value. These plants and their essential oils are used in medicine, cosmetics, and the food industry. One of the main limiting factors in peppermint and chamomile commercial cultivation is weed competition since weeds are able to decrease both oil amount and biomass yield. The purpose of the present study was to determine the effect of parasitism by field dodder (Cuscuta campestris Yunck.) on peppermint and chamomile dry weight and their essential oil yield and composition. Essential oils from both noninfested and infested peppermint and chamomile plants were obtained by hydrodistillation and characterized chemically by gas chromatography (GC) coupled with mass spectrometry (MS). The amount of dry matter accumulated by peppermint and chamomile plants infested by field dodder was lower (25% and 63%, respectively) compared to noninfested plants. Essential oil yield increased for peppermint (3.87% (v/w) and 3.63% (v/w)), but decreased for chamomile (0.2% (v/w) and 0.5% (v/w)) both from infested and noninfested plants, respectively. The oil composition profile significantly differed in terms of content. In peppermint plants, field dodder infestation increased menthone content by 23%, and decreased the content of both menthol by 11% and pulegone by 67%. Furthermore, δ-cadinene was detected only in oil extracted from infested peppermint plants. Compared to peppermint, chamomile plants were significantly more affected by field dodder in terms of essential oil yield, as well as oil composition and plant dry weight. In chamomile plants, (E)-dendrolasin was detected in the oil of noninfested plants, and 1,4-dimethyl-7-(1-methylethyl)-azulen-2-ol was detected only in the oil of infested plants.

Leaf extract of Asclepyas syriaca was utilized to investigate the phytotoxic effects on the germination and seedling growth of Amaranthus retroflexus L., Chenopodium album L., and Iva xanthifolia Nutt. and to evaluate the relative chlorophyll content and oxidative stress of these weeds growing in pots. The total content of phenols in the extract and its antioxidant activity were determined before bioassays. The TPC was 43.3 ± 1.5 mg GAE g⁻¹ dry extract, while IC50 value of DPPH radical scavenging activity was 0.651 mg ml⁻¹ and FRAP value was 38.7 ± 1.5 μmol Fe²⁺ g⁻¹ d.e., UHPLC-MS analysis was carried out to identify and quantify the major compounds in it. A total of 15 phenolic compounds in the extract were quantified, including 4 flavonoids, 7 hydroxybenzoic acids, and 4 hydroxycinnamic acids. Delphinidin was the most abundant (32.5 mg g⁻¹ d.e.), followed by 4-hydroxybenzoic acid (1.810 mg g⁻¹ d.e.), rutin (1.533 mg g⁻¹ d.e.), p-coumaric acid (1.131 mg g⁻¹ d.e.), quercetin (1.071 mg g⁻¹ d.e.), gallic acid (0.946 mg g⁻¹ d.e.) and protocatechuic acid (0.598 mg g⁻¹ d.e.). The seed bioassay confirmed the inhibitory effect of leaf extract on the seed germination and early seedling growth of the tested weeds at all concentrations. In all tested plants the inhibition of seed germination ranged from 68 to 100%, while the inhibitory effect on early seedling growth (radical and shoot length) was ≥ 72% and up to 97–100% for root and 74 to 100% for shoot growth. The results obtained from the pot bioassays revealed significant oxidative stress and a decrease in the relative chlorophyll content. These findings evinced that A. syriaca leaf extract had a significant phytotoxic effect on tested weeds. However, further studies under field conditions are needed to confirm the presented results and to check its herbicidal potential.

A bioassay test was conducted to evaluate the sensitivity of maize, sunflower and barley to clomazone residues in sandy loam soil. Clomazone was applied at different rates from 0.12 to 12 mg a.i./kg of soil. The parameters measured 14 days after treatment were: shoot height, fresh and dry weight, and content of pigments (carotenoids, chlorophyll a and chlorophyll b). The results showed that the lowest clomazone concentration caused a significant reduction in all measured parameters for barley and sunflower shoots. Fresh weight of maize shoots was not sensitive to clomazone residual activity in soil while the other parameters were highly inhibited. Nomenclature: clomazone (2-(2-chlorbenzyl)-4,4-dimethyl-1,2-oxazolidin-3-one), maize (Zea mays L.), sunflower (Helianthus annuus L.), barley (Hordeum vulgare L.).

The purpose of this study was to examine in vitro the herbicidal effect of an essential oil of dill (Anethum graveolens) on germination and early seedling growth of two parasitic flowering plants in the genus Cuscuta (Cuscuta campestris and C. epithymum), as well as its phytotoxic impact on germination and early seedling growth of two host plants (alfalfa and red clover). Chemical analysis of the essential oil extracted from dill leaves and flowers showed that carvone (51.69%) and limonene (39.88%) predominated. The results of a seed bioassay showed inhibitory effects of different concentrations (1%, 0.5%, 0.1%, and 0.01% v v-1) of the essential oil of dill leaves on germination and early seedling growth of both tested species of the genus Cuscuta. Germination of C. campestris seeds was inhibited between 67% and 94%, while seed germination of C. epithymum was inhibited between 67% and 100%. A similar inhibitory effect was observed regarding the seedling length parameter. Moreover, the bioassay results indicated significant phytotoxic effects of dill essential oil on the seed germination and early seedling growth of host plants. Red clover proved more sensitive as even the lowest concentration inhibited germination by 35%, while higher concentrations caused 70-100% inhibition. On the other hand, lower concentations inhibited germination of alfalfa seeds by 5%, while higher concentrations caused greater inhibition (36-100%). Future research should include both pot experiments and field micro-trials to determine herbicidal, i.e. phytotoxic, effects of dill essential oil on parasitic flowering plants and cultivated species.

Secondary plant metabolites with allelopathic activity or phytotoxicity could be biotechnologically important, serving as a source of allelochemicals, and thus contributing to the agro-industrial sector. The objective of this study was to use the obtained common walnut (Juglans regia L.) and bitter dock (Rumex obtusifolius L.) leaves extracts rich in phenolic compounds, i.e. with high antioxidant potential, and to identify their phytotoxicity to Setaria glauca (L.) P. Beauv. and Sorghum halepense (L.) Pers. weed seedlings grown in vitro. The obtained plant extracts had remarkably high affinity for scavenging free radicals, having DPPH IC50 values of 0.127 mg/ml for common walnut leaf extract and 0.194 mg/ml for bitter dock leaf extract. Ferric reducing antioxidant power of the extracts was also high, FRAP value of the common walnut leaf extract was 384.4 ?} 8.1 ?mol Fe2+/g dry mass, and of the bitter dock leaf extract 321.6 ?} 2.5 ?mol Fe2+/g dry mass. At the highest used concentration, common walnut leaf extract reduced germination of S. glauca by 67.3%, while bitter dock leaf extract reduced germination of that weed by 54.5%. Shoot length of S. glauca was inhibited 80.7% when subjected to common walnut leaf extract, and 78.2% under the influence of bitter dock leaf extract, and its root length was inhibited 96.4% and 93.1% respectively. Germination of S. halepense was inhibited 100% under the influence of the obtained common walnut leaf extract at its highest test concentration, and 79.2% when subjected to bitter dock leaf extract at the same concentration. Shoot length of this weed was reduced 100% after treatment with common walnut leaf extract, and 93.7% when subjected to bitter dock leaf extract. Root length was reduced 100% and 99.3%, respectively. Overall, the extracts demonstrated pronounced antioxidant activity and remarkable allelopathic potential.