Essential oils (EOs) from the aerial parts of three Launaea species collected from two different habitats were extracted [Launaea mucronata collected from coastal desert (LMC) and from inland desert (LMD), Launaea nudicaulis collected from coastal desert (LNC) and from inland desert (LND), Launaea spinosa (LS)]. The EOs were analysed by gas chromatography mass spectrometry (GC‐MS) and tested for antioxidant activity as well for allelopathic activity against purslane. A total of 117 compounds were identified from all samples, including 64 from LS, 50 from LMC, 39 from LMD, 24 from LNC, and 20 from LND, representing 100% of the total oil mass. The oxygenated sesquiterpenes were the major compounds of the EOs from LS, LMC, LMD, LNC, and LND, representing 62.73, 61.02, 58.35, 43.22, and 48.33%, respectively, whereas the monoterpene hydrocarbons were minor compounds in all samples. α‐Acorenol (31.42%), trans‐longipinocarveol (12.04%), and γ‐eudesmol (6.31%) represented the major compounds of the EO from LS, whereas hexahydrofarnesyl acetone and n‐heneicosane represented the major compounds from LMC, LMD, LNC, and LND. Correlations between the EOs of the five samples were studied via principal component analysis and agglomerative hierarchical clustering, based on the chemical composition of the EOs. There was a significant variation between Launaea species regarding both the quality and the quantity of EO composition, whereas no significant difference was observed based on the habitats (coastal and inland deserts). The EOs of the five Launaea samples exhibited meaningful antioxidant activities, as follows: LND > LNC > LMD > LMC > LS. Moreover, the EOs of the five Lanaea samples exhibited significant allelopathic activity against purslane weed in a concentration‐dependent manner. LMD and LMC showed the highest effect, with germination inhibition of 96.1 and 87.9% at 250 μL L−1, and radicle growth inhibition of 92.6 and 89.7%, but LS was the least effective extract. Our results showed that the variation in the quality and quantity of EO composition is mainly species specific, with a slight correlation with habitats.

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Herbicides are natural or synthetic chemicals used to control unwanted plants (weeds). To avoid the harmful effects of synthetic herbicides, considerable effort has been devoted to finding alternative products derived from natural sources. Essential oils (EOs) from aromatic plants are auspicious source of bioherbicides. This review discusses phytotoxic EOs and their chemical compositions as reported from 1972 to 2020. Using chemometric analysis, we attempt to build a structure-activity relationship between phytotoxicity and EO chemical composition. Data analysis reveals that oxygenated terpenes, and mono- and sesquiterpenes, in particular, play principal roles in the phytotoxicity of EOs. Pinene, 1,8 cineole, linalool, and carvacrol are the most effective monoterpenes, with significant phytotoxicity evident in the EOs of many plants. Caryophyllene and its derivatives, including germacrene, spathulenol, and hexahydrofarnesyl acetone, are the most effective sesquiterpenes. EOs rich in iridoids (non-terpene compounds) also exhibit allelopathic activity. Further studies are recommended to evaluate the phytotoxic activity of these compounds in pure forms, determine their activity in the field, evaluate their safety, and assess their modes of action.

Plants are considered green resources for thousands of bioactive compounds. Essential oils (EOs) are an important class of secondary compounds with various biological activities, including allelopathic and antimicrobial activities. Herein, the present study aimed to compare the chemical profiles of the EOs of the widely distributed medicinal plant Calotropis procera collected from Saudi Arabia and Egypt. In addition, this study also aimed to assess their allelopathic and antimicrobial activities. The EOs from Egyptian and Saudi ecospecies were extracted by hydrodistillation and analyzed via GC-MS. The correlation between the analyzed EOs and those published from Egypt, India, and Nigeria was assessed by principal component analysis (PCA) and agglomerative hierarchical clustering (AHC). The allelopathic activity of the extracted EOs was tested against two weeds (Bidens pilosa and Dactyloctenium aegyptium). Moreover, the EOs were tested for antimicrobial activity against seven bacterial and two fungal strains. Ninety compounds were identified from both ecospecies, where 76 compounds were recorded in Saudi ecospecies and 33 in the Egyptian one. Terpenes were recorded as the main components along with hydrocarbons, aromatics, and carotenoids. The sesquiterpenes (54.07%) were the most abundant component of EO of the Saudi sample, while the diterpenes (44.82%) represented the mains of the Egyptian one. Hinesol (13.50%), trans-chrysanthenyl acetate (12.33%), 1,4-trans-1,7-cis-acorenone (7.62%), phytol (8.73%), and myristicin (6.13%) were found as the major constituents of EO of the Saudi sample, while phytol (38.02%), n-docosane (6.86%), linoleic acid (6.36%), n-pentacosane (6.31%), and bicyclogermacrene (4.37%) represented the main compounds of the Egyptian one. It was evident that the EOs of both ecospecies had potent phytotoxic activity against the two tested weeds, while the EO of the Egyptian ecospecies was more effective, particularly on the weed D. aegyptium. Moreover, the EOs showed substantial antibacterial and antifungal activities. The present study revealed that the EOs of Egyptian and Saudi ecospecies were different in quality and quantity, which could be attributed to the variant environmental and climatic conditions. The EOs of both ecospecies showed significant allelopathic and antimicrobial activity; therefore, these EOs could be considered as potential green eco-friendly resources for weed and microbe control, considering that this plant is widely grown in arid habitats.