Imad El-Alam

(1) Background: The use of natural products based on essential oils (EO) is nowadays arousing great interest as an alternative method to control plant pathogens and weeds. However, EO possess low bioavailability and are highly volatile, and their encapsulation in hydroxypropyl-ß-cyclodextrin (HP-β-CD) could be a means to enhance their stability and maintain their bioactivity. Thus, the current study aims at investigating, in the presence and the absence of HP-β-CD, the antifungal and phytotoxic activities of nine EO, distilled from plant species belonging to Alliaceae, Apiaceae, and Cupressaceae families, with considerations for their chemical composition. (2) Methods: EO antifungal activity was assessed by direct contact and volatility assays against Fusarium culmorum, a major phytopathogenic fungi, while phytotoxic effects were evaluated against lettuce (Lactuca sativa L.) and rye-grass (Lolium perenne L.), by seedling’s emergence and growth assays. (3) Results: These EO inhibit fungal growth in both direct contact and volatility assays, with half-maximal inhibitory concentrations (IC50) ranging from 0.01 to 4.2 g L⁻¹, and from 0.08 up to 25.6 g L⁻¹, respectively. Concerning phytotoxicity, these EO have shown great potential in inhibiting lettuce (IC50 ranging from 0.0008 up to 0.3 g L⁻¹) and rye-grass (IC50 ranging from 0.01 to 0.8 g L⁻¹) seedlings’ emergence and growth. However, the EO encapsulation in HP-β-CD has not shown a significant improvement in EO biological properties in our experimental conditions. (4) Conclusion: All tested EO present antifungal and phytotoxic activities, with diverse efficacy regarding their chemical composition, whilst no increase of their biological effects was observed with HP-β-CD.

Background: Origanum syriacum L. is an aromatic plant growing wild in Lebanon. This species is highly used in Lebanese traditional medicine and is a staple food in Lebanese gastronomy. Due to the over-harvesting, this species has become a cultivated crop rather than being collected from the wild. This study aims to evaluate the chemical polymorphism according to soil type. Methods: Plant samples were cultivated in different soil types including manure, potting mix, professional agriculture mixture, vegetable compost, nursery soils, and natural agricultural soil inoculated with arbuscular mycorrhizal fungi. After 16 weeks of culture, fresh shoot biomass was measured. Root colonization rate was evaluated and foliar biomasses were used for essential oil (EO) extraction. EO yield was calculated and the identification of the main chemical compounds of EO samples was performed by gas chromatography (GC) and gas chromatography–mass spectrometry (GC/MS). Results: Our findings revealed that the soil type affects the O. syriacum chemotype. Indeed, the EO samples could be divided into two groups: thymol chemotype group including manure and vegetable compost soils and non-sterilized non-inoculated EO samples, and the thymol/carvacrol chemotype including potting mix, professional agriculture mixture, nursery mixture, sterilized non-inoculated, non-sterilized inoculated, and sterilized inoculated EO samples. These results showed that manure and vegetable compost soils promoted thymol synthesis, whereas potting mix, professional agriculture mixture, and nursery mixture soils were thymol/carvacrol chemotype. Moreover, mycorrhizal inoculation increased carvacrol and reduced thymol productions in comparison to non-inoculated conditions. Additionally, mycorrhizal inoculation showed significant enhancements in mycorrhizal rates and shoot biomass production with respect to the non-sterilized soil. Conclusions: These variations confirm the influence of the edaphic conditions on the chemical components biosynthesis pathways of oregano plants. The results of this investigation could be used for determining optimal soil type, leading to a good quality herb production.