Phytotoxic Effects of Nepeta meyeri Benth. Extracts and Essential Oil on Seed Germinations and Seedling Growths of Four Weed Species

Essential oil isolated from the aerial parts of Nepeta meyeri Benth. by hydrodistilation was analysed by GC and GC-MS methods. A total 18 components were identified in the oil representing 100.0% of the oil. Main components were 4aα,7α,7aβ-nepetalactone (80.3%), 4aα,7α,7aα–nepetalactone (10.3%), trans-pulegol (3.1%), 1, 8-cineole (3.0%) and β-bourbonene (2.0%). In addition, n-hexane extract of N. meyeri was analysed by using GC and GC-MS methods and 18 components were identified. Likewise, nepetalactones, 4aα,7α,7aβ-nepetalactone (83.7%), 4aα,7α,7aα–nepetalactone (3.6%), 1, 8-cineole (1.9%) and α-terpinene (1.5%) were the predominat compounds in the hexane extract. Three concentrations (0.5, 1.0 and 2.0 mg/mL) of the essential oil and n-hexane, chloroform, acetone and methanol extracts isolated from the aerial partsand roots were tested for the herbicidal effects on the germination of the seeds of four weed species including Amaranthus retroflexus L., Chenopodium album L., Cirsium arvense L. and Sinapsis arvensis L. The essential oil of N. meyeri completely inhibited the germination of all weed seeds whereas the extracts showed various inhibition effects on the germination of the weed species. Herbicidal effect was increased with the increasing application concentrations of the extracts. In general, the acetone extract was found to be more effective as compared to the other extracts. All extracts also exhibited various inhibition effects on the seedling growths of the weed species. All extracts also tested for their phytotoxic effects on the weeds at greenhouse condition and the results showed that the oil and extracts caused mortality with 22.00-66.00% 48h after the treatments. These findings suggest that the essential oil and the extracts of N. meyeri have potentials for use as herbicides against those weed species.