Calamintha nepeta L. (Savi) as source of phytotoxic compounds: bio-guided fractionation in identifying biological active molecules

Calamintha nepeta L. (Savi), known as Lesser Catmint, is a Mediterranean species belonging to the Labiatae family, considered an important source of natural compounds. Since little is known about phytotoxic potential of Lesser Catmint, the bio-guided fractionation method was employed to isolate and identify some compounds, prerequisite for their possible future use in weed management. Leaves and stems of catmint were extracted with methanol and fractionated using n-hexane, chloroform, ethyl acetate and n-butanol, solvents with different polarity. The potential phytotoxicity of the methanolic extract and its fractions, evaluated by ED50 values comparison, was assayed in vitro on seed germination and root growth of lettuce (Lactuca sativa L.). Germination and root growth of lettuce were strongly inhibited by catmint methanolic extract and its fractions, showing the following hierarchy of phytotoxicity for both physiological processes: ethyl acetate ≥ n-hexane > chloroform ≥ n-butanol. In the most active fraction, analyzed by HPLC, 5 poliphenols, gallic, vanillic, syringic, p-coumaric and ferulic acids, were identified and quantified. Whereas, the n-hexane fraction was a mixture of 32 chemicals, mainly composed of terpenoids and fatty acids, as analyzed by gas chromatography–mass spectrometry (GC–MS). Further, GC analysis allowed to quantify 5 compounds: camphor, trans-caryophyllene, menthol, farnesene and pulegone. Furthermore, both fractions inhibited seed germination and root growth of two of the most common weeds, Amaranthus retroflexus and Echinochloa crus-galli. The results confirmed the phytotoxic activity of C. nepeta L. (Savi) due to the presence of different molecule classes with biological activity and their potential future application as bio-herbicides.