Insecticidal Potential of Ocimum basilicum Leaves: Metabolite Distribution in Different Leaf Tissues

Leaves serve as essential plant organs that facilitate photosynthesis and consist of several layers, such as the mesophyll and epidermis, each of which possesses unique metabolite compositions. These metabolites play a role in the plant’s defensive system against insects. For instance, the leaves of Ocimum basilicum L. (basil) possess biocidal properties against a variety of insects. Although the insecticidal properties of these leaves have been well documented, the distribution studies on the leaf metabolites are inadequate. Thus, this study examined the metabolite profiles of the two leaf layers, epidermis and mesophylls. The separation of epidermis and mesophyll extracts was accomplished using whetstone powder, followed by gas chromatography-mass spectrometry to analyze the obtained metabolite profiles. The leaf trichomes were examined by scanning electron microscopy. Certain chemicals were only detectable within the epidermal or mesophyll tissues. For example, tricosane (16.37%) and geraniol (7.88%) were exclusively detected in the epidermis, whereas limonene oxide (1.26%) and α-humulene (1.04%) were only detected in the mesophyll. Furthermore, certain components were found in higher quantities in the epidermis and mesophyll layers, whereas others were more prevalent in the opposite layer. Our findings relevant to the trichome types, specifically glandular and non-glandular trichomes, indicated that both play a role in the initial defenses against herbivorous insects. This study offers significant insights into the chemicals that serve as plant defenses in basil leaf tissue and trichomes. Future studies on the distribution of chemical compounds in different leaf tissues can provide further insights into the mechanisms of plant-insect interaction and facilitate the development of strategies for identifying compounds that play a role in defense.