Essential Oils Obtained from Sicilian Citrus reticulata Blanco By-Products: Antibacterial and Allelopathic Activity

Mandarin, one of the winter fruits commonly used in the preparation of foods and juices, is a fruit native to China and Southeast Asia. In this work, essential oils (EOs) obtained from byproducts of the Citrus reticulata Blanco flavedo of five cultivars present and cultivated within the Botanical Garden of Palermo were chemically and biologically studied: C. reticulata ‘Avana’ (C1), C. reticulata ‘Tardivo di Ciaculli’ (C2), C. reticulata ‘Bombajensis’ (C3), C. reticulata ‘Aurantifolia’ (C4), and C. reticulata ‘Padre Bernardino’ (C5). The GC and GC-MS analysis performed on all the extracted samples clearly highlighted the notable presence of limonene, a characteristic hydrocarbon monoterpene of EOs of the Citrus genus. C1, C2, C3, C4, and C5 were tested in relation to their possible antibacterial and allelopathic activity, also highlighting the activity of limonene, the main compound. For the antibacterial activity, eight different bacterial strains were used, both Gram-positive and Gram negative (Staphylococcus aureus, Proteus vulgaris, Klebsiella pneumoniae, Enterobacter cloacae, Pseudomonas aeruginosa, Escherichia coli, Salmonella typhi, and Enterobacter aerogens). For the allelopathic effect, two model systems were chosen: the germination of radish seeds (Raphanus sativus L.) and of spores in the moss Tortula muralis (Hedw.). The EOs from all cultivars showed pronounced antibacterial effects against all strains with an MIC comprised in the range of 16–256 μg/mL. Limonene showed the highest activity with an MIC between 4 and 16. The allelopathic effects showed a decrease in the percentage of seed germination, root, and epicotyl growth on Raphanus and a strong reduction in the germination of Tortula spores with an alteration in the development of the protonema. Limonene showed the same but more intense allelopathic activity.