The term “pesticide” indicates a wide range of compounds such as insecticides, fungicides, herbicides, rodenticides, molluscicides, nematicides and plant growth regulators. In Tunisia, weeds and phytopathogenic fungi can cause high yield loss, attaining 80%. Fungi as plant pathogens cause a variety of plant diseases resulting in losses both in food crop production and after harvest. Weeds have been identified as one of the most aggressive agricultural problems, reducing the quality and yield of crop production.In fact, weeds are the main biotic stresses on crop production; they are in competition with crops for natural resources: water, sunlight, space and nutrients. The available literature suggests the substantial yield and economic losses due to weeds. In Tunisia’s cereal production, weeds continue to be an ongoing problem and are one of the limiting factors for growth. To fight against these pests, farmers use synthetic pesticides with negative, harmful effects on the environment and human health. Herbicides are increasingly found in groundwater and surface water due to their extensive use in agricultural systems. In addition, the intensive use of pesticides allows the constant emergence of resistance by all types of organisms. Therefore, biologically active compounds from plants can be used as a potential source of potential fungal, pest and weed control agents. EOs represent a source of compounds with anti-fungal, pest and weed control potential to be used as an alternative to fill the role of synthetic products. Consequently, commercial hydroponic systems for several years there has been a great deal of interest in plant-derived EOs, rich in active compounds which have very important biological properties, as a source of bio-pesticides.
This is the case for the species of the Eucalyptus genus, which is native to Australia, contains approximately 900 species and is a member of the Myrtaceae family. Since 1957, 117 different species of Eucalyptus have been introduced to Tunisia. Eucalyptus species are now widely distributed around the world, owing principally to their advantageous wood properties for the paper industries. Historically, only a few species belonging to this genus have been employed to extract EOs, primarily from the leaves, for application mostly in the cosmetic and pharmaceutical industries. Eucalyptus leaves are rich in EOs, which consist mainly of monoterpenes and sesquiterpenes as well as other secondary metabolites such as phenols, alkaloids, flavonoids and tannins. As well, Eucalyptus extracts have been reported to have herbicidal effects on the seedling growth and germination of many weeds. Hence, the present study aimed to elucidate the chemical composition of the EOs obtained from the leaves of eight Tunisian Eucalyptus species and to investigate their anti-fungal and, for the first time, phytotoxic effects. The anti-fungal activity of the EOs was assessed against four plant pathogenic Fusarium strains, and their herbicidal effects were tested by evaluating their inhibition of the seed germination and seedling growth of three common weeds and three cultivated species.The results showed that the EO yield in E. cladocalyx, grown in the arboretum of Zerniza in the region of Sejnene in northwest Tunisia, was comparable to those obtained from Algeria and Morocco, but much lower than those obtained in another Tunisian site. For E. ovata, the richness of the EO is the same as in those grown in Algeria. E. sargentii showed the highest yield in comparison to other species in our study, but this yield was significantly lower than that obtained from the same species in other regions in Tunisia.
The differences between these yields and those reported in the literature could be attributed to many factors, such as the age of the tree, the climatic conditions, the edaphic conditions and the method of extraction. In total, 41 components were determined, and the highest number were identified in E. ovata EO, while only 17 components were identified in E. angulosa EO. Eucalyptol was the main component in all EOs except in those from E. resinifera and E. diversicolor. These results confirm what has been described by authors who reported that the monoterpene eucalyptol was the dominant component for the main species of the genus Eucalyptus. Kouki et al. reported that eucalyptol was the main component in some Eucalyptus species growing in Tunisia with percentages between 44.9 and 78.1%. Our analysis showed that the α-pinene content takes the second position in some species such as E. ovata, E. sargenti and E. angulosa, which agrees with several previous works. These results were partially in agreement with the data described by an Algerian team showing E. ovata EO contents of 51.2 and 7.8% for 1,8-cineole and α-pinene, respectively, whereas for E. saligna EO, the major component was α-phellandrene followed by p-cyrnene. In Morocco, two studies reported the composition of E. cladocalyx EO; the first identified 24 compounds representing about 81.1% of the EO, with α-pinene , p-cymene and 1,8-cineole as the most prominent components, followed by β-pinene , trans-pinocarveol and α-terpineol. In the second study, Fouad et al. identified 29 different components representing 79% of the total oil. The major components detected were spathulenol and 1,8-cineole , followed by p-cymene . In Tunisia, the work of Ben Hassine et al. described only six compounds in the EO of E. cladocalyx, whose major compounds were 1,8-cineole and α-thujene. Ameur et al. identified 23 compounds including globulol . In our study, the EO of E. cladocalyx was rich in eucalyptol as the major component, followed by β-pinene.
Regarding the EOs of E. diversicolor, the results described by Elaissi et al. partly agree with our conclusions concerning the presence of p-cymene. On the other hand, a Moroccan EO was characterized by the high content of 1,8-cineole and the absence of p-cymene. In vitro tests revealed that all EOs inhibited fungal growth in a dose-dependent way, which agrees with many reports in the literature on the concentration-dependent anti-fungal activity of essential oil. In this study, the EOs tested elicited anti-fungal activity for the four phytopathogenic fungal strains , and this mycelial growth inhibition was variable depending on the concentration and nature of the essential oil. These findings agree with those of previous studies that evaluated the anti-fungal activity against various phytopathogenic fungi of the EOs of some Eucalyptus species with different chemical profiles. Recently, we reported that some EOs inhibited the growth of eight phytopathogenic fungi. E. citriodora Hook. completely inhibited the growth of seven strains belonging to the genus Fusarium at a concentration of 4 µL/mL. Another study showed that EOs from E. oleosa F. Muell. ex Miq. inhibited fungal growth with MICs of 6 µL/mL for five strains of Fusarium. In our work, the EO of E. cladocalyx showed fungicidal activity against all four Fusarium strains tested at low dose, whereas the other EOs were fungicidal only against one or two of the four fungal strains. The EOs of E. ovata at the four concentrations studied did not exhibit fungicidal activity. Consequently, higher concentrations are needed to obtain the MFC. In vitro tests revealed that all EOs inhibited fungal growth in a dose-dependent way, which agrees with many reports in the literature on the concentration-dependent anti-fungal activity of essential oil In particular, the EO of E. cladocalyx showed a remarkable anti-fungal activity against phytopathogenic fungi that have a devastating effect on several species cultivated in Tunisia. This EO is characterized by the abundance of some compounds such as eucalyptol, pinene, trans-pinocarveol and α-terpineol, cannabis racking systems which may explain its anti-fungal power. Several previous works have shown the involvement of 1,8-cineole in the inhibition of phytopathogenic fungi. Based on the literature and according to Table 1, several compounds detected in the oils studied are known to have anti-fungal potential. According to Kim et al. , 16 pure compounds of EOs were tested for their anti-fungal potential; among these compounds were α-pinene, 1,8-cineole and p-cymene, which constitute the major compounds of Eucalyptus tested in our present study. These compounds showed significant anti-fungal potential. This could explain the results of the current study. Similarly, α-pinene, a major compound for the oils studied , is a monoterpene that has been demonstrated to inhibit respiration and ion transport and to act on cell integrity by increasing membrane permeability. In addition, 1,8-cineole, an oxygenated monoterpene and major compound in this study , has been described for its anti-fungal potential against phytopathogenic fungi. Similarly, according to Kim et al. , 1,8-cineole showed a significant inhibition of growth and the production of aflatoxin B1 and aflatoxin B2 of several fungi strains, and these anti-fungal properties were explained by a dramatic downregulation of 1, 8-cineole on the expression of afl E and afl L. This can explain the activities observed in this present study without neglecting the role of other compounds, thus without neglecting the interactions of synergism and antagonism. A research study reported that some components of Eucalyptus EOs such as α-terpineol, terpinolene, and 1,8-cineole are fungitoxic against phytopathogenic fungi. Regarding the anti-fungal mechanism, previous work has reported that the apolar terpenes can penetrate the lipid bilayer of the fungal membrane using their apolar properties. Hence, terpenes induce fungal membrane disruption by increasing the membrane’s permeability. Our report showed that Eucalyptus EOs exhibit herbicidal activity for all species tested, with more noticeable effects on weeds.
In agreement with our findings, previous works showed the herbicidal effects of Eucalyptus EOs against several weeds and crop plants. Thus, some components of Eucalyptus EOs are known for their phytotoxic effects and can be used as natural herbicides. Nevertheless, the phytotoxic activity of Eucalyptus EOs may affect some crops. It is therefore important to develop research to select the EOs with maximum herbicidal activity against weeds while minimizing negative impacts on crop growth. Seven of the eight EOs studied contain appreciable percentages of eucalyptol correlated with significant herbicidal activity. The high levels of eucalyptol, a monoterpene with phytotoxic properties, may partly explain the results obtained. The greatest inhibition was obtained using the EO of E. cladocalyx, which has appreciable levels of 1,8-cineole, α-pinene, β-pinene, trans-pinocarveol and α-terpineol. These findings suggest that 1,8-cineole combined with other terpenes may provide significant phytotoxic effects. According to the chemical composition of the oils of the eight Eucalyptus trees studied , we can notice the presence of several compounds known for their herbicidal activities, such as 1,8-cineole, α-pinene and p-cymene, detected as major compounds of the tested oils. These findings confirm a synergy between the various constituents of EOs for the observed phytotoxic effects. Although some studies have tried to explain the mechanisms of action of the EOs on germination and inhibition of the growth of seedlings, these modes of action remain unclear. Previous reports have suggested a number of effects and hypotheses; the majority of researchers working on this topic agree that EOs have phytotoxic effects that can cause anatomical and physiological changes in plant seedlings leading to accumulation of lipid globules in the cytoplasm, reduction of certain organelles such as mitochondria, inhibition of DNA synthesis or disruption of membranes surrounding mitochondria and nuclei. In this way, the development of natural pesticides and herbicides would help to reduce the negative impact of chemicals, such as the development of resistance in pathogens and parasites and resistant weeds. To this end, bio-pesticides and bioherbicides can be effective, selective, biodegradable and less harmful to the environment and human health. This study reports the potential anti-fungal and herbicidal effects of the essential oils from eight Eucalyptus species.Rice is the major calorie source for a large proportion of the world’s population and is one of the most commonly grown agricultural commodities in the world . California is the second largest rice-growing state in the USA, with around 200,000 ha production, most of which is concentrated in the Sacramento Valley . The majority of California’s rice production consists of short- and medium-grain japonica varieties and a few long-grain indica varieties, including cultivars developed for both the local climate and a continuously-flooded cropping system, where rice is pre-germinated and seeded by airplane onto fields with a 10-15 cm standing flood . The flooded conditions in which California rice is grown favor flood-adapted, competitive grass weeds such as watergrass species Beauv. spp., bearded sprangletop [Leptochloa fusca Kunth ssp. fascicularis N. Snow], and weedy rice f. spontanea Roshev.. The continuously flooded system also promotes sedges such as rice field bulrush [Schoenoplectus mucronatus Palla] and small flower umbrellas edge as well as aquatic broad leaf weeds such as ducksalad [Heteranthera limosa Willd.] and redstems spp..