Chemotaxonomic study on Thymus villosus from Portugal

The composition of the essential oils of four populations of Thymus villosus subsp. lusitanicus (Boiss.) Coutinho from Portugal was investigated by GC and GC-MS. To study the chemical polymorphism the results obtained from GC analyses of the volatile oils from individual plants from four populations were submited to Principal Component and Cluster analyses. A comparision with the essential oil of T. villosus subsp. villosus, previously studied by us was done. Important differences with regard to the major constituents in these two taxa were found. Linalool, geranyl acetate, geraniol and terpinen-4-ol were the main components of the essential oils of T. villosus subsp. lusitanicus, whereas in the oil of T. villosus subsp. villosus p-cymene, myrcene and alpha-terpineol were the major ones. Although, both taxa showed chemical polymorphism, different types of essential oils were characterized in each one: linalool; linalool/ terpinen-4-ol/trans-sabinene hydrate; linalool/1,8-cineole; geranyl acetate/geraniol; geranyl acetate/geraniol/1,8-cineole in T. villosus subsp. lusitanicus and p-cymene/camphor/linalool; p-cymene/borneol; linalool/geraniol/geranyl acetate; alpha-terpineol/camphor/myrcene in T. villosus subsp. villosus. Thus, the two subspecies of T. villosus can be easely differenciated by the composition of their essential oils.     

Screening of antibacterial activities of twenty-one oxygenated monoterpenes

Plant essential oils are widely used as fragrances and flavours in the cosmetic, perfume, drug and food industries. Oxygenated monoterpenes are widespread components of the essential oils, usually occurring in high amount. In this paper, the antibacterial activities of twenty-one oxygenated monoterpenes (borneol, borneol acetate, camphor, carvone, 1,8-cineole, citronellal, beta-citronellol, dihydrocarvone, fenchol, fenchone, geraniol acetate, isomenthol, limonene oxide, linalool, linalool acetate, nerol, nerol acetate, terpinen-4-ol, alpha-terpineol, menthol and menthone) and penicillin (standard antibiotic) were determined using a disc diffusion method (in vitro) against 63 bacterial strains, belonging to 37 different genera and 54 species (plant, food and clinic origins). The results showed that the oxygenated monoterpenes exhibited a variable degree of antibacterial activities. These compounds also inhibited the growth of bacterial strains by producing a weak zone of inhibition from 7 to 11 mm in diameter, depending on the susceptibility of the tested bacteria. Among the tested compounds, nerol, linalool alpha-terpineol, fenchol and terpinen-4-ol showed antibacterial activity at a broad spectrum. However, their antibacterial activities were lower than those of penicillin. In contrast to these compounds, camphor and 1,8-cineole exhibited no inhibition effects on the growth of all tested bacteria.     

Evaluation of extracts and oils of tick-repellent plants from Sweden

Abstract. Leaves of Myrica gale Linnaeus (Myricaceae), Rhododendron tomentosum (Stokes) H. Harmaja (formerly Ledum palustre Linnaeus: Ericaceae) and Artemisia absinthium Linnaeus (Asteraceae) were extracted with organic solvents of different polarities and the essential oils of leaves were obtained by steam distillation. The extracts or oils were tested in the laboratory for repellency against host-seeking nymphs of Ixodes ricinus Linnaeus (Acari: Ixodidae). Rhododendron tomentosum oil, 10%, diluted in acetone, exhibited 95% repellency; R. tomentosum and A. absinthium extracts in ethyl acetate, > 70% repellency; A. absinthium extract in hexane, approximately 62% repellency; and M. gale oil, 10%, approximately 50% repellency on I. ricinus nymphs. Compounds in the leaf extracts or in the oils were collected by solid phase microextraction (SPME) and identified by gas chromatography-mass spectrometry (GC-MS) and/or MS. Characteristic volatiles detected from oil or extract of M. gale were the monoterpenes 1,8-cineole, alpha-terpineol, 4-terpineol and thujenol; and of R. tomentosum myrcene and palustrol. Characteristic volatiles from leaf extracts of A. absinthium were sabinene, oxygenated monoterpenes, e.g. thujenol and linalool, and geranyl acetate. Each plant species synthesized numerous volatiles known to exhibit acaricidal, insecticidal, 'pesticidal' and/or arthropod repellent properties. These plants may be useful sources of chemicals for the control of arthropods of medical, veterinary or agricultural importance.     

Antimicrobial activity of essential oils isolated from Portuguese endemic species of Thymus

AIMS: Thymus species are wild species mostly found in the arid lands of Portugal. Possible antimicrobial properties of Thymus essential oils have been investigated. The chemical composition of the essential oils and the antimicrobial activity of Thymus mastichina (L) L. subsp. mastichina, T. camphoratus and T. lotocephalus from different regions of Portugal were analysed. METHODS AND RESULTS: Hydrodistillation was used to isolate the essential oils and the chemical analyses were performed by gas chromatography (GC) and GC coupled to mass spectrometry. The antimicrobial activity was tested by the disc agar diffusion technique against Candida albicans, Escherichia coli, Listeria monocytogenes, Proteus mirabilis, Salmonella spp. and Staphylococcus aureus. Pure linalool, 1,8-cineole and a mixture (1 : 1) of these compounds were included. Linalool, 1,8-cineole or linalool/1,8-cineole and linalool/1,8-cineole/linalyl acetate were the major components of the essential oils, depending on the species or sampling place. The essential oils isolated from the Thymus species studied demonstrated antimicrobial activity but the micro-organisms tested had significantly different sensitivities. CONCLUSIONS: The antimicrobial activity of essential oils may be related to more than one component. SIGNIFICANCE AND IMPACT OF THE STUDY: Portuguese endemic species of Thymus can be used for essential oil production for food spoilage control, cosmetics and pharmaceutical use. Further studies will be required to elucidate the cell targets of the essential oil components.     

Analysis of the volatile components of Lavandula canariensis (L.) Mill., a Canary Islands endemic species,growing in Australia

The essential oil of a Canary Islands endemic species, Lavandula canariensis (L.) Mill., gathered from Australia, was extracted by hydrodistillation and solid phase micro-extraction (SPME). The oils have been studied by GC and GC–MS. A total of 38 compounds have been identified in the leaf oil extracted by hydrodistillation, the principal components being carvacrol (23.6%), β-bisabolene (20.8%), (E,E)-α-farnesene (11.3%), β-caryophyllene (7.6%) and carvacrol methyl ether (7.3%), while the oil extracted by SPME showed carvacrol (42.6%) as the principal component with moderate amounts of (E,E)-α-farnesene (9.1%), β-bisabolene (7.5%), cis-hex-3-en-1-ol (5.6%) and carvacrol methyl ether (4.6%). SPME extracts indicated that carvacrol, cis-hex-3-en-1-ol and linalool were in greater concentration in the head space vapours than in the oil. This is the first report describing the essential oil composition of this species.     

Terpenoid compositions, and antioxidant and antimicrobial properties of the rhizome essential oils of different Hedychium species

A phytochemical study of the rhizome essential oils of four different Hedychium species was performed by means of GC and GC/MS analyses. H. ellipticum mainly contained 1,8-cineole, sabinene, and terpin-4-ol, while H. aurantiacum possessed terpin-4-ol, para-cymene, and bornyl acetate as the major entities. Similarly, trans-meta-mentha-2,8-diene and linalool were noticed in H. coronarium. Three different collections (I-III) of H. spicatum showed amazing differences in the relative contents of their essential oils, 1,8-cineole and 10-epi-gamma-eudesmol being identified as markers for samples I and II, terpin-4-ol and sabinene being the major compounds in sample III. The rhizome essential oils of the above species were studied for their antioxidant activities by different methods, including their effect on the chelating properties of Fe(2+), DPPH radical-scavenging activity, and reducing power. Antimicrobial screenings of the oils by the paper-disc method were performed against Staphylococcus aureus, Shigella flexneri, Pasteurella multocida, Escherichia coli, and Salmonella enterica enterica, and the respective minimum-inhibitory-concentration (MIC) values were determined. The rhizome essential oils from all Hedychium species exhibited moderate-to-good Fe(2+) chelating activity. H. spicatum from collection site III showed a completely different DPPH radical-scavenging profile than the samples from the other collection sites.     

Volatile oil variability in Thymus serpylloides ssp. gadorensis growing wild in Southeastern Spain

Volatile oils from single plants of Thymus serpylloides ssp. gadorensis were collected from Southeastern Spain and studied to check for chemical variability using gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). Many of the samples showed a phenolic chemotype, while another important group had significant levels of linalool. Geraniol, myrcene, caryophyllene oxide, terpinen-4-ol and 1,8-cineole were commonly present. Principal Component Analysis (PCA) and Cluster Analysis (CA) of this chemical variability separated two groups of plants characterized by either phenols or linalool, and an isolated third type with geraniol. A few samples were found to have both phenolic and non-phenolic compounds in high quantities, thus showing a mixed chemotype. Multidimensional scaling analysis (MDS) of the percentage concentration for each component of the essential oil showed that thymol, linalool, 1,8-cineole, borneol and geraniol have clear divergent vectors.     

Essential oil composition and variability of Thymus lotocephalus and Thymusxmourae

The composition of the essential oils of four populations of Thymus lotocephalus G. López and R. Morales and one population of T.xmourae Paiva and Salgueiro, two endemic taxa from Portugal, was investigated mainly by GC and GC-MS. Txmourae is a natural hybrid between T. lotocephalus and T. mastichina (L.) L. subsp. donyanae R. Morales, which essential oil was analysed for the first time. In its oil, it was possible to find compounds of both parents, which could enable us to confirm its intermediate status between those two taxa. 1,8-Cineole and borneol were the main constituents in the essential oil of T.xmourae, whereas linalool, geranyl acetate and 1,8-cineol were the major ones in T. lotocephalus. Intermedeol was also an important constituent in the oils of both taxa. Nevertheless, the volatile oils of the four populations investigated of T. lotocephalus showed important differences among the main constituents. In order to study their infraspecific variability, the results obtained in the analysis of individual plants were submitted to a Principal Component and Chemometric Cluster Analyses. Five types of essential oils were found: linalool, 1,8-cineole, linalool/1,8-cineole, linalyl acetate/linalool and geranyl acetate.     

RNAi targeting the gene for 1,8-cineole synthase induces recomposition of leaf essential oil in lavandin (Lavandula × intermedia Emeric)

Recomposition of volatile compounds is an effective way to alter the fragrance of an essential oil. Using RNA interference (RNAi), we attempted to reduce the production of 1,8-cineole, a major monoterpene in essential oils, to alter the composition of the essential oil in lavandin. We obtained 12 transgenic regenerants via inoculation with Agrobacterium tumefaciens, including an RNAi inducing vector targeting five regions of the gene for 1,8-cineole synthase (CINS). Of these, two regenerants, targeting a coding region about 250 nt upstream with 5′-UTR and a coding region about 300 nt downstream of CINS mRNA (CINS I and V), respectively, showed a significant decrease of 1,8-cineole production in leaf essential oil, although the overall composition was barely altered because the production of other compounds decreased concurrently. By contrast, in two other regenerants, targeting a coding region about 1000 nt in the middle and a coding region about 300 nt downstream of CINS mRNA (CINS IV and V), respectively, 1,8-cineole production could be barely observed, without any decrease in production of other compounds. Expression of CINS in these transgenic regenerants was extremely suppressed to 0.02 and 0.08 of that of a nontransgenic regenerant control. The composition of leaf essential oil in the transgenic regenerants was changed by the RNAi. Specifically, the major compounds changed from 1,8-cineole, camphor, and borneol, to linalool, camphor, and borneol. Consequently, the fragrance of essential oils of these plants was perceived as more citrusy than the fragrance of the nontransgenic regenerant. These results suggest that the knockdown strategy was a useful tool for altering the fragrance in lavandin.

     

Chemical compositions of Casuarina equisetifolia L., Eucalyptus toreliana L. and Ficus elastica Roxb. ex Hornem cultivated in Nigeria

Essential oils were obtained by separate hydrodistillation of three different plants cultivated in Nigeria and analysed comprehensively for their constituents by means of gas chromatography (GC) and gas chromatography-mass spectrometry (GC–MS). The leaf essential oil of Casuarina equisetifolia L. (Casuarinaceae) comprised mainly of pentadecanal (32.0%) and 1,8-cineole (13.1%), with significant amounts of apiole (7.2%), α-phellandrene (7.0%) and α-terpinene (6.9%), while the fruit oil was dominated by caryophyllene-oxide (11.7%), trans-linalool oxide (11.5%), 1,8-cineole (9.7%), α-terpineol (8.8%) and α-pinene (8.5%). On the other hand, 1,8-cineole (39.4%) and α-terpinyl acetate (10.7%) occurred in large quantities in the essential oils of the leaf of Eucalyptus toreliana L. (Myrtaceae). The oil also features high levels of sabinene (5.9%), caryophyllene-oxide (4.7%) and α-pinene (4.2%). The main compounds identified in the leaf oil of Ficus elastica Roxb. ex Hornem. (Moraceae) were 6,10,14-trimethyl-2-pentadecanone (25.9%), geranyl acetone (9.9%), heneicosene (8.4%) and 1,8-cineole (8.2%).     

Volatile oil comparison of cotyledon leaves of chemotypes of Melaleuca alternifolia

Oil quantity and quality were measured for the cotyledon leaves of the commercial terpinen-4-ol chemical variety of Melaleuca alternifolia. Oil yield obtained by ethanolic extraction was 3.8 micrograms per leaf or 2.6% (dry weight basis). The major components of the oil were alpha-pinene (7.4%), beta-pinene (12.0%) and terpinolene (27.3%). The non-commercial terpinolene chemical variety was found to be rich in 1,8-cineole (12.5%) and terpinolene (25.4%). The non-commercial 1,8-cineole chemical variety was rich in 1,8-cineole (37%) with significant quantities of alpha-pinene (15.5%), beta-pinene (23.3%) and terpinolene (10.9%). The cotyledon leaf composition, when compared with that of mature leaf from the same chemical variety, was found to be biased toward pinene and terpinolene biogenetic pathway constituents and hence not a good indicator of mature tree quality especially for the commercial terpinen-4-ol chemical variety. The implications of these analyses for the determination of M. alternifolia plantation quality and the understanding of oil formation, are discussed.     

Aromatic components of the leaves of the New Zealand lemonwood tree Pittosporum eugenioides

The major aromatic components of the essential leaf oil of the New Zealand lemonwood tree Pittosporum eugenioides are octyl acetate (33%), terpinen-4-ol (13%), decanol (6%) and (Z)-hex-3-enol (5%). These products are responsible for the characteristic Citrus-like aroma which is detected when the leaves are crushed, a phenomenon which provided the species with its common name. The major component of the oil, octyl acetate is also an abundant component of the essential oils of Heracleum and Boswellia species.     

Influence of growth phase on the essential oil composition of Hyptis suaveolens

The chemical composition of the essential oils of seven populations of Hyptis suaveolens in vegetative, flowering and fruiting stages and their interpopulation variability were investigated by GC–MS. Sabinene, limonene, 1,8-cineole, (E)-caryophyllene and spathulenol were the principal constituents. The results from the chemical analysis were submitted to Principal Component and Chemometric Cluster Analysis which allowed five groups of populations to be distinguished with respect to the stage of growth and high content of bicyclogermacrene/terpin-4-ol, sabinene, 1,8-cineole/spathulenol, limonene/γ-terpinene and spathulenol/(E)-caryophyllene. Pattern of geographic-variation in essential oil composition indicated that monoterpene hydrocarbons were mainly produced in plants from sampling sites located in higher latitudes and altitudes regardless of the phase of growth, while sesquiterpenes were mainly produced in fruiting samples grown at lower ones. The Canonical Correlation Analysis between the soil sampling sites with the populations revealed a significant relationship between oil components and edaphic factors. Sesquiterpenes and potential acidity, Al, and Al saturation load fairly strong onto the first canonical variate and are related to fruiting samples collected at lower latitudes. On the other hand, monoterpene hydrocarbons are strongly related to chemical balance in soils (P, Zn, Cu, Mn, base saturation, neutral pH), which is related to the vegetative/flowering sampling at higher latitudes.     

Variation in volatile leaf oils of seven eucalyptus species harvested from Zerniza arboreta (Tunisia)

Leaves of seven species of the genus Eucalyptus L'Hér., viz., E. cladocalyx F. Muell., E. citriodora Hook., E. diversicolor F. Muell., E. fasciculosa F. Muell., E. grandis W. Hill, E. ovata Labill., and E. botryoides Sm., were harvested from Zerniza arboreta (region of Sejnene, northwest of Tunisia) in June 2007. Of the latter species, leaves were collected from trees having two origins, Morocco and Italy. Hydrodistillation of the dried leaves provided essential oils in yields varying from 0.4±0.0 to 3.3±0.1%, according to the species. E. citriodora had the highest mean percentage of essential oil amongst the species examined, whereas the lowest one was obtained for E. botryoides originating from Morocco. Analysis by GC (RI) and GC/MS allowed the identification of 140 compounds, representing 92.5 to 99.4% of the total oil composition. The contents of the different samples varied according to the species. The main components were 1,8-cineole (2), followed by α-pinene (1), p-cymene, borneol, α-terpineol, cryptone, spathulenol, trans-pinocarveol (4), bicyclogermacrene (5), caryophyllene oxide, and β-phellandrene. Principal components analysis and hierarchical cluster analysis separated the eight Eucalyptus leaf essential oils into five groups, each constituting a chemotype.     

Chemical composition and antibacterial activity of essential oil from Artemisia feddei

The chemical components of the essential oil from Artemisia feddei LEV. et VNT. were analyzed using GC-MS. Ninety-nine compounds, accounting for 96.23% of the extracted essential oil, were identified. The main oil compounds were 1,8-cineole (16.86%), chamazulene (9.04%), alpha-terpineol (8.18%), alpha-phellandrene (5.78%), alpha-thujone (5.51%), alpha-terpinyl acetate (5.07%), borneol (5.08%), beta- caryophyllene (4.71%), camphor (4.04%), and terpinen-4-ol (3.04%). The antimicrobial activity of the essential oil and some of its compounds was tested against 15 different genera of oral bacteria. The essential oil from A. feddei had a considerable inhibitory effect on all the obligate anaerobic bacteria tested (MICs, 0.025 to 0.05 mg/ml; MBCs, 0.025 to 0.1mg/ml), whereas the major compounds demonstrated different degrees of growth inhibition.     

Chemical composition and fumigant toxicity of the essential oils from 16 species of Eucalyptus against Haematobia irritans (Diptera: Muscidae) adults

Oils extracted from various species of Eucalyptus (Eucalyptus badjensis Beuzev & Welch, Eucalyptus badjensis x Eucalyptus nitens, Eucalyptus benthamii variety dorrigoensis Maiden & Cambage, Eucalyptus botryoides Smith, Eucalyptus dalrympleana Maiden, Eucalyptus fastigata Deane & Maiden, Eucalyptus nobilis L.A.S. Johnson & K. D. Hill, Eucalyptus polybractea R. Baker, Eucalyptus radiata ssp. radiata Sieber ex Spreng, Eucalyptus resinifera Smith, Eucalyptus robertsonii Blakely, Eucalyptus rubida Deane & Maiden, Eucalyptus smithii R. Baker, Eucalyptus elata Dehnh, Eucalyptus fraxinoides Deane & Maiden, E. obliqua L'Hér) were obtained by hydrodistillation. The chemical composition of essential oils was determined by gas chromatography coupled to mass spectrometry. Essential oils were mainly composed of 1,8-cineole, alpha-pinene, alpha-terpineol, 4-terpineol, and p-cymene. Vapors from these essential oils and their major components were found to be toxic to Haematobia irritans (L.) (Diptera: Muscidae) adults. An aliquot of each oil was placed in a cylindrical test chamber, and the number of knocked down flies was recorded as a function of time. Knockdown time 50% was then calculated. Results showed that essential oil of E. polybractea had the highest knockdown activity of 3.44 min. A correlation was observed between the content of 1,8-cineole in the Eucalyptus essential oils and the corresponding toxic effect.     

Investigation of monoterpenoids rich essential oils of two Ocimum basilicum L. varieties at different agro-climatic conditions in India

India is an emerging basil essential oil producer in South-east Asia. Two high essential oil yielding hybrids, namely one inter specific hybrid between of O. basilicum and O. kilimandscharicum Gürke (HYBL-1) and another intraspecific hybrid of O. basilicum × O. basilicum (OBL-1) of basil were analyzed using GC, enantiomeric GC, NMR, enantio-GC–MS and GC–MS methods. Inter specific hybrid HYBL-1 contained high essential oil-rich in linalool (68.5%), camphor (8%), and 1,8-cineole (4.6%) as characteristic constituents among monoterpenoids, whereas β-caryophyllene (1.9%), germacrene D (1.0%), and epi-α-cadinol (1.9%) were the sesquiterpenoids at the Lucknow (North Indian conditions) and linalool (71.8%), camphor (9.4%) and 1,8-cineole (4.3%) at Hyderabad (South Indian conditions) locations. Intraspecific hybrid (OBL-1) possessed linalool (66.1%), 1,8-cineole (5.4%) and geraniol (8.6%) with sesquiterpenoids in low proportions. Inter specific hybrid HYBL-1 showed superiority over OBL-1 in the multi-location trials conducted at Lucknow and Hyderabad. Average mean performance of inter specific hybrid over locations was: herb yield 44.80 t/ha, oil content 0.63%, oil yield 188.50 kg/ha, linalool content 67.65%, camphor content 8.90% v/s OBL-1 herb yield 21.32 t/ha, oil content 0.53%, oil yield 97.50 kg/ha, linalool content 65.55%, camphor content 0.00%, respectively. The essential oil of these two hybrids subjected to enantiomer differentiation revealed a high enantiomeric excess for (3R) -(?)-linalool, whereas (1R)- (+)-camphor was recorded exclusively in inter specific hybrid. The extensive NMR experiments were performed to confirm constituents in these hybrids and found that NMR spectroscopy could also be an ideal tool for the differentiation of essential oils from commercial samples declared as natural.     

Survival in different habitats: Extreme ultramafic and calcareous soils affect Stachys recta essential oils composition

Two populations of Stachys recta growing in Italy on ultramafic and calcareous soils have been studied for their essential oils. Although the yields were comparable, the composition of the essential oils differed significantly. Plants growing on ultramafic soil produced mainly non-terpene derivatives (55.7%), of which the most abundant ones were 1-octen-3-ol (38.2%) and (E)-3-hexen-1-ol (5.9%); the terpenes α-cadinol (6.1%) and δ-cadinene (5.6%) were also significantly represented. In contrast, the populations living on calcareous soil produced an essential oil dominated by terpenes (93.8%), with germacrene D (18.8%), β-caryophyllene (17.7%), 1,8-cineole (15.9%) and α-pinene (14.2%) among the main components.     

Composition of the volatile oils of two Anthemis L. taxa from Turkey

The essential oils of water-distilled aerial parts of Anthemis pseudocotula and Anthemis cretica subsp. pontica (Asteraceae) were analysed by GC-MS. As a result thirty-five and forty compounds were identified representing 93.1% and 89.0% of the oils, respectively. The main compounds of A. pseudocotula were 1,8-cineole (39.40%), camphor (9.36%), artemisiaketone (5.68%), filifolene (5.15%), and a-terpineol (4.69%), whereas beta-caryophyllene (20.26%), azulene (14.98%), spathulenol (6.03%), and germacrene D (5.82%) were the major constituents of A. cretica subsp. pontica.     

Chemical composition of essential oils of four Eucalyptus species and their phytotoxicity on silverleaf nightshade (Solanum elaeagnifolium Cav.) in Australia

Phytotoxicity and chemical composition of essential oils from four selected Eucalyptus species in Australia were investigated. Essential oils had stronger inhibitory effects on germination and seedling growth of silverleaf nightshade (Solanum elaeagnifolium Cav.) when compared with a commercial eucalyptus oil and with 1,8-cineole. E. salubris oil had the highest inhibition index for silverleaf nightshade germination, root growth and shoot growth, while E. spathulata had the lowest inhibitory effect except root growth. Gas chromatography-mass spectrometry analysis revealed 56 compounds present in E. salubris oil, with 1,8-cineole (57.6?%), ??-pinene (10.9?%) and p-cymene (8.3?%) predominant. E. dundasii oil contained 55 identified compounds with 1,8-cineole (65.5?%) and ??-pinene (19.9?%) being the richest fractions. There were 56 compounds identified from E. brockwayii oil with ??-pinene (31.1?%), isopentyl isovalerate (20.2?%) and 1,8-cineole (16.9?%) as the most abundant components. E. spathulata oil contained 60 compounds, predominantly 1,8-cineole (52.9?%) and ??-pinene (31.0?%). Further study is required to determine the phytoxicity of the individual identified compounds on silverleaf nightshade and whether the observed phytotoxicity is attributable to a single compound or to the synergistic effects of several compounds.