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Leaf oil chemistry of Eucalyptus nitens and the Tasmanian native eucalypts

­­Brad Potts, Paul Tilyard and Julianne O’Reilly-Wapstra
School of Plant Science and CRC for Forestry, University of Tasmania

The compositions of volatile oils from juvenile and adult leaves of the 29 Tasmanian native species of Eucalyptus (encompassing 138 populations) have been studied.  The results were compared to similar studies of Tasmanian grown E. nitens.  The meta-analysis showed that while the relative proportions may vary, the major components of the leaf oils in E. nitens were found in virtually all native species of subgenus Symphyomyrtus (to which E. nitens belongs).  In most cases the two dominant chemical components were the same.

Recent media stories have raised interest in the similarity of the foliar chemistry of Eucalyptus nitens compared to that of the Tasmanian native eucalypts.  The suggestion was raised that a chemical(s) leaching from E. nitens plantations may be the cause of toxicity detected in laboratory testing of some foam and surface water samples collected from the George River in North-eastern Tasmania ( read more).  A report by Analytical Services Tasmania identified toxicity in a foam sample collected in the headwaters of the river as well as surface samples lower in the catchment (read more).  While no pesticides were detected, they did identify compounds ‘known to be naturally occurring in many plant species including eucalypts’.  Two compounds in particular, 1,8 cineole and alpha-pinene are major components of the leaf volatile oils of many eucalypt species, including E. nitens.  This raised the question as to how similar chemically are the leaves of E. nitens and the native Tasmanian species?  

Eucalypt leaves contain many different types of secondary chemical compounds, many of which are believed to play a role in the tree’s natural defence against herbivores (e.g. insects and marsupials) and fungi.  Some eucalypt species (mainly from subgenus Symphyomyrtus) have cyanogenic glycosides in their leaves, but these compounds have not been reported from E. nitens and, among the Tasmanian species, to date have been reported only in E. viminalis and E. ovata (Gleadow et al. 2008).  There is also a group of compounds called FPCs (formylated phloroglucinol compounds) which are well-known to play a role in the defence of eucalypts from marsupial browsing (Eschler et al. 2000; Moore and Foley 2005; O'Reilly-Wapstra et al. 2004).  Two of the key groups of FPC compounds are the sideroxylonals and macrocarpals, which have both been reported to occur at variable levels within (e.g. E. globulusO'Reilly-Wapstra et al. 2004) and between species (Eschler et al. 2000).  The most-well known group of leaf chemicals, however, are the volatile oils - mainly consisting of monoterpenes and sesquiterpenes - which include the compounds 1,8 cineole and alpha-pinene (Boland et al. 1991; Coppen 2002).  The volatile oils of eucalypts are well-known for their medicinal and industrial uses and have been studied widely in eucalypts (Boland et al. 1991; Coppen 2002).  They are the group of chemicals most extensively surveyed in the native Tasmanian eucalypts and thus there is a good base against which to compare the introduced E. nitens.

The levels of the chemical components in the volatile oils from eucalypt leaves may vary quantitatively due to numerous factors including genetics (e.g. species, provenance or family), environment (e.g. nutrition - O'Reilly-Wapstra et al. 2005; Loney et al. 2006), the season and physiological age of the leaf (Li 1993; Loney et al. 2006), as well as the ontogenetic leaf type (e.g. seedling, juvenile or adult leaf types) (Li et al. 1994; O'Reilly-Wapstra et al. 2007; McArthur et al. 2010).  However, the published literature on E. nitens grown in Tasmania from seed collected from native stands on mainland Australia (see Hamilton et al. 2008 for the natural distribution of E. nitens) suggests that, in general, the composition of the major components of the leaf oils of this species is similar to that of the Tasmanian native eucalypt species from the subgenus Symphyomyrtus.

There are 29 eucalypt species recognised on the island of Tasmania (read more and see Williams and Potts 1996) and one more, E. nebulosa, was described in 2008 (Gray 2008; Table 1 and see related article in Biobuzz 7).  These species belong to two of the 13 major evolutionary lineages of eucalypts that are recognised at the level of subgenus or genus (Eucalyptus and Corymbia).  The Tasmanian species belong to subgenus Eucalyptus (ashes and peppermints; formally known as subgenus Monocalyptus) and subgenus Symphyomyrtus (gums) (Williams and Potts 1996).  These subgenera differ in many characteristics, including flower anatomy, and they are reproductively isolated (i.e. hybridisation does not occur between species from the different subgenera).  Differences in leaf chemistry exist between subgenera, for example, Eschler et al. (2000) did not detect FPCs in species in subgenus Eucalyptus and cyanogenic glycosides, when they occurred, were mainly detected in Symphyomyrtus species (Gleadow et al. 2008).  The Tasmanian Symphyomyrtus species have been classified into five taxonomic series (Table 1).  Eucalyptus nitens belongs to series Globulares along with E. globulus, but in a different subseries (Brooker 2000; McKinnon et al. 2008).

Much of our knowledge of the composition of the volatile oils in the leaves of the Tasmanian eucalypts and E. nitens comes from a PhD project undertaken at the University of Tasmania by Haifeng Li (Li 1993) nearly 20 years ago.  This work was undertaken prior to the major expansion of the E. nitens plantation estate in Tasmania and was aimed at understanding better the role of leaf phytochemistry in insect host selection.  An extensive sampling of both juvenile and adult foliage from native populations of all the Tasmanian species was undertaken.  The more widespread species were represented by up to 11 populations and for each population and foliage type a pool of foliage was made from between 15 and 20 trees.  The E. nitens foliage was sampled from (i) four species trials established by CSIRO along an altitudinal gradient in southern Tasmania where E. nitens was grown with E. globulus, E. delegatensis and E. regnans (Li 1993; Li and Madden 1995) (adult foliage), (ii) one of the original trials established by Forestry Tasmania in southern Tasmania testing the genetic differences between native provenances of E. nitens (Li et al. 1994) (adult and juvenile foliage), and (iii) two family trials from native forest seed established by Associated Pulp and Paper Mills (and subsequently incorporated into the estates of North Ltd. and then Gunns Ltd.) in northern Tasmania (adult foliage) (see Hamilton et al. 2008 for history of E. nitens breeding).    


Figure 1.  Environmental variation in the percentage composition of major volatile oil components of adult leaves in progeny of E. nitens from the Penny Saddle (Victoria) provenance grown at four different sites along an altitudinal gradient in southern Tasmania (from Li 1993).  The ‘all-Eudesmol’ category combines the alpha, beta and gamma isomers of eudesmol.


­From this large-scale sampling, oils were extracted from the leaves by steam distillation (faster methods are available these days).  The amount of oil extracted per dry weight of leaf was estimated and, depending upon the study, the percentage composition of up to 36 chemicals were quantified (Li et al. 1995; Li et al. 1996).  We have compiled these research data from the published papers and additional data in the thesis.  We plotted average percentages of 13 major oil components across the various studies to allow visualisation of the general trends in variation amongst these eucalypt species.  While genetic differences between species may be confounded by differences in the timing of sampling and environmental differences between sites, these effects will be smoothed in our meta-comparisons by the large numbers of samples and different sites studied (Li et al. 1995; Li et al. 1996).  Furthermore, differences between species (E. nitens, E. globulus, E. delegatensis and E. regnans; Li and Madden 1995) when ­

Figure 2.  Variation in the mean oil yield (+ 1 s.e.) from (a) adult and (b) juvenile leaves expressed on a leaf dry weight basis (g g-1) for the Tasmanian native species from subgenera Symphyomyrtus (grey) and Eucalyptus (black) in the wild and E. nitens (blue) grown in field trials in Tasmania (from Li 1993 and resulting publications).  The means are derived from an average of five populations per native species and for E. nitens adult foliage was sampled from seven sites and juvenile foliage samples from one site. Only one sample was obtained from each of the rare species E. perriniana and E. radiata, and no standard errors are shown.


­ grown in common environment field trials demonstrate a genetic basis to the variation in leaf chemistry.  Environmental factors may also affect both yield and composition of the oil, but this tends to be of a quantitative rather than qualitative nature and the major trends in the oil composition of, for example, E. nitens are maintained across diverse sites (Figure 1).

The average oil yield from E. nitens juvenile leaves was reported to be higher than adult leaves (Li et al. 1994), but both were at the bottom end of the range of the species-average from the native forest samples (Figure 2).  


Figure 3.  The percentage composition of major oil components of adult leaves averaged across all samples of all Tasmanian species of the subgenus Symphyomyrtus (17 species, 67 populations) and subgenus Eucalyptus (formerly subgenus Monocalyptus; 12 species and 71 populations) compared with the average of E. nitens sampled from field trials at seven sites in Tasmania.    



While the possibility of qualitative differences in minor components that were not quantified cannot be dismissed, of the 36 chemical components that were quantified by Li et al. (1996), none was found in the leaf oils of E. nitens that was not also found in one or other native eucalypt species.  The levels of the chemicals varied both within and between species in a quantitative manner and the composition of E. nitens oil was most similar to that of the 17 native Symphyomyrtus species, both on average (Figures 3 & 4) and at the individual species level (Figure 5).  The major components of the leaf oils in E. nitens juvenile or adult foliage were found in virtually all native species of the subgenus Symphyomyrtus, and in most cases the dominant chemical components were 1,8, cineole followed by alpha-pinene.  These chemicals are only a minor component of the oils of most Tasmanian species from the subgenus Eucalyptus (e.g. Figure 5).



Figure 4.  The percentage composition of major oil components of juvenile leaves averaged across all samples of all Tasmanian species of the subgenus Symphyomyrtus (17 species, 67 populations) and subgenus Eucalyptus (formerly subgenus Monocalyptus; 12 species and 71 populations) compared with a study of E. nitens juvenile foliage collected from a multi-provenance field trial in Tasmania.  NB: Only 9 of the major chemical components were quantified in the study of E. nitens juvenile foliage.



Figure 5.  The average percentage of major oil components in adult leaves for eight native eucalypt species that are common in north-eastern Tasmania (subgenus Symphyomyrtus, left; subgenus Eucalyptus, right) and E. nitens growing in field trials in Tasmania (Plantation).  The E. nitens values are averaged across seven sites.  The number of populations averaged for the native species are: E. globulus (6); E. ovata (6); E. viminalis (6); E. amygdalina (8); E. delegatensis (10); E. obliqua (9); E. regnans (6) and E. sieberi (3).


The profile of major components of the E. nitens oil from adult leaves would appear to deviate no more from that of the Tasmanian Symphyomyrtus species which occur in north-eastern Tasmania than the native species differ amongst themselves (Figure 5).  While there was only one study of E. nitens juvenile leaves, a similar conclusion is reached, suggesting that it would be difficult to differentiate E. nitens from the Tasmanian native species as a source for the 1,8 cineole and alpha-pinene in the water samples from the George River.  Another major chemical reported in the toxic samples by Analytical Services Tasmania was beta-pinene.  Beta-pinene has only been reported as a very minor component (<1.3%) of the leaf oils of the Tasmanian native eucalypt species or E. nitens (Li et al. 1996; Brophy and Southwell 2002).  There is no obvious reason why beta-pinene should be preferentially concentrated in the assayed samples more than alpha-pinene (Dr. Noel Davies, pers com.).  Thus, as beta-pinene is listed as a major - as opposed to minor - compound detected in the river samples assayed by Analytical Services Tasmania, the data reviewed here suggest that it is not likely to originate from a eucalypt leaf oil source.  


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Biobuzz issue eleven, May 2010