Medicinal and Aromatic Plants—Industrial Profiles


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part of The Gordon and Breach Publishing Group.

GARY BAKER
138
selection to ensure that the quality of oil from those trees meets industry standards.
Similarly, seedlings are often purchased from commercial nurseries with no certificate
of origin. As oil yields and oil quality from plantations are variable, there is a need to
improve both the amount and quality of the yield through an efficient breeding
programme.
GENETIC VARIATION
A number of studies have assessed variation in natural stands of M. alternifolia for oil
concentration (Bryant 1950; Butcher 1994) and oil composition (Penfold et al. 1948;
Southwell et al. 1992; Butcher et al. 1994). Oil concentration can vary considerably between
trees both within and between native stands. Bryant (1950) measured an oil concentration
variation of 40–89mg/g for a single stand. Butcher (1994) found a similar variation (25–
88mg/g) between 109 trees sampled from 11 populations throughout the species’ natural
distribution, with the total between-tree variation being evenly distributed within and between
the different populations.
Melaleuca alternifolia produces an essential oil that can vary in composition. Three
chemical forms, based on the proportion of 1,8-cineole in the oil were identified by
Penfold  et al. (1948). The three forms were classified as the terpinen-4-ol ‘Type’
containing 6–15% 1,8-cineole, ‘Variety A’ 31–40% 1,8-cineole and ‘Variety B’ 54–
64% 1,8-cineole. A terpinolene-rich form of M. alternifolia has also been reported by
Southwell et al. (1992). After the analysis of 109 trees from 11 different populations
throughout NSW and Queensland, Butcher (1994) has proposed five chemical forms.
Three forms, ‘Type’ (0.1–11% 1,8-cineole), ‘Variety A’ (36–48% 1,8-cineole) and
‘Variety B’ (65–71% 1,8-cineole), correspond approximately to those of Penfold et al.
(1948). Two additional forms ‘Variety C’ and ‘Variety D’ are distinguished from within
the terpinolene-rich form identified by Southwell et al. (1992). ‘Variety C’ (15–20%
terpinen-4-ol; 30–36% 1,8-cineole; 10–18% terpinolene) and ‘Variety D’ (1–2%
terpinen-4-ol; 17–34% 1,8-cineole; 28–57% terpinolene) are based on the relative
proportion of terpinen-4-ol and terpinolene. The chemical variation of the oil produced
by this species is of direct economic importance as commercial production of tea tree
oil is limited to terpinen-4-ol rich or ‘Type’ oils.
Several progeny trials of M. alternifolia have recently been undertaken (Butcher et al.
1996; Doran et al. 1997) in northern NSW to examine the variation in oil yield and oil
composition under plantation conditions. When assessing the potential to improve
plantation productivity, variation in leaf yield and leaf oil concentration together with oil
composition need to be considered. Progeny from 5 parent trees in each of 12 populations
(Butcher et al. 1996) and from 200 parent trees representing 26 areas within 15 populations
(Doran et al. 1997) were trialed. Significant variation was measured between populations
for leaf yield, oil concentration and oil quality. Doran et al. (1997) concluded that seed
collected from populations east of 153°E longitude out-performed seed collected from
‘inland’ trees. The partitioning of variation components between population, family, plot
and tree for oil concentration (Butcher et al. 1996) revealed that the majority of variation
occurred between populations while for growth traits the dominant source of variation
was between individual trees.
Copyright © 1999 OPA (Overseas Publishers Association) N.V. Published by license under the Harwood Academic Publishers imprint,
part of The Gordon and Breach Publishing Group.

TEA TREE BREEDING
139
BREEDING PROGRAMMES
The Need
In economic terms, a breeding programme is successful if revenues generated through the
sale of genetically improved planting stock exceeds the cost of the programme. Additional
benefits accrue to industry when the improved stock increases plantation productivity,
profitability and thus competitiveness in the world market place. To be efficient, a breeding
programme needs to match potential genetic gain with available genetic, physical, human
and financial resources.
Australia is the custodian of the genetic resources of M. alternifolia and has the expertise
and technology to utilise and improve this genetic resource (Doran 1992). The opportunity
exists for the industry to undertake and maintain breeding programmes that will meet the
needs of the industry and to maintain an advantage over competitors.
Breeding Strategy
The development of a tree improvement programme involves setting a breeding objective.
From this a breeding strategy is developed to manage the genetic improvement and then a
breeding plan evolves to achieve the set objectives. A breeding strategy works with a genetic
base population and through the ongoing processes of selection and mating manages the
mass propagation of the genetically improved population either as seed or cuttings.
Populations in a Breeding Strategy
An effective breeding strategy accumulates genetic gain over each successive generation in
the cycle of progeny testing, selection and mating (Figure 2). This successive gain is achieved
through maintenance of the three populations needed in the cycle to supply the genetically
Figure 2 The populations and activities involved in tree breeding
Copyright © 1999 OPA (Overseas Publishers Association) N.V. Published by license under the Harwood Academic Publishers imprint,
part of The Gordon and Breach Publishing Group.

GARY BAKER
140
improved material for a fourth population (Eldridge et al. 1993), which for M. alternifolia
is the oil producing plantation. These four populations are—
1.
Base population—The gene resource, includes trees in natural stands and some
plantations which are suitable for selection. This base resource of genetic variation
will continue to be a source for selection to meet future breeding needs.
2.
Breeding population—The trees and their progeny which are repeatedly tested, selected
and mated over many generations to progressively improve genetic gain.
3.
Propagation population—The trees selected from the breeding population to mass
produce genetically improved planting material.
4.
Production population—The trees in a plantation for the production of tea tree oil.
Selection and Mating
The identification of superior trees to mate is essential in the production of genetically
improved seed. This process of selection and mating is needed for each generation to achieve
the desired progressive genetic gain.
To enable selection in any trait there must be substantial variation of that trait in the
population. To then achieve genetic gain for that trait in the next generation the trait must be
readily passed on to the progeny. Genetic gain can also be captured asexually by cloning
superior trees.
Efficient methods of selection include progeny testing on appropriate sites using suitable
statistical designs, appropriate techniques to measure selectable traits and suitable selection
technology such as index selection to take into account a number of selectable traits. Index
selection can use genetic, economic and pedigree information to maximise the probability
of selecting trees with the best genes (Eldridge et al. 1993).
Superior trees are selected for their greater number of favourable genes. Mating of these
trees allows the favourable genes to recombine so that new and better trees arise, carrying
even greater numbers of favourable genes (Eldridge et al. 1993). Mating or crossing of
parents may be natural (open pollinated) or artificial (controlled pollinated).
A mating design is the pattern in which the female and male parents are crossed. Care is
needed to minimise the potential of inbreeding from related parents, although the design
may allow material from other sources to be incorporated. The adoption and then success of
the mating design is therefore dependent on the availability of resources, particularly the
knowledge and skills of the workers. Many mating designs exist. A general description of
several designs is given by Zobel and Talbert (1984) who grouped them into incomplete
pedigree designs (open pollinated and polycross) and complete pedigree designs (nested,
factorial, bi-parent and diallel). Open pollinated progeny tests are inexpensive, simple and
informative, while the value of known pedigree designs from controlled pollinated crosses
lies in the additional information to be gained.
Breeding Plan
Once the breeding strategy has been developed to manage the genetic improvement of the
species with the available resources, then the breeding plan is undertaken to detail the
Copyright © 1999 OPA (Overseas Publishers Association) N.V. Published by license under the Harwood Academic Publishers imprint,
part of The Gordon and Breach Publishing Group.

TEA TREE BREEDING
141
implementation of the breeding strategy. The typical breeding plan details goals and the
methods needed to achieve objectives within the timeframe and resources of the programme.
Once implemented, plans are subject to regular revision.
BREEDING STRATEGY FOR MELALEUCA ALTERNIFOLIA
Objectives
The objective of a tea tree breeding strategy would be to progressively improve oil yield
and quality by increasing the yield of leaves per tree and the oil concentration in those
leaves and ensuring that oil produced is of consistent quality to maximise desirability and
price.
Selection Criteria
Plantations of M. alternifolia are managed as coppice crops to produce large quantities of
terpinen-4-ol rich tea tree oil. To increase plantation production by improving the genetics
of the plant, the selection criteria should deal with those plant characteristics that increase
oil yield and quality. Those characteristics are—
(1)
High leaf biomass as a seedling and when coppiced.
(2)
High leaf oil concentration.
(3)
High oil quality.
(4)
Adaptable to different growing conditions.
(5)
Resistant to diseases and pests.
Oil yield is a function of leaf biomass and the oil concentration of that leaf. To maximise
gains from breeding, the selection should aim to achieve gains in both these traits. This is
achieved when both traits are either independent of each other or if they are dependent, then
they are positively correlated. This implies that gains in one trait will result in gains in the
other. When a negative correlation occurs between selectable traits it is difficult to improve
traits concurrently using recurrent selection (Eldridge et al. 1993).
A strategy is therefore needed to accommodate negative correlation in order to maximise
genetic gains. Butcher (1994) has proposed three strategies for consideration. Firstly, the
effect of the correlation can be minimised, by using a combined index selection (Cotterill
and Dean 1990) with a restriction imposed on one trait at pre-selection levels (Cotterill and
Jackson 1981). An alternative would be to select trees which are correlation breakers (Eldridge
et al. 1993) and then to mass propagate those individuals. The third alternative would be to
cross two separate breeding populations for plant dry weight and oil concentration. The
hybrid progeny may express higher oil yields than could be achieved through simultaneous
selection (Dean et al. 1983).
Oil quality should be determined on its antimicrobial activity against microorganisms.
Currently, there is a market demand for oils with the lowest possible 1,8-cineole: terpinen-
4-ol ratio (i.e. 1,8-cineole<4% and terpinen-4-ol>36%). The variation in oil composition
between populations (see above) indicates the potential for selection to meet this demand.
Copyright © 1999 OPA (Overseas Publishers Association) N.V. Published by license under the Harwood Academic Publishers imprint,
part of The Gordon and Breach Publishing Group.

GARY BAKER
142
However, there are doubts about the need to lower the 1,8-cineole level in oils. Recent
evidence has shown that 1,8-cineole is not detrimental to the oil’s bioactivity or safety
(Southwell et al. 1996, 1997) and that it may have beneficial effects (Southwell et al. 1993;
Chapter 9). Other research indicates that relatively minor components in the oil (e.g. sabinene)
may influence the efficacy of the oil against specific microorganisms (Williams et al. 1990).
Selection for oil quality must meet current market demands, while allowing the flexibility
to cater for the on-going changes in the market place.
Broad adaptability and resistance to diseases and pests are both selection criteria that
influence leaf biomass more than oil concentration. Results from several progeny trials
established in NSW show that, when seedlots were ranked for oil yield and again for oil
quality, the order of ranking was similar for different sites (Doran et al. 1997). This indicates
that there is a reasonable degree of stability for oil traits such that selected seedlots may
retain their elite status over sites. From the 200 seedlots trialed by Doran et al. (1997), nine
were selected to assess productivity in northern Queensland. Early results (Drinnan 1997)
suggest that seedlot rankings for oil concentration are consistent to that when grown in
NSW.
The major pest that defoliates tea tree plantations in NSW is Paropsisterna tigrina
(Chapuis) or Pyrgo beetle (Campbell and Maddox 1996). Estimates of leaf loss to Pyrgo
can vary with many reports of complete loss of crop (Maddox 1996a). The use of chemicals
to control this pest is declining as the industry adopts a zero chemical use policy to ensure
residue free oils. Thus the need for an alternative to manage this pest will increase. Selection
for genetic resistance would offer the industry significant benefits. Pyrgo activity coincides
with flush growth. Selection for traits to alter the flushing time, or rate at which the flush
leaf matures may reduce leaf loss (Maddox 1996b). Maddox (1996a) reported also that
feeding damage from Pyrgo can decline with an increasing proportion of a-terpinene and
terpinolene in flush leaf oils.
Selection for disease resistance is currently not considered worthwhile as no serious
diseases have been recorded on tea tree (Colton and Murtagh 1990).
To maximise genetic gain, the number of traits for selection should be kept to a minimum
as the gains in individual traits decline as the number of traits for selection increases (Cotterill
and Dean 1990).
Species Information
To develop a breeding strategy for M. alternifolia it is important to know certain aspects of
the biology of the species. These include both sexual and asexual reproduction and the
range of variation in economically important traits along with their relationships and
heritabilities. These aspects are listed.
1.
Morphology. The flowers of M. alternifolia are morphologically bisexual and are
insect pollinated. Outcrossing predominates with less than 10% self-pollination
(Butcher et al. 1992). This low rate of self-pollination is highly advantageous for a
breeding strategy based on open pollination as it reduces the likelihood of inbreeding.
2.
Flowering. The abundance and periodicity of flowering is variable both within and
between populations (Doran et al. 1997). Flowering usually occurs during October
Copyright © 1999 OPA (Overseas Publishers Association) N.V. Published by license under the Harwood Academic Publishers imprint,
part of The Gordon and Breach Publishing Group.

TEA TREE BREEDING
143
and November, for trees that are more than 2–3 years old. Flowering is heaviest during
these months in years that have wet winters (Baker 1996, 1997). It appears that a wet
winter is associated with both an increase in the number of trees flowering in a
population and with the number of flowers per tree.
3.
Pollination. Although the controlled pollination of M. alternifolia is possible (Moncur
1997), pollinating by hand is inherently slow and expensive. To undertake controlled
crosses in a breeding strategy, the potential gains from such crosses have to be balanced
against the costs, particularly when resources to a breeding program are limited.
Potential gains from such crosses include the use of the progeny as a source of pedigree
material for genetic studies as well as producing unrelated families for further selection.
4.
Propagation. Mass propagation of this species is by seed. Cloning is possible by
micropropagation (Hartney and Svensson 1992) or cuttings (Whish 1993) but the use
of clones in yield trials over several years is needed to test if clones are suitable for
plantation production. Cuttings from selected trees can be used in the establishment
of clonal seed orchards to capture greater genetic gains.
5.
Distribution. Information on the natural distribution of M. alternifolia (see above) is
used when collecting seed for progeny trials. Seed is collected from the terpinen-4-ol
rich oil trees located throughout the natural distribution of the species in NSW. Although
seed normally matures 12–18 months after flowering (Colton and Murtagh 1990),
mature seed can be collected at any time as some trees retain their seed crops for
several years.
6.
Genetic parameters. Genetic parameters express estimates of genetic and non-heritable
variations of a population in respect to some characteristic (Allard 1960). In addition
to being needed for the evaluation of different breeding strategies, these estimates
together with their nature, magnitude and inter-relationships are necessary to assess
improvement by selection. Heritability is a measure of how strongly a trait is influenced
by genetics (Hanson 1963). When heritability is high, gain from selection will be
high. Genetic parameters and expected gains from selection and breeding as estimated
in several M. alternifolia progeny trials are—
(a) Heritabilities of 0.51 (Doran et al. 1997) and 0.67 (Butcher 1994) for oil
concentration, 0.21 for plant height and 0.14 for stem diameter (Doran et al.
1997), 0.25 for plant dry weight and 0.27 for coppicing ability (Butcher 1994).
These estimates indicate that improvement, particularly in oil concentration would
follow selection for single traits.
(b) The absence of genetic correlation between oil concentration and growth traits
(e.g. basal diameter, which is highly correlated with leaf yield (Doran et al. 1997)).
This suggests that oil concentration and leaf biomass should be able to be improved
simultaneously in a breeding programme. Butcher (1994), however, reported a
negative genetic correlation of 0.42 between oil concentration and plant dry weight
implying that genetic gain would have to be balanced between these two traits.
(c) Calculated gains of 17% for oil concentration and 14% for coppicing (Butcher
1994). These estimated gains were derived from one generation of breeding at a
selection intensity of one tree in ten when a combined index selection was used,
restricting plant dry weight to pre-selection levels.
Copyright © 1999 OPA (Overseas Publishers Association) N.V. Published by license under the Harwood Academic Publishers imprint,
part of The Gordon and Breach Publishing Group.

GARY BAKER
144
Resources to Implement the Strategy
Tree breeding is a long-term activity that requires both a considerable commitment and
balanced use of genetic, physical, human and financial resources (Doran 1992). All breeding
programmes need to efficiently use limited resources. In the case of a M. alternifolia
programme, analytical requirements for the determination of leaf oil traits are a major criteria
in the selection of a breeding strategy.
The major genetic resource available for use in a M. alternifolia breeding programme is
seed collected from natural stands. Plantation trees do not represent a desirable genetic
resource in terms of seed, however, they could be used as a source of clones for inclusion in
the programme. Collection of seed from plantations is generally not feasible or desirable,
as plantations are usually harvested annually and so do not produce mature seed. Additionally
any seed that did result from a plantation, is likely to be highly inbred as plantations are
usually established using seed from only a limited number of parent trees.
The physical resources include suitable sites to establish and maintain progeny trials and
orchards. Sites for open pollinated orchards need to be located in isolation from stray pollen.
Physical resources also include the facilities used to implement the breeding strategy, such
as a nursery to grow seedlings and clones, an equipped laboratory to determine leaf oil
characteristics and appropriate room and equipment to store, process and analyse samples
and results.
For a strategy to be successful, funding has to be both adequate and long-term. An
appropriate budget allows for the initial capital and then on-going costs of tree breeding,
while providing funds for the employment of personnel, their transport and communication.
Breeding for tree improvement is becoming a highly specialised science, particularly in
the field of mathematical statistics for efficient selection and quantitative genetics when
predicting the consequences of selection (Eldridge et al. 1993). To develop and manage a
strategy efficiently, a team of specialists and technical assistants is needed.
Monitoring Progress
Strategies should have the capacity for review. Progress needs to be documented and reviewed
on a regular basis. Communication with research partners and industry facilitates feedback
leading to greater efficiency in the chosen breeding strategy.
Determinants of a Breeding Strategy
Breeding strategies can range from simple and cheap to complex and expensive (Eldridge
et al. 1993). Limited resources will often determine the complexity of the strategy and
hence the potential gain. The right strategy for a breeding programme will maximise the
genetic gain from available resources. When choosing the appropriate breeding strategy,
there are four basic components to consider.
1.
Method of selection (mass or recurrent). Recurrent selection maintains family identity
for the base, breeding and propagation populations while mass selection does not.
With fewer measurements, records or statistics, mass selection provides a cheaper
Copyright © 1999 OPA (Overseas Publishers Association) N.V. Published by license under the Harwood Academic Publishers imprint,

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