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.
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).
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