In this chapter, the term oil concentration is used to describe the amount of oil in a unit weight of the plant, or part of the plant such as the leaf. The standard unit is milligrams of oil per gram of dried leaf (mg/g). When the amount of oil was given as a volume in the referenced publications, it was converted to a weight by multiplying by the average oil density of 0.9 (Penfold and Morrison 1950). Another common unit of concentration expresses the weight of oil as a percentage of the plant weight. This can be converted to mg/g by multiplying by ten. To provide consistency, all published results were converted to the standard unit in this chapter.
When the weight of the plant or oil is referenced to a unit area where it grew, it is referred to as a yield with units of kg/ha or g/m2. Some authors use the term oil yield to describe the concentration as defined above, but in this chapter such use was altered to maintain consistency.
Much agronomic work refers to the yield of above-ground parts and does not include the weight of roots. The above-ground growth is referred to as biomass. As tea tree oil is found only in leaves, it is useful to subdivide the total biomass into three components; leaves, fine stems and main stems. Fine stems are defined as stems of less than 2.5mm diameter (Murtagh 1988), and main stems are the remainder. Fine stems carry virtually all the leaves. The total of leaf plus fine stems are called twigs. Such measurements are expressed on a dry-weight basis using separate estimates of the moisture content in each fraction to make the conversion. When twigs were distilled and the oil concentration was expressed per unit weight of twig, it was converted to a concentration per unit leaf weight by dividing by the measured ratio of leaf in twig. If this was unavailable and the measurements being compared were taken over a short interval, a typical ratio of 0.68 (Murtagh 1996) was used.
Tea tree oil is stored in subepidermal glands that are adjacent to the epidermis and equally distributed on both sides of a leaf (list et al. 1995). Oil glands are first apparent in immature leaves, with the number per leaf increasing as the leaf expands, to reach a maximum just before the leaf is fully expanded. The oil gland density appears to be under some degree of genetic control (List et al. 1995).
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