Optimisation of biological and nursery system factors affecting cloning of tea tree

Gail Elizabeth Lowe

doi:10.25918/thesis.239

Compared to many trees, Tea Tree (Melaleuca alternifolia), is relatively easy to propagate both by rooted cuttings and tissue culture. But like most woody species, the rate of rooting and the quality of the root system that develops on the scion, declines as the stock plant from which it is derived matures. This is because the tissues of a plant specialise into organs with different functions (i.e., leaves, roots, stems, flowers etc.), as it undergoes ontogenetic development. As they specialise, tissues lose their capacity for totipotency, i.e., the ability to dedifferentiate into callus and then re-differentiate into other cell type. This loss of totipotency is an impediment to clonal propagation in tea tree from all but very juvenile material because the roots on cuttings must form adventitiously from stem tissue. This has major implications for the viability of commercially deploying clones for tea tree oil production. The aim of my research was to reduce the impact of stock plant maturation upon propagule quality during clonal propagation of tea tree. The project addressed factors identified during a literature review that were most likely to ameliorate the effects of maturation, by either avoiding more specialised tissue during the selection of scion material, or by modulation of the aging process in stock plants by manipulating nutrition, or other nursery system factors. Specifically, four experiments were carried out: 1. A cutting type experiment, where the performance of two cutting types at two stages of differentiation were contrasted; 2. A stock plant nutrition experiment, where the impact of nutritional stress (absence of major elemental components) upon stock plant health and scion performance were studied; 3. A stock plant density and system factor experiment, where competition for essential resources among stock plants for growth, and the propagation characteristics of scion, were analysed, and 4. A study of the effect of exogenous application of an analogue of the plant hormone strigolactone, upon the plant architecture of clonal tea tree plantlets. Tip cuttings, rather than nodal stem cuttings rooted at higher rates, had superior root systems including a tap root, superior shoot architecture (single stem due to preservation of apical bud) for machine planting, and were more vigorous, all attributes consistent with less tissue specialisation associated with ontogeny. The use of tip cuttings (called mini cuttings), eliminated any meaningful genotype difference among lines for propagation success, dispelling the notion of genotype-dependent vegetative propagation in this species. Stock plant nutrition was shown to impact cutting rooting rates and vigour. High nitrogen application proved to be beneficial because of the use of mini cuttings that utilise lush juvenile regrowth for scion material. Low phosphorus treatments produced purple pigmented and more lignified scion, a sign of specialisation, that may be due to accelerated maturation as a consequence of nutritional stress, thus revealing a possible nexus between ontogenetic stage and propagability. Despite significant differences in cutting rooting and growth due to treatments in the stock plant density and media trial, none could be linked to maturation or specialisation. Tea tree stock plants were amenable to high intensity management systems and had high survivorship, and persistent productivity over one year of cropping, provided they had adequate nutrition. In this first attempt, system productivity (13,440 cuttings/m2/year) was about half the productivity that has been achieved for some intensive eucalypt systems. Finally, application of strigolactone analogue, a hormone known to impact plant architecture and developmental processes, significantly reduced lateral branch initiation in tea tree cuttings where apical buds were removed. Lateral branching is one manifestation of seedling maturity. Encouraging apical, rather than lateral growth, possibly encouraged retention of juvenility, resulting in more vigorous growth, and may produce a plantlet with a single upright stem more amenable to machine planting. Targeting technical and biological factors likely to impact tissue specialisation and maturation identified ways to increase the amenability of tea tree to vegetative propagation and generate propagules more closely aligned with the requirements imposed on them by the propagator, mechanical planting, and the grower. Integrating these techniques may provide synergies to increase propagation system efficiency and plant quality further.

Optimisation of biological and nursery system factors affecting cloning of tea tree

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Optimisation of biological and nursery system factors affecting cloning of tea tree

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