Aspects of the physiological and chemical control of adventitious root formation in Eucalyptus nitens (Deane & Maiden) Maiden

Eucalyptus nitens is an important species in plantation forests in Tasmania and is becoming important in other cool temperate regions of the world. Reliable initiation of roots in cuttings is needed, to enhance forest productivity through the establishment of clonal plantations. Adventitious root initiation is dependent on the re-polarisation of cells to form a new root meristem. Failure to initiate roots may be due to the inability of the plant to undergo re-polarisation of cells to form a new meristem. The research undertaken has been directed at the manipulation of some of the factors that are implicated in the establishment of cellular polarity in plants. Several new techniques for enhancing root initiation have been investigated and shown to promote root formation in cuttings of E. nitens. The principle conclusions of the research are outlined below. It was demonstrated that a high proportion of E. nitens seedlings have the ability to initiate adventitious roots in cuttings collected from very young seedlings. This ability is lost quickly as the plant ages. The loss of rooting ability does not appear to be related to any specific changes in the stem morphology. Histological examination demonstrated that the loss of rooting ability is not correlated with the formation of structural barriers within the stem or callus. Root primordium formation appears to occur in regions of undifferentiated callus in most cuttings but can also occur directly from the cambium, with little or no intervening callus phase. Root initiation was found to be sensitive to the timing and method of auxin applications. It was possible to increase the proportion of cuttings that initiate roots by delaying the application of auxin until several weeks after the cuttings were first placed on the mist-bed. Calcium ions are part of the mechanism by which auxin signals are translated into cellular actions. Attempts to increase the rate of root initiation in cuttings by altering calcium levels in the surrounding medium were inconclusive. Electric potential differences are known to play a role in the establishment of cellular polarity and in organogenesis from callus. It was demonstrated that low voltage electric currents could be used to stimulate root initiation in some cuttings. It was hypothesised that cuttings with substantial basal callus would be most receptive to this treatment, but this was found not to be the case. The experiments using electric currents were difficult to replicate and were subject to unexplained variation in results. Further development of the technique is required to quantify the effects of such currents and to identify the mode of action. Stigmasterol and vitamin D were demonstrated to act as auxin synergists in the promotion of root initiation in cuttings and also to stimulate the growth of tissue cultures. ATPase extracts were prepared to investigate whether the action of sterol potentiation of auxin is associated with changes in membrane bound I-1+-ATPase activity. The treatment of cuttings with G compounds, as a method of promoting root initiation, was investigated. G compounds are a group of naturally occurring chemicals found in some eucalypt species and are known to inhibit and to boost root initiation in some species. In cuttings of E. nitens, they were found to have no significant effects on root initiation. The results obtained are discussed in terms of the role that the treatments might play in root morphogenesis through the re-polarisation of cells to create a new axis. The results support the general hypothesis that treatments aimed at enhancing the development of cellular polarity are useful in stimulating root initiation in cuttings. Some suggestions are made for future research, to develop these initial findings into practical treatments for the propagation of E. nitens