A study of the efficacy of organ cultures to examine wood formation in Pinus radiata D. Don
Thesis DisciplinePlant Physiology
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Pinus radiata D. Don is an economically important plantation species to New Zealand that is susceptible to the wood quality flaw 'intra-ring checking'. Intra-ring checking is a term used to describe radial fractures that can occur in the earlywood portion of a growth ring, altering the appearance and resilience of the wood, thereby decreasing its economic value. This thesis presents a study that was part of a broad, ongoing collaborative investigation directed at understanding wood quality issues, with the long term goal of enhancement of future radiata pine crops. These investigations are funded by the Wood Quality Initiative Ltd., and involve basic science, field trials and engineering studies related to intra-ring checking. Specifically, the present study was designed to establish the effects of the mineral nutrients boron, calcium and magnesium on wood formation, to determine whether they are associated with intra-ring checking. This research was carried out in three stages. Firstly, the ultra-structural and biochemical properties of wood with intra-ring checking were examined to determine if specific features of the cell wall were associated with the incidence of intra-ring checks. Electron microscopy techniques revealed that the CML/S1 region of the cell wall often showed a decrease in CML lignin staining and S1 striations in wood with intra-ring checks. However, Klason and acetyl bromide assays did not show a change in lignin content. In order to understand how changes in the CML/S1 region of the cell wall may occur, methods were required that would allow for the observation of wood formation in a controlled environment. In the second stage of this study, an organ culture technique was successfully developed to allow for the growth of radiata pine cambial tissue, sandwiched between phloem and xylem, on a defined nutrient medium. This nutrient medium was manipulated, using ion-binding resins, to control the amount of boron, calcium and magnesium available to the growing tissues, to determine if variations in wood formation could be induced. In the final stage of this research, an extensive comparative examination of different techniques that could be used for the observation and measurement of selected wood properties was undertaken, in order to determine the efficacy of the organ cultures to study wood formation in an altered nutrient environment. Wood properties were examined for various stages of xylogenesis, beginning with cell division and expansion, followed by cell wall deposition, and lastly with the onset of lignification in order to define the success of the culture technique. Electron microscopy investigations suggested that in the presence of very little boron the CML/S1 wall showed darker striation deposits, while an increase in calcium availability, resulted in a more defined CML/S1/S2 wall region compared to the controls. Further examination of the cell walls suggested that pectin esterification and possibly lignification could also be increased by limited boron availability. However, in many of the observed and measured parameters of wood properties, a great deal of complex 'between-tree' and 'within-culture' variation was observed. The results show that elucidation of the association between nutrient availability and the incidence of intra-ring checking can not be established from this organ culture study. In a concurrent study, the preliminary investigation of arabinogalactan-proteins (AGPs) in radiata pine was undertaken. Radiata pine AGPs were positioned in the compound middle lamella of xylem cells, suggesting potential roles in cell-cell adhesion or cell-cell signalling. For the first time, radiata pine AGPs were isolated and characterized in terms of their protein and carbohydrate composition, both of which yielded features typical of AGPs in other plant species. Unique to radiata pine AGPs was the presence of a large proportion of 5-linked arabinose. While the precise function(s) of AGPs are unknown, the results obtained in this research have established a basis for further investigation into the potential for their involvement in wood formation. Overall, new tools have been established to facilitate future research on radiata pine, a commercially important species, and novel results have been obtained concerning the mechanisms of wood formation therein.