Growth Stresses (Thesis intro)
It has been shown previously that growth stresses are induced during secondary cell wall formation, however the mechanism responsible is unknown. Two hypotheses exist to explain the stress generation; 1) deposition of lignin between the fibril aggregates, or 2) a mechanism resulting in a length change of the cellulose itself causing the cell to expand or contract axially depending on the orientation of the fibrils (MFA). A combination of the two has also been proposed.
The proposed research will increase our understanding of the mechanisms behind growth stress formation through the development of a mathematical model taking into account a realistic cell wall supramolecular architecture to simulate cell wall maturation. The geometric parameters of the molecular cell wall architecture and chemical composition will be studied using x-ray diffraction, atomic force microscopy and wet chemistry. Experimentation will also be used to investigate the role of the gelatinous (G) cell wall layer in the formation of tension wood.
The New Zealand Dryland Forest Initiative (NZDFI) is currently breeding Eucalyptus bosistoana as a high value timber crop alternative to Pinus radiata. One of the goals of the breeding trials is to select for minimal growth stresses so that the timber does not loose value when harvesting and milling due to the release of internal stresses. The proposed research will select families at young ages which are producing the lowest possible growth stresses improving the E. bosistoana breeding stock.
As trees grow they produce wood in order to become taller, wider or orientate stems, branches and roots. Becoming taller or reorienting stems and branches can be an effective way to outcompete other plants for light.
With increasing height, width or off axis stems comes increasing gravitational force, wind drag and internal pressures (for water transport), which requires either enough redundant strength in the existing structure (such as in monocotyledons) or for the tree to strengthen its structure as it increases its size. In woody plants size increase and reorientation occurs in two ways, apical and cambial growth on branches, roots and the stem(s).