|dc.description.abstract||This research contributes to our understanding of the development and genetic variation of Sophora prostrata Buchanan (Fabaceae). This has provided a basis for further study into the evolution and development of divaricate branching, and relationships within Sophora. A comprehensive review of divaricate literature and assessment of published indices has identified the next steps in understanding and defining divaricates. Assembling and reviewing the available literature on S. prostrata has highlighted gaps in our knowledge, as well as issues that need to be addressed.
Sophora prostrata is one of approximately 60 species displaying the divaricate habit in New Zealand. Many passionately debated hypotheses have been put forward to explain the evolution of divaricates, including adaptation to climatic factors, browsing by moa, or light intensities. The theories proposed to explain divaricate evolution are reviewed, and the indices of Atkinson (1992) and Kelly (1994) that are used to quantify divarication are assessed, using divaricate and non-divaricate species pairs. Despite Atkinson’s focus on branching parameters, and Kelly’s focus on aspects of the leaves, the species distinguished as divaricates were correlated in both indices. However, a more robust definition, and therefore a more comprehensive index, would allow the exact number of divaricates within the various potential divaricate subtypes to be established. Clarifying what exactly defines and unifies divaricate subtypes is essential to be able to properly address the evolution of this habit in New Zealand.
The differences in branching and decapitation response were measured and compared between S. prostrata and S. tetraptera (using three decapitated individuals and three intact of each species), and patterns in axillary bud development along three branches of S. prostrata were investigated. Proportions of established branches were found to be the same in both species, contradicting the idea that divaricates have more branches than non-divaricating species. However, in S. prostrata short shoots were found to be a significant contributor to the architecture, which allows S. prostrata to be able to exploit space that has already been colonised. Sophora prostrata also displayed increased growth and node activation in response to decapitation, which could be due to weak apical control resulting in no new leader shoot gaining absolute dominance over the overall plant. Further investigation into the architecture and development of decapitated and intact S. prostrata and S. tetraptera, along with other divaricate and non-divaricate species pairs, could help in understanding the complexities of apical control and apical dominance in woody species, and contribute to our knowledge of branching in general.
Inter Simple Sequence Repeat (ISSR) markers were used in effort to further test the previously shown distinctness of S. prostrata, using a larger sample size across a wider geographic range than had previously been used, and assess the genetic variation across the geographic range of S. prostrata (throughout the Canterbury and Marlborough regions). The nine ISSR primers and 29 samples (18 of S. prostrata, 5 of S. tetraptera, and 6 of S. microphylla) were unable to support the distinctness of S. prostrata, or identify distinct relationships between the localities. Further research into the population genetics of S. prostrata is necessary to clarify any distinct populations, which could then educate ecosourcing. Variation that is shared across taxa could be a result of hybridisation, which has been widely documented in New Zealand Sophora species. The high number of fragments shared among all species (55%) is evidence of the close relationship between New Zealand Sophora species, which has been previously demonstrated.
Information relevant to S. prostrata from the present thesis as well as other published and unpublished sources is presented as a contribution to the New Zealand Biological Flora series. In some areas such as the lower Waimakariri catchment, S. prostrata populations are declining due to browse damage, and there is a lack of seedling recruitment due to grazing and competition from grasses. Sophora prostrata populations will continue to decline without artificial replacement and other conservation measures.
Increased interest and conservation effort involving S. prostrata and other divaricates can ensure that these valuable and iconic plants remain a unique and significant part of New Zealand’s native flora. Divaricates are individually and collectively a valuable resource that should be further utilised to answer many important ecological, evolutionary, and developmental questions.||