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Abstract
Plant functional traits are hypothesized to co-vary and have been often interpreted as reflecting resource strategies for acquisition, transport, and use of carbon, water, and nutrients. These trait combinations are expected to range along a continuum from fast, resource-acquisitive trait values to slow, resource-conservative values. This dissertation focuses on leaf and stem functional traits related to tissue structure and water transport for the sunflower genus, Helianthus, which encompasses wild species from diverse habitats across the North American continent and cultivated H. annuus. Using a comparative approach and common garden greenhouse studies, expected stem and leaf trait co-variation was examined from three complementary perspectives: evolutionary diversification of wild species, evolutionary responses to artificial selection, and ecological responses. At the evolutionary scale, a comparison of stem and leaf traits for 14 wild species provided evidence of correlated trait evolution and adaptive differentiation associated with habitat climate. The effects of crop domestication were assessed by comparison of two varieties of domesticated H. annuus (ancient landraces and modern improved cultivars) with its wild progenitor. This comparison revealed that this suite of leaf and stem traits did not shift in a coordinated fashion in response to the artificial selective pressures of crop domestication. Additionally, trait shifts were found to be inconsistent in comparison of these two forms of domestications (i.e. wild to ancient landraces v. wild to improved cultivar). In response to the abiotic stress of water limitation, leaf and stem traits of six wild sunflower species shifted in a coordinated fashion towards more resource-conservative trait values. In conclusion, this dissertation provides evidence for correlated evolution of a suite of stem and leaf functional traits and the plastic responses of this suite of traits are observed to co-vary when species are subjected to water stress; however, these traits are not found to co-vary during the artificial selective process of crop domestication. This suggests that co-variation of these traits across wild taxa may be primarily due to selective pressures rather than hypothesized biophysical or genetic constraints.