| This thesis examines, firstly, what ecological pressures may be important in shaping characteristic levels of phenotypic plasticity, and secondly, what underlying physiological processes a plant may have to control its plasticity. Within the Stellaria longpipes complex, a model system was developed. The status of an alpine tundra ecotype within the complex was investigated and subsequently described as S. longpipes var. monantha. It was concluded that the alpine ecotype was well suited for intraspecific comparisons with other forms of S. longpipes subsp. longpipes. A series of populations was located along an elevational gradient and fine-scale differentiation between alpine and prairie ecotypes was described genetically and morphologically. Analysis of isozymes revealed two distinct clusters separating high-elevation from low-elevation populations. Differences in means and plasticities of traits used to describe the tundra ecotype paralleled genetic separation, and reciprocal transplants and analyses of habitat stresses indicated that the high plasticity of the prairie form is adaptive for high interspecific competition, while the low plasticity of the alpine ecotype is at least a partial response to extreme wind and temperature stress. The ecotypes showed contrasting abilities to synthesize and respond to the hormone ethylene, which may partially account for their differences in plasticity. In the alpine ecotype, ethylene production was elevated in response to wind causing an inhibition of growth, whereas in the prairie ecotype, ethylene stimulated growth and was produced in a constant daily rhythm regardless of wind stress. Gibberellin (GA) metabolism and sensitivity was investigated, but no direct role was defined for differences in elongation between unstressed ecotypes. In comparison to the prairie ecotype, the alpine ecotype produced higher quantities of GAs and was equally sensitive to all applied GAs except GA... |
