In the past, botanists have reasoned that the separation of the male and female flowers on different plants (dioecy) may have prevented self-fertilization and the inherent problems associated with continuous inbreeding. Recently, however, other suggestions concerning the evolution and maintenance of dioecy have been proposed. Several of these hypotheses are based on the differential costs of reproduction in males and females. Females are hypothesized to have higher reproductive costs because they supply the energy, and water, and nutrients for seed and fruit development. Males, in contrast, only experience the costs of producing pollen for fertilization. Because of this difference in reproductive costs, it has been assumed that females would experience greater stress under low resource conditions such as shading, low nutrient availability and low water situations and would therefore be found with less frequency in unfavorable ecological sites. This prediction has been borne out qualitatively in many descriptive studies which have indicated that in some species males and females are spatially segregated along ecological stress gradients. However, differences between male and female plants in degree of stress tolerance has been experimentally studied in only three species and results have been inconclusive. This project will focus on differences in stress tolerance between male and female plants of several dioecious species under conditions of low availability of light, water and nitrogen. In order to gauge the level of stress tolerance the response of plant growth and photosynthesis rate to the stress treatments will be measured. Individuals of three dioecious species, Thalictrum dasycarpum, Silene alba, and Spinacia oleracea, will be used. Individuals will be monitored for differences in photosynthetic rates and growth patterns within each species. Water and nitrogen contents of leaves will also be measured.
