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|Title:||Effects of Some Events Experienced by Seedlings in Competition|
|Author(s):||Hartgerink, Antoinette Philippine|
|Department / Program:||Plant Biology|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Abstract:||The objectives of the study were (1)to demonstrate that randomly occurring factors that affect seedlings can be dominant forces in determining the fitness of individuals, (2)to examine the effects of one such factor, defoliation, on final plant size and (3)to examine the dynamics of the growth response of defoliated and undefoliated plants.
Populations of Abutilon theophrasti were grown under uniform glasshouse conditions on heterogeneous or homogeneous substrates. The substrate was made heterogeneous by the addition of factors, termed "patch types", such as stones, localized nutrients or sand, and simulated footprints or denser soil, all on the size scale of individual seedlings. Patch type exerted a significant effect on the final biomass and number of seeds produced by plants grown on heterogeneous soil. Plants encountering footprints or sand were the most suppressed by the end of the season, and plants encountering extra resource in the form of nutrients or denser soil became the dominants. At low density, populations grown on heterogeneous substrate had a greater variance in final biomass per plant than populations on homogeneous substrate due to the effect of the patch types on competitive release or suppression.
Abutilon theophrasti seedlings experience some cotyledon herbivory in the field. The response of Abutilon theophrasti to early defoliation by 50% was measured in the field as well as in greenhouse experiments. The effect of defoliation as measured by final stem diameter in the field or above ground biomass in the greenhouse was evaluated at 2 densities, and with or without the presence of a heterospecific competitor, Setaria faberi.
In the field, defoliated plants were about 30% smaller than undefoliated plants at the end of the season. There was no difference in response of seedlings defoliated at the cotyledon stage versus when they were one week older. There was no effect of presence of Setaria or of differences in density on the response to defoliation. In the greenhouse, plants defoliated at the cotyledon stage were 75% smaller than their undefoliated competitors, which experienced competitive release of 25%. Plants that were defoliated by 50% at 3 weeks were 50% smaller than their undefoliated competitors at the end of the season, and there was no competitive release of the latter. There was a slight positive effect of the presence of Setaria and a strong positive effect of lower density on the response to defoliation.
The dynamics of the growth response of defoliated and undefoliated plants was examined with successive harvests of shoots and roots throughout the season. Initially, there was a dramatic decrease in relative growth rate (RGR) of roots and stems, followed by a higher than normal RGR a few days later. After this point, in competitive situations, defoliated plants' RGR decreased steadily, but in non-competitive situations their RGR remained high. Leaf RGR was maintained at all times in both situations.
The data suggest that external factors, that may affect seedlings by chance, can allow individuals to become dominant, and contribute most to the seed pool, irrespective of their genetic makeup. This may be one mechanism by which genetic diversity is maintained in populations.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1981.
|Date Available in IDEALS:||2014-12-14|