| While aging in many species is associated with declining reproduction and increasing mortality rates, it is not at all certain whether these patterns also occur in plants. Traditional life history theory provides explanations of why aging may be expected, but the redundancy, resiliency and constant turnover of tissues may exempt plants from the negative effects of aging. The work presented in this dissertation examines the expression of aging in plants at multiple levels of organization. At the population level, effects of aging were investigated in two populations of the herbaceous perennial Cryptantha flava which were followed for over twenty years. Plant size and annual reproduction declined in older plants in one population, and reproduction alone declined at the other. In contrast, annual survival rate was generally constant, regardless of age, indicating that aging may not affect all components of fitness in the same way. At the level of the individual, two studies were conducted. The first investigated whether trade-offs existed between growth, reproduction and longevity in two annual species, Coreopsis tinctoria and Linum usitatissimum, when growth was increased through nitrogen addition. Trade-offs did occur in Linum, but the stronger finding from this work was that nitrogen addition changed the amount of time allocated to growth and reproduction in both species. A second study on Coreopsis tinctoria documented that declines in flower size over the life span of the plant were due to positional effects within the plant in unpollinated plants, but these effects disappeared when flowers were pollinated and became resource sinks as they developed seeds. Oxidative damage to lipids and proteins was measured in leaves from two congeneric annual-perennial pairs of plants over two years. Temporal variability influenced levels of damage to lipids, but plant age also played a role in determining damage to proteins as older plants had more damage than younger ones. There were no consistent differences between annuals and perennials. The results from these various studies show that some measures of plant performance decline with age, but others do not. This suggests that plants may possess properties which buffer them in part against effects of aging. |
