Perennial cereal crops constitute a plant life form rare either in nature or in cultivated agroecosystems. While woody plants and annual herbaceous plants have been successfully selected to achieve high levels of reproductive allocation, herbaceous perennial plants with very high levels of sexual reproduction do not presently exist. The lack of high yielding herbaceous perennials is of intense interest to agronomists, horticulturalists, evolutionary biologists, population ecologists, ecophysiologists, and breeders. Within the last few decade, renewed attention has been devoted to the development of high yielding, herbaceous perennial plants that can meet the demand for food, forage and fiber in environmentally sustainable ways. However, perennial cereal crops face serious challenges if they are to be economically feasible, including genetic incompatibilities, resource tradeoffs between sexual reproduction and multi-year survival, and potential for disease buildup. In a series of studies between 2008 and 2012, I explore these issues within the context of three perennial cereal species: perennial wheat (Triticum aestivum x Thinopyrum spp.), perennial rye (Secale cereale x montanum) and intermediate wheatgrass (Thinopyrum intermedium). Chapter 1 of this dissertation is a literature review dealing with the history of perennial cereal breeding, potential environmental benefits, and challenges that perennial cereals face. Chapter 2 reports on a two-year agronomic study of grain yield, biomass production and phenology of four accessions of perennial wheat and one accession of perennial rye. Chapter 3 describes a series of experiments exploring differences in photosynthetic rates, and in hydraulic, morphological and biochemical traits affecting resource accumulation, between perennial and annual cereals. Chapter 4 explores the extent to which photosynthesis and metabolism in perennial cereals is responsive to changes in carbohydrate supply / demand ratio (source / sink ratio), and whether this responsiveness differs between perennial and annual cereals. Finally, Chapter 5 explores the resistance of three perennial cereal species to the fungal disease Fusarium head blight (FHB) in a greenhouse inoculation experiment. In brief, we find the following. * Perennial wheat achieves grain yields of approximately 50% of annual wheat, while perennial rye achieves 75% of annual rye. Both species show an ability to maintain these yields into their second year, and show later flowering than annuals. * Perennial wheat, perennial rye and intermediate wheatgrass show 10-60% higher photosynthetic rates than their annual analogues. Intermediate wheatgrass, differently than the other two species, shows declining photosynthetic rates with increasing plant age. These differences are driven primarily by biochemical rather than hydraulic changes. * Perennial wheat and rye are more sink-limited than their annual analogues, and show an ability to mostly compensate for moderate source/sink changes. The perennial species appear to show a more conservative reproductive strategy than their annual relatives, which is explainable on ecological and evolutionary grounds.* Intermediate wheatgrass shows high resistance to FHB, while perennial rye is moderately susceptible and perennial wheat accessions vary in susceptibility. Thus our studies provide novel contributions to the growing literature on perennial cereals, illustrating some physiological traits of perennial cereals as well as some of the problems they face. We hope that our results can contribute to advancing efforts to achieve high yielding herbaceous perennials, as well as to improving our understanding of how life history, source-sink balance and whole-plant age interact to affect resource acquisition rates.
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