Abstract

Variable mortality of Pyrenophora semeniperda--infected Bromus tectorum seeds has been referred to as a "race for survival", stating that seeds that germinate quickly are more likely to escape pathogen-caused mortality. Dormancy status is not the only variable determining outcomes within the Bromus-Pyrenophora pathosystem. Varying temperature and exposure to water may strongly influence germination outcomes of B. tectorum when in the presence of P. semeniperda. Low water potentials characteristic of semi-arid soils are often over-looked in the context of seed pathogens, and are ecologically relevant- especially for plant species that inhabit intermittently dry environments. To adequately characterize the Bromus tectorum-Pyrenophora semeniperda pathosystem, four studies were conducted to address the following questions: (1) do temperature, water potential, and dormancy status influence germination outcomes in the Bromus-Pyrenophora pathosystem, (2) do repeated wetting-drying scenarios influence germination outcomes of infected B. tectorum seeds following dehydration at low water potentials similar to those found in the field (i.e., -4 through -150 MPa), (3) can we accurately characterize the asexual life cycle of P. semeniperda on a dormant B. tectorum seed, determining when infection takes place, and what occurs during disease development in continuously hydrated conditions, and (4) how does disease development of P. semeniperda influence the B. tectorum seed embryo and endosperm. All studies were conducted using dormant and/or non-dormant B. tectorum seeds and an intermediate strain of P. semeniperda. Study one used varying temperatures (5-20°C), and five water potentials (0, -0.5, -1, -1.5, -2 MPa) (achieved using PEG 8000). Inoculated seeds were exposed to low water potentials at various temperatures for 7, 14, 21, or 28 days then re-hydrated for 28 days. In the second study, seeds were incubated at 20°C at four nominal water potentials (-4, -10, -40, or -150 MPa) following 8 or 24 hours of initial hydration. Seeds were dehydrated for 1, 7, 14, or 21 days, then re-hydrated. In study three, inoculated seeds were chemically fixed between days 0 and 21 and viewed with a scanning electron microscope. In the fourth study, infected seeds were frozen with liquid nitrogen following 3, 8, and 14 days of disease development, then cross sectioned longitudinally and laterally prior to chemical fixation. Results indicate that non-dormant seeds escape death by germinating rapidly under favorable conditions, that incubation at low water potentials greatly increases seed mortality, that -10 MPa is near the threshold for full pathogen activity, and at water potentials lower than -40 MPa, P. semeniperda may successfully survive severe dehydration if previous hydration resulting in infection has occurred. SEM images indicate that mycelia penetration occurs within 8-24 hours, and that mycelium may penetrate all opening in the seed (i.e., stomata, cracks). Development of P. semeniperda is shown to cause significant damage to the endosperm and embryo within 8 days. As starch is consumed, the endosperm collapses leaving a hollow middle. The embryo is more resilient, but gradually deforms and deteriorates.

Degree

College and Department

Life Sciences; Plant and Wildlife Sciences

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