Graduate Thesis Or Dissertation
 

The influence of certain environmental and edaphic factors on germination and emergence of Bromus tectorum L.

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  • The influence of certain environmental and edaphic factors on the germination and emergence of downy brome (Bromus tectorum L.) was investigated in the laboratory. A preliminary study was conducted to determine the osmotic stability of mannitol and polyethylene glycol 20,000 (PEG) solutions, which are commonly used to simulate water stress in seed germination studies, and to compare osmotic versus matric potential effects on the germination of winter wheat (Triticum aestivum L.). Lacking a weed seed source of known genetic uniformity and high percentage germination, wheat, which has both of these traits, was used as the test plant. Solutions of mannitol, PEG, and KCL (standard), with water potentials ranging from -3.5 to -18.0 bars, were incubated at 10, 20, and 30 C and analyzed periodically for water potential using thermocouple psychrometry. In addition, percentage and rate of germination of wheat seeds placed in moist soil or in 27-day-old or freshly prepared solutions of mannitol and PEG were compared. The osmotic potential of the different mannitol solutions and the -9.1 and -17.4 bar PEG solutions did not change with time. However, the osmotic potential of the -4.0 and -6.4 bar PEG solutions decreased about 1.0 bar. Percentage and rate of germination of winter wheat was the same in the 27-day-old and freshly prepared mannitol and PEG solutions; but at equal potentials, the germination rate was most rapid in the mannitol solutions. Wheat emergence rate from watered soil was linearly related to germination rate in PEG, but not mannitol solutions. Hence, the slight instability of PEG solutions appears to be of no biological consequence in seed germination studies. The interactive influence of soil matric potential and temperature on the percentage of downy brome seedling emergence was determined using soil ranging in matric potentials from -2 to -16 bars, and incubated at alternating and constant mean temperatures from 5.1 to 20 C. The interactive effects of soil bulk density, ranging from 0.9 to 1.3 g cm⁻³, and soil matric potentials, from -2 to -13 bars, on the percentage of seedling emergence was also examined. Reductions in soil matric potential markedly reduced the percentage of emergence. Overall, emergence was better at constant than at alternating temperatures. At higher matric potentials, downy brome emerged faster at warmer temperatures, while at very low matric potentials the percentage of seedling emergence was least restricted at cooler temperatures. Cold soil temperatures markedly reduced emergence at all levels of soil moisture. Soil matric potentials did not affect the percentage of emergence of seedlings grown from seed lots harvested during climatologically diverse years. Emergence, but not germination, was inhibited by increased levels of soil compaction. No significant soil compaction x moisture interaction was observed as measured by final seedling emergence. Under rangeland and wasteland conditions, the successful seedling establishment of downy brome is probably most limited by warm, dry soils or very cold soils (subzero temperatures for part of the day). All other moisture-temperature conditions appear intermediate to these two extremes in effect on establishment. Under cultivated field conditions soil compaction appears to be the major factor controlling successful seedling establishment. The effect of high-temperature on overcoming initial postharvest dormancy, and the possible occurrence of natural endogenous germination rhythms in downy brome seeds were investigated. Afterripening temperatures from 0 to 50 C, for periods of 0 to 28 days, had little effect on downy brome germination in petri dishes at 15 and 20 C incubation temperature. However, at 30 C germination temperature, the percentage of germination was significantly increased by short periods of afterripening at 50 C. Similar results occurred at 20 to 40 C afterripening after 14 to 28 days exposure. In general, high-temperature afterripening conditions (40 to 50 C) initially increased downy brome germination at 30 C incubation temperature, with prolonged exposure tending to decrease germination. No endogenously controlled germination rhythms were observed in downy brome seeds.
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