| Abstract |
- 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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