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The influence of soil moisture stress, temperature, and bulk density on the germination and seedling emergence of downy brome (Bromus tectorumL.) was investigated in the laboratory. Reductions in soil matric potential from -2 to -16 bars markedly reduced the percentage and rate of emergence. Seedling emergence was better at constant than at alternating temperatures. At high matric potentials, the rate of emergence was accelerated by warmer soil temperature (20 C), while at very low matric potentials, the percentage and rate of seedling emergence were least restricted at cooler temperatures (10 and 15 C). Soil matric potential did not influence the percentage or rate of emergence of seedlings grown from seed lots harvested during climatologically diverse years. Seedling emergence but not germination was inhibited by increased levels of soil compaction. Soil compaction times moisture interaction were not observed, as measured by final seedling emergence.

Solutions of mannitol and high molecular weight polyethylene glycols (PEG) of different osmotic potentials are often used to simulate water stress in seed germination studies. However, little has been reported about the osmotic stability of these solutions. The objectives of this study were to determine the osmotic stability of mannitol and PEG 20,000 solutions, to examine the biological significance of any observed instabilities, and to compare osmotic vs. matric water potential effects on germination of winter wheat (Triticum aestivum L., ‘Nugaines’). Four solutions of mannitol, PEG, and KC1 (standard) with water potentials ranging from −3.5 to −18.0 bars were incubated at 10, 20, and 30 C and water potentials were measured periodically with thermocouple psychrometry. In addition, comparisons were made of the percent and rate of germination of wheat seeds placed in 28‐day‐old and freshly prepared solutions of mannitol and PEG and the percent and rate of seedling emergence from a watered soil media. The osmotic potential of different mannitol solutions and of 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 1.0 bar. Percent and rate of germination of wheat was the same in the 28‐day‐old and in 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 the germination rate in PEG, but not in mannitol solutions. Hence the slight instability of PEG solutions appears to be of no biological consequence in seed germination studies, and wheat seed emergence rates at different soil matric potentials can be approximated from germination rates in PEG osmotic solutions.

Afterripening requirements and the possible occurrence of endogenously controlled germination rhythms in downy brome (Bromus tectorumL.) were investigated in the laboratory. Afterripening temperatures from 0 to 50 C, maintained for periods of 4 to 28 days, had little effect on the percentage and rate of downy brome germination when seeds were incubated at 15 and 20 C. However, at 30 C germination temperature, the percentage and rate of germination were significantly increased by short periods (4 to 7 days) of afterripening at 50 C. Similar results occurred at 20 to 40 C afterripening temperatures 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, but prolonged exposure tended to decrease germination. No endogenously controlled germination rhythms were observed in downy brome caryopses.