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Steinbauer & Grigsby (19576)

Notes:

a +Factor: seeds germinated in light, in alternating temperature regime, and in nitrate solution. -Factor: seeds germinated in dark, at constant temperature equal to the mean of the alternating regime, and in water. b The numbers given for percentage germination are mostly means taken over several chilling treatments, populations, seed types, etc., but not over treatments that involve variation in other factors listed in the table. In a few cases, the numbers are from selected treatments that demonstrate the effect.

mination (Wesson & Wareing, 1969b; Holm, 1972), the effect could not be attributed to improved oxygen availability alone. Holm (1972) further observed that imbibed Abutilon theophrasti and Ipomoea purpurea produced ethanol, acetone, and acetaldehyde when oxygen concentrations dropped below 6% and demonstrated that these compounds inhibited germination of seeds, even in normal air. He therefore proposed that moderate reduction in oxygen by respiration in the soil results in anaerobic seed metabolism, which produces volatile germination inhibitors. In the absence of air exchange, these enforce seed dormancy. Thus, tillage probably prompts germination of weed seeds both by venting volatile inhibitors from the surface soil and by moving deeply buried seeds to near-surface conditions where air exchange is improved.

Although ethylene and carbon dioxide concentrations are also commonly elevated in undisturbed soil, these compounds appear to play a small role in inhibiting seed germination. Ethylene affects germination of only a small proportion of weed species, and usually promotes, rather than inhibits, germination (Taylorson, 1979). Similarly, concentrations of carbon dioxide up to 5% tend to enhance, rather than inhibit, germination (Baskin & Baskin, 1987; Egley, 1995).

One of the most important cues promoting germination of seeds in the seed bank is light. In a classic study, Wesson and Wareing (1969a) collected soil at night, screened it in the dark, and then placed it in trays in a greenhouse in either the light or dark. Averaged over three experiments at different times of year, they found 12 times more dicot seedlings and 26 times more grass seedlings in the light treatment. Many subsequent studies have shown that germination of a great range of weed species is promoted by light (Taylorson, 1972; Stoller & Wax, 1974; Froud-Williams, 1985; Baskin & Baskin, 1986). Some species of weed seeds germinate in response to very small amounts of light. For example, conditional dormancy in Datura ferox and Amaranthus retroflexus can be broken by the equivalent of a few milliseconds of sunlight (Scopel, Ballaré & Sánchez, 1991; Gallagher & Cardina, 1998). Moreover, many species, like Spergula arvensis and Stellaria media, that lack light sensitivity when shed from the parent plant quickly develop it after incorporation into the soil (Wesson & Wareing, 1969b; Holm, 1972).

Because light-sensitive germination is controlled by the phytochrome system, light depleted in red wavelengths by passage through a plant canopy is inhibitory to germination of light-sensitive species (Górski, 1975). In fact, even some species with moderately high germination in the dark are severely inhibited by light that has passed through plant leaves (King, 1975; Silvertown, 1980). Thus, germination under established vegetation is held in check not only by the amount of light but also by its spectral composition.

Although the several factors discussed above promote germination individually, the effects are most pronounced when several factors combine. Vincent & Roberts (1977), Bostock (1978), Roberts & Benjamin (1979), and Kannangara & Field (1985) demonstrated that two- and three-way interactions among light, nitrate, and fluctuating temperature enhanced germination of 13 out of the 15 weed species they studied. Presumably, the several factors acting in concert provide a more certain signal that competition has been eliminated than any of the factors acting singly.

Germination in response to tillage is both a fact that must be dealt with in the design of agricultural systems and a tool for manipulation of weed populations. For example, shallow cultivation between crop rows is often preferable to deep cultivation. A shallow cultivation tends to eliminate the weeds that were prompted to germinate in response to seedbed preparation without cueing germination of many additional seeds. In contrast, a deep cultivation tends to bring up seeds that are then prompted to germinate by disturbance-related cues. Dynamics of the seed bank in response to tillage is discussed further in Chapter 4.

The germination response of weeds to soil disturbance can also be used to induce inappropriate germination. For example, species with broad seasonality of germination can be stimulated to establish at times that are unsuitable for survival to reproduction, thereby depleting the seed bank. A more common application is to use shallow cultivations with intervening rests before planting to flush out and kill many of the weeds that would otherwise establish with the crop. Use of this "false seedbed" method is analyzed in Chapter 4.

Not all species of weeds are sensitive to germination cues associated with soil disturbance. Most of these are relatively large seeded species (Table 2.4, below) that presumably have sufficient resources in the seedling stage to establish in the face of some competition from established vegetation. Many have hard, impermeable seed coats that prevent water uptake and germination, or other dormancy mechanisms that prevent germination until the seed coat is physically altered (Table 2.4). In the field, temperature extremes or desiccation typically break physical dormancy (Baskin & Baskin, 1998a, pp. 114-20). Response to these factors spreads germination over several years and, to some extent, also cues germination to appropriate times of the year. Some large-seeded weeds also have innate physiological dormancy mechanisms (Wareing & Foda, 1957). Thus, large-seeded weeds have mechanisms that match germination to appropriate environmental conditions, but only a few (e.g., Solanum viarum - Akanda, Mullahey & Shilling, 1996) sense the removal of competitors through a strong response to light, alternating temperature, or nitrate.

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