Weed life history: identifying vulnerabilities - Ecological Management of Agricultural Weeds - page 52

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, ger-
mination (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 (1969 a ) 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 seed-
lings in the light treatment.Many subsequent studies have shown that germi-
nation 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, 1969 b ; 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 indi-
vidually, 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 interac-
tions among light, nitrate, and fluctuating temperature enhanced germina-
tion 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 popu-
lations. For example, shallow cultivation between crop rows is often prefer-
able to deep cultivation. A shallow cultivation tends to eliminate the weeds