Agriculture Reference
In-Depth Information
species of the temperate zone (Baskin & Baskin, 1988) and provide means for
matching the period of germination to weather conditions that are suitable
for establishment and growth of the plant. With time, seed coats break down,
chemical inhibitors are leached away, and cold or heat requirements are satis-
fied by winter or summer temperatures, depending on the species. The seed
then becomes capable of germination.
Nondormant seeds still may not germinate, however, if environmental
conditions are unfavorable.Frequently,seeds remain in a quiescent state until
appropriate temperatures, water, light, and other germination cues indicate
that conditions are favorable for germination and establishment. For some
species, seeds that can not germinate because appropriate conditions are
lacking may enter a secondary state of innate dormancy (e.g.,
Ambrosia artemi-
siifolia
- Baskin & Baskin, 1980;
Arabidopsis thaliana
- Baskin & Baskin, 1983).
In that state,the seed must undergo another period of chilling,heating,leach-
ing, etc., before germination is again possible.The transition into (and out of)
innate dormancy is gradual: the seed passes through a series of conditional
dormancy states in which the range of environmental conditions that trigger
immediate germination becomes increasingly narrow (Baskin & Baskin,
1998
a
, pp. 50-64). Seeds of many species may cycle between innate dormancy
and non- or conditional dormancy for several years before the environment
happens to favor germination in an appropriate season.
Due to these dormancy processes, most weed species germinate at particu-
lar times of year. For example, Chepil (1946) observed the timing of emer-
gence of 59 species in Saskatchewan and grouped the species into five
categories.The categories of peak emergence were (i) early spring (e.g.,
Bromus
tectorum
,
Chenopodium album
,
Plantago major
), (ii) mid spring (e.g.,
Setaria viridis
,
Cirsium arvense
), (iii) summer (e.g.,
Amaranthus retroflexus
,
Capsella bursa-pastoris
,
Portulaca oleracea
), (iv) autumn (e.g.,
Sophia multifida
,
Lepidium perfoliatum
), and
(v) no consistent period of peak emergence (e.g.,
Taraxacum officinale
,
Sinapis
arvensis
,
Medicago lupulina)
. Other authors have found similar variation in the
emergence times of temperate weeds (Figure 2.2) (Lawson, Waister &
Stephens, 1974; Roberts & Neilson, 1980; Roberts & Potter, 1980; Håkansson,
1983; Roberts, 1984), although some have also found species with bimodal
germination in spring and fall (e.g.,
Veronica hederifolia
in Figure 2.2).
Several points can be made regarding these studies. First, most “winter
annual”agricultural weeds are only facultatively tied to autumn germination.
Only a few have secondary dormancy mechanisms that prevent spring germi-
nation (e.g., winter annual races of
Arabidopsis thaliana
, Baskin & Baskin,
1983), and so most agricultural weeds that show a winter annual phenology
are found in spring-sown crops as well (Hald, 1999). Second, little overlap in
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