Weed life history: identifying vulnerabilities - Ecological Management of Agricultural Weeds - page 46

Figure 2.1 Dormancy/germination states ofweed seeds.(Redrawn from Egley
(1995) based on the concepts ofBaskin & Baskin (1985,1998 a ).)
ecology and management of parasitic weeds (Musselman, 1987; Pieterse,
Verkleij & ter Borg, 1994; Hosmani, 1995).
Dormancy and germination
Seasonal and aseasonal germination
Weed seeds often undergo several changes in dormancy state between
seed shed and germination (Figure 2.1) (Baskin & Baskin, 1985). These
changes represent an adaptive response to the problem of immobility: a seed
has little control over where it lands, but through dormancy response to envi-
ronment, it can choose when to germinate. When first shed from the parent
plant, seeds may lack dormancy and be ready to germinate if environmental
conditions are favorable. This is commonly the case for those winter annuals
like Galium aparine that commonly shed seeds in mid to late summer
(Håkansson, 1983). Seeds of these species need to be ready for immediate ger-
mination since winter annuals usually do best when they establish early in the
autumn.
Alternatively, seeds may have innate dormancy when shed (primary innate
dormancy). Innate dormancy may be due to impermeable (hard) seed coats,
chemical germination inhibitors in the seed coat or embryo, a cold or heat
requirement, or other physiological mechanisms (Povilaitis, 1956; Baskin &
Baskin, 1985; Taylorson, 1987). Such mechanisms are found in most weedy