2.27.97
Weedy Life Cycles
dorm.html
Seed Dormancy
Introductory concepts
Dormancy
1: A state in which viable seeds (or buds; spores) fail to germinate under
conditions of moisture, temperature and oxygen favorable for vegetative
growth
2: A state of relative metabolic quiescence
Dormancy can be seen as "dispersal in time"
Dispersal phase usually dormant; dehydrated seeds weight less (esp. wind
dispersal) and are metabolically slower
Dormancy:
-rhythmic adaptation of weeds to the temporal rhythms in the environment
-strategic alternative to dispersal
Somatic Polymorphism and Dormancy
Production of seeds of different morphologies or behavior (phenotypes)
on different parts of the same plant
Seed somatic polymorphism represents an allocation of different fractions
of the seed output of a plant to different ends (different germination requirements)
-adaptive advantage to producing seed on one plant with different qualities
-seed dormancy somatic polymorphism is common in weedy species of Gramineae,
Compositae, Chenopodiaceae and Cruciferae families
-somatic polymorphism allows sensitive adjustment to environment in the
number of morphs it produces
Seed and dormancy polymorphisms are so common amongst weed species that
it is dangerous to ascribe one set of dormancy mechanisms or germination
breaking requirements to any one species
Seed somatic polymorphism examples:
Example: Rumex crispus (curled dock)
-progeny of individual plants vary enormously in ability to germinate in
darkness or at common temperature
-variation is greater between plants than between habitats, no one germination
response
Example: Xanthium spp. (common cocklebur);
-seed borne in pairs in capsule: large and small seed dispersed together
-upper seed in capsule usually dormant, lower germinates first
-dormancy breaking requirements different for 2: 12 month difference insurance
second will become established if first year unfavorable
Example: Avena fatua (wild oat), and Avena ludoviciana
-grains borne on different parts of the spikelet have different germination
requirements
-first grain of spikelet lacks dormancy, remainder have deep dormancy
Example: Compositae germination behavior differentiated by seed size, seed
formed in ray versus disc flowers
Example: Chenopodium album (common lambsquarters) may produce 4 different
kinds of seed on same plant
-two color categories: brown and black; two seed coat categories: reticulate
and smooth
-brown: thin-walled, larger, germinate quicker than black, even at low temperatures;
killed by winter, but if they survive have the capacity to produce very
large plants with high reproductive output; only 3% of seed on a plant;
among the first to be produced by a plant
-black: require cold treatment, nitrate to break dormancy
-ratio of brown:black govered by environmental conditions
Example: common purslane seed varies from nondormant to dormant on same
plant)
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