  The dormancy status of S. faberi seed at abscission was assessed with
reference to tiller and panicle development. Seed from a single genetic line
were grown under field, greenhouse and controlled environment growth chamber
conditions. At abscission, a small fraction (<10%) of S. faberi seed germinated
under favorable conditions. Seed were dissected and germination of caryopses
and embryos also tested. Removal of seed structures exterior to the embryo
increased percentage germination. As the seed rain progressed mean percentage
germination and variation among samples increased. Changes in germination were
correlated with tiller development and relative time of seed maturity within a
panicle. Seed produced on tillers that developed earlier were more likely to be
dormant than seed from later-developing tillers. Seed that matured later on a
panicle were more likely to germinate than seed that matured earlier on the
same panicle. A consistent trend toward later maturing seed having less
dormancy was found for seed grown under different environments which implies an
inherent or parental source for variation in giant foxtail seed dormancy. The
variation in percentage germination at abscission and following stratification
treatments indicates that the S. faberi seed rain consists of individual seeds,
possibly each with a different degree of dormancy.
