Journal of Experimental Botany, Vol 50, 1403-1419, Copyright © 1999 by Oxford University Press
C Ren and A Kermode
Yellow cedar (Chamaecyparis nootkatensis) seeds
exhibit prolonged dormancy following their dispersal from the parent plant.
Embryos excised fully from their enclosing seed tissues exhibit 100%
germination, indicating that the seed tissues enclosing the embryo (the
testa, remnants of the nucellus and the megagametophyte) play an inhibitory
role and prevent radicle emergence. As part of an assessment of the role of
seed tissues in the dormancy mechanism of yellow cedar seeds, light
microscopy was used to examine changes within the major structures of the
seed following a 90 d war (26
ARTICLES
Analyses to determine the role of the megagametophyte and other seed tissues in dormancy maintenance of yellow cedar (Chamaecyparis nootkatensis) seeds; morphological, cellular and physiological changes following moist chilling and during germination
Department of Biological Sciences, Simon Fraser University, Burnaby, BC, Canada V5A 1S6; Corresponding author e-mail: kermode@sfu.ca
C)/cold
(4C) moist treatment ('stratification') and
during germination. In the micropylar tip of the seed, the nucellus forms a
hard nucellar cap covering the radicle. The nucellar cap is composed
primarily of degenerated cells; histological staining with ruthenium red
revealed a predominance of pectins. There were no obvious cellular or
morphological differences (detected by light microscopy) between mature
seeds subjected to a 3 d soak and seeds subjected to a 3 d soak and the 90
d dormancy-breaking treatment. However, just prior to germination there was
an outward projection of the nucellar cap through the micropyle, which
appeared to be caused by the extension of highly folded proteinaceous
strands lying immediately in front of the radicle. When the testa was
removed, the embryo enclosed within the intact megagametophyte was
incapable of germination. If, however, the megagametophyte surrounding the
embryo was slit or the embryo surrounded by an intact megagametophyte was
subjected to a 3d rinse in water, some germination occurred, perhaps as a
result of an enhanced release of inhibitors from the megagametophyte. After
stratification, dormancy of yellow cedar seeds is broken; concurrent with
dormancy breakage, there was a mechanical weakening of the megagametophyte.
The embryo also underwent changes that included an increase in turgor and a
reduced sensitivity to highly negative osmotic potential. It is concluded
that coat-imposed dormancy of yellow cedar seeds is enforced by mechanical
restraint of the megagametophyte as well as a leachable chemical inhibitor
(most probably ABA).Key words: Coat-imposed dormancy,
embryo, megagametophyte, nucellar cap, seed germination, yellow cedar.
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