- High levels of promoters are required only temporarily at the beginning of the “trigger” phase
that ends dormancy [123]. - ABA blocks the action of GAs; if both are present, cytokinin must also be present to permit GA
to act [61,62]. - Auxin
Nikolaeva [124] determined the content of presumed IAA (wheat coleoptile segment and mustard seed
germination assays) in seeds and/or embryos of several tree species before, during, and after cold strati-
fication. Activity (promotion of coleoptile section growth, inhibition of germination) declined as stratifi-
cation was prolonged. Inhibitor activity in nondormant seeds was approximately half of that observed in
dormant seeds. Nondormant seeds treated with the naturally occurring auxin IAA produced seedlings
with symptoms similar to those of seedlings from insufficiently chilled seeds. From these and other data
she concluded that high levels of IAA prevented germination of nonchilled seeds, and that chilling re-
duced the IAA concentration to the levels found in seeds that did not require chilling. Subsequent inves-
tigators have found little support for the role of auxin in dormancy. Most later research on hormones has
focused on GAs and ABA.
- Gibberellins
Amen [123] proposed that seed dormancy could be divided into four phases. During the inductionphase,
levels of growth promoters decline and/or the seed coat becomes impermeable to oxygen; therefore, the
seed becomes dormant. During the ensuing maintenancephase, germination is prevented by endogenous
inhibitors. In the triggerphase, a factor that elicits germination but whose continued presence is not es-
sential (the trigger, e.g., light) induces the production of a germination agent, whose continued presence
is required for germination. In the final phase (germination), the germination agent [growth promoter(s)]
provides the stimulus for radicle protrusion.
Much of the evidence for this scheme is based on the effects of exogenous growth regulators on
germination; only a few studies have supported the hypothesis in terms of actual increases in seed hor-
mone content following action by “triggers,” including chilling, and light. In one such study, Williams
et al. [122] could detect little change in GA content of hazel seeds during moist chilling at 5°C. How-
ever, levels rose rapidly once dormancy had been broken, provided that the seeds were returned to
20°C.
The gibberellin (GA 4 ) content of apple seeds rises during chilling but is no higher in fully chilled
seed than in nonchilled seed [125]. This could, of course, be interpreted as supporting a “trigger” role for
GA. Similar roles for both GA and cytokinin have been suggested in maple seeds [126].
- Abscisic Acid
Considerable effort has been directed toward elucidating the role of ABA in controlling dormancy in
seeds. The ABA content of immature seeds of several species, including wheat [127] and rapeseed [128],
rises to a maximum, then falls as the seeds mature and dry out. Although the concentration of ABA in the
mature seed is low, desiccation reduces water content, thereby preventing germination.
The effects of ABA in preventing the germination of immature embryos in vitro plus the evidence
for the role of ABA in viviparity, noted above, strongly imply that ABA is one of the factors preventing
embryo germination. Seeds of the species investigated in these studies (e.g., maize, rapeseed) are non-
dormant or have only a shallow dormancy at harvest; similar relationships may not apply in seeds that ex-
hibit deep dormancy.
In ash (Fraxinus) seeds, ABA content is low in F. americanarelative to that in F. ornus[129]. Seeds
of the former are nondormant, whereas the latter require moist chilling to break their dormancy. This dor-
mancy again is correlated with ABA content. While the ABA content of seeds of three species of Rosais
negatively correlated with their germinability [130], the ABA content of seeds of several species of pear
bears no relationship to depth of dormancy [131], nor does the ABA content of immature or mature seeds
ofAvena fatua(dormant) differ from that of seeds of A. sativa(nondormant) [132]. Differences in sensi-
tivity to ABA could, of course, explain some of these discrepancies but have seldom been tested experi-
mentally. Early results indicated that the levels of ABA or ABA-like inhibitors fell during moist chilling
of ash [129] and several other species, including apple [133]. However, subsequent investigations indi-
cated that ABA content either did not decline during low-temperature stratification [134] or that the de-
DORMANCY: MANIFESTATIONS AND CAUSES 175