dent in the cotyledons at 5°C or in either axes or cotyledons at 20°C. The authors suggested that protein
reserves might be mobilized to the axis during the breaking of dormancy.
Despite much research in this field, the picture remains confusing. Some workers have suggested that
proteins found in dormant, but not in nondormant, seeds inhibit germination [106–108]. Mahhou and
Dennis [109] reported reduced levels of large proteins (36 and 41 kDa) in the cotyledons of peach seeds
stratified at 5°C, even when the embryonic axis was excised. These changes did not occur at 20°C. In
some tissues, chilling increases the content of certain proteins (e.g., Ref. 110); in others, some proteins
increase during chilling while others decrease [111–113]. Ried and Walker-Simmons [114] have pre-
sented evidence for heat-stable proteins in embryonic axes of dormant wheat seeds that are induced by
treatment with ABA. Much higher concentrations of ABA are required to produce similar levels of pro-
teins in nondormant embryos, suggesting that sensitivity to ABA may play a role in dormancy.
- Synthesis of Nucleotides
The limited ability of dormant tissues to convert adenosine to nonadenylic nucleotides (NTP sum of
triphosphates of guanidine, cytosine, and uridine) has been suggested as a possible cause of dormancy.
Correlations between the ability to convert adenosine to NTP and the dormant state have been reported in
Jerusalem artichoke (Helianthus tuberosumL.) tubers [115], in apple embryos [116], and in buds or sub-
apical tissues of ash [117], willow, and hazel [118].
B. Permeability Changes
Several investigators have proposed that changes in membrane permeability are responsible for dor-
mancy. To test this hypothesis, tissues are incubated with a weak acid [5,5-dimethyl-2,4-oxazolidinedione
(DMO)]; only the undissociated form can pass through the cell membrane. Use of radioactive DMO per-
mits determination of the ratio of the concentration of DMO within the cell (Ci) to the concentration in
the intercellular spaces (Ce). Relative membrane permeability parallels the Ci/Ceratio. Using this method,
Gendraud and Lefleuriel [119] observed a higher Ci/Ceratio in dormant than in nondormant tubers of
Jerusalem artichoke. This implies less movement of nutrients to the meristematic tissues of dormant tu-
bers. In similar studies, Ben Ismail [120] compared Ci/Ceratios in bud versus shoot tissues of apple dur-
ing the dormant period. Higher ratios occurred in shoots than in buds during the fall and early winter, sug-
gesting limited movement of solutes from shoots to buds. Thereafter, the ratio in the buds rose to levels
higher than those observed in the shoots. Although the results parallel the expected response of intact trees
or isolated shoots, bud development in single-node cuttings exposed to laboratory conditions was reduced
only in samples collected in November.
C. Role of Hormones
The role of hormones in seed dormancy is supported primarily by the effects of applied hormones in both
inhibiting the germination of nondormant seeds (ABA) and stimulating the germination of dormant seeds
(cytokinins, GAs). However, effective concentrations are often much higher than those found in the seeds
themselves, and the response is seldom as great as one might expect. Although treatment with GA is ef-
fective in breaking dormancy in lettuce seeds, germination of peach seeds can be maximized only after
some chilling has occurred [121]. Even then, the symptoms of insufficient chilling (abnormal leaves, etc.)
are not eliminated. Furthermore, despite early reports to the contrary, few good correlations have been es-
tablished between content of endogenous hormones and dormancy status.
Several hypotheses have been proposed regarding the role of hormones in seed dormancy. Germi-
nation is prevented by:
- High concentrations of growth inhibitors, (e.g., ABA)
- Inhibitory concentrations of auxin [indole-3-acetic acid (IAA)]
- Insufficient concentrations of growth promoters (GA, cytokinins)
- Both (1) and (3)
Modifications of these hypotheses propose that:- Promoters are synthesized in seeds requiring chilling only following their return to warm tem-
peratures [122].
174 DENNIS