cline was not temperature-dependent [135]. The concentration of ABA declines during soaking of lettuce
seed, regardless of their germination capacity [136].
In many of these studies the entire seed was extracted. Karssen et al. [79] proposed that the GA
and/or ABA content of the embryo may be more important than that of the whole seed. Using selected
mutants of Arabidopsis thaliana, they demonstrated that embryos recessive for ABA production were
nondormant even when the seed coat contained high levels of ABA. Later work with GA-deficient mu-
tants led Karssen et al. [80] to propose that GA content is the critical factor in germination. The seeds of
genotypes that cannot synthesize GAs remain dormant regardless of their ABA content.
Inhibitor content of buds has also been quantified in relation to dormancy. Again initial results were
promising; the inhibitor content of buds of several species, as measured by bioassay, appeared to rise
when plants were transferred from long to short photoperiods [137,138]. This work led to the identifica-
tion of ABA by Ohkuma et al. [139] and Cornforth et al. [140]. As analytical instruments have become
more sensitive and experiments more critical, however, the negative correlation between ABA content
and growth response has not been confirmed [141,142]. In fact, one laboratory reported [143,144] that
rapidly growing apices contained more ABA than did subapical tissues.
Coleman and King [145] reported a positive correlation between ABA content of tubers of 10 potato
cultivars following 2 months of storage at 10°C and the time to 50% sprouting at 20°C. However, ABA
content of three other cultivars actually increasedduring storage at three temperatures (2, 10, and 20°C),
yet dormancy was broken in all cases, often when ABA content was near maximal.
D. New Approaches to the Understanding of Dormancy
Relatively little is known about how genes control seed and bud dormancy, but research in molecular bi-
ology is beginning to open the “black box.” Studies of apical dominance, for example, are under way us-
ing transgenic plants that differ in the relative amounts of IAA and cytokinin synthesized. Plants with high
IAA/cytokinin ratios exhibit strong apical dominance, and vice versa [146,147], suggesting that these hor-
mones may indeed be responsible for this phenomenon. Genes for hormone synthesis in plants that ex-
hibit seed and/or bud dormancy have been identified and can now be cloned. Once these can be inserted
in the same or other species, rapid progress may be expected in elucidating the roles of such compounds
in controlling dormancy.
VII. SUMMARY
Dormancy serves a protective function in permitting plant survival under extremes of temperature, water
deficit, and other environmental stresses, and species differ in their manifestations of dormancy. Several
types of dormancy are known, with control sometimes residing within the dormant organ, sometimes out-
side the organ. As would be expected, the conditions required to break dormancy differ with the type of
dormancy exhibited and vary from changes in light intensity or photoperiod to exposure to low or alter-
nating temperatures. Many theories have been proposed to explain the physiological basis of dormancy,
but none has proved valid in accounting for all the facts known. New approaches, especially molecular
biology, should provide new information in this important field.
REFERENCES
- Simpson GM. In: ME Clutter, ed. Dormancy and Developmental Arrest. New York: Academic Press, 1978,
p 167. - Lang GA, Early JD, Aroyave NG, Darnell RL, Martin GC, Stutte GW. HortScience 20:809, 1985.
- Lang GA, Early JD, Martin GC, Darnell RL. HortScience 22:371, 1987.
- Lang GA. HortScience 22:817, 1987.
- Juntilla O. HortScience 23:805, 1988.
- Martin GC. HortScience 22:824, 1987.
- Cline MG. Bot Rev 57:318, 1991.
- Khan AA, ed. The Physiology and Biochemistry of Seed Dormancy and Germination. New York: North-
Holland, 1977. - Khan AA, ed. The Physiology and Biochemistry of Seed Development, Dormancy, and Germination. New
York: Elsevier Biomedical Press, 1982. - Bewley JD, Black M. Physiology and Biochemistry of Seeds in Relation to Germination. Vol 2. Viability,
Dormancy and Environmental Control. New York: Springer-Verlag, 1982.
176 DENNIS