As noted earlier, in some areas of the tropics or subtropics, peach, apple, and grape can be multiple
cropped, although a brief dormant period intervenes between foliations. The leaves must be removed to
stimulate bud break, and chemicals, such as sodium chlorate, copper sulfate, or urea, are often applied to
injure the leaves and induce premature abscission. In areas where multiple cropping is impossible, but
chilling inadequate to completely relieve dormancy, other chemicals, such as combinations of dinitro-O-
cresol and oils, are used to hasten bud break and concentrate the bloom period. Hydrogen cyanamide
(H 2 NCN), which releases HCN within the tissues, is a relatively new compound that has similar effects
and has been extensively tested for this purpose [55,56].
In arid regions bud dormancy of some species can be broken by withholding water for several weeks,
then irrigating. Asparagus growers in California and Peru can produce crops year-round using this
method. Irrigation is also used in combination with rest-breaking chemicals and/or defoliation of decidu-
ous fruit trees in tropical regions [57].
In areas where chilling is adequate but spring freezes often damage flowers and/or fruits, delaying
bloom could provide protection. Evaporative cooling by misting with water can delay bloom; delays of 3
weeks or more are possible in arid climates [58,59]. However, side effects, such as poor fruit set, have
limited commercial application. This method has also been tested in warm climates for cooling buds dur-
ing the winter [60], thereby hastening the breaking of dormancy; again, commercial application has been
limited.
Methods of weakening the integuments of seeds with hard seed coats to allow water to penetrate have
already been discussed, as well as the effects of light and temperature on seeds with “shallow” dormancy.
Several growth regulators, including both gibberellins (GAs) and cytokinins, promote germination in dor-
mant or partially dormant seeds. GA is effective in stimulating germination in seeds with a shallow dor-
mancy. Light-sensitive lettuce seeds, for example, will germinate in darkness when supplied with GA.
Cytokinin, although generally effective in stimulating dark germination, can overcome the inhibitory ef-
fects of high temperatures. Abscisic acid (ABA) blocks germination in many seeds, regardless of envi-
ronmental conditions. Khan [61] tested the effects of all three hormones and their combinations on the
germination of light-sensitive lettuce seeds. The action of GA was blocked by ABA, but cytokinin coun-
teracted the effect of ABA, thereby permitting germination when all three hormones were applied. From
these data, Khan [62] proposed that the roles of GA, ABA, and cytokinin were primary, preventive, and
permissive, respectively; GA is the primary stimulus, with cytokinin being essential only when ABA is
present. Khan and others [63,64] have confirmed and extended these observations by using inhibitors of
GA synthesis to block germination and demonstrating that in some cases, cytokinin and/or ethylene is re-
quired, in addition to GA, to overcome the inhibitory effects of stress caused by water deficit, salinity,
and other conditions.
GA will also stimulate germination in some cold-requiring seeds, although some chilling is usually
required before maximum response is obtained. Cytokinins are usually less effective. Both GA and cy-
tokinins can hasten release from dormancy in buds of woody plants, as well as overcoming apical domi-
nance during the early growing season. A combination of GA4/7and benzyladenine, for example, is cur-
rently available commercially to stimulate growth of lateral buds of conifers used for Christmas trees,
thereby providing a more pleasing form.
Ethylene promotes germination in some weed species [e.g., redroot (Amaranthus retroflexus) and
lamb’s quarters (Chenopodium album)], but many species are not responsive [65]. Gibberellins and cy-
tokinins have more general effects. A few cases are known in which ethylene breaks bud dormancy, but
again, response is species dependent.
Several chemicals are effective in prolongingbud or seed dormancy. Potato tubers are regularly
fumigated with 1-methyl-3-chlorophenylcarbamate (CIPC) to delay their sprouting during storage.
Scientists are testing naturally occurring compounds as potential substitutes. Andean natives store pota-
toes in pits together with leaves of muña (plants of the genera MinthostachysandSatureja) to delay
sprouting and reduce both weight loss and insect injury [66]. Trials with volatile components of read-
ily available essential oils demonstrated that 1,8-cineole, found in eucalyptus oil, has promise in in-
hibiting both sprouting and fungal growth [67]. Application of maleic hydrazide to the foliage of onion
plants several weeks before harvest inhibits sprouting of the stored bulbs [68]. The naturally occurring
plant growth inhibitor ABA inhibits seed germination in many species [10], although its cost prohibits
commercial use. It is less effective on buds, perhaps because of limited penetration and/or rapid
metabolism.
DORMANCY: MANIFESTATIONS AND CAUSES 171