8
Dormancy: Manifestations and Causes
Frank G. Dennis, Jr.
Michigan State University, East Lansing, Michigan
161I. IMPORTANCE OF DORMANCY
During their life cycles, plants are exposed to periods of stress caused by low or high temperatures,
drought, or other environmental factors. In the course of evolution, complex defense mechanisms have
developed for protection against such stresses. One such mechanism is dormancy. Simply defined, dor-
mancyis the inability of an otherwise viable seed, whole plant, or meristem (a bud, apex, etc.) to grow.
Many plants adapted to the tropics do not become dormant; shoot growth occurs whenever environ-
mental conditions permit. However, growth often occurs in flushes, and certain branches may be grow-
ing while others are not. In the dry topics, rainy seasons alternate with dry ones; here plants are adapted
to growing when water is available, but growth slows or ceases during the dry season. Where cold and
warm seasons alternate, as in the temperate zones, continuous growth is similarly impossible. Plants stop
growing in the late summer or autumn, then resume growth again in the spring. In both the temperate and
the polar regions another adaptation has occurred—plants develop resistance to low temperatures, or
“cold hardiness,” to permit survival at temperatures as low as 40°C or below. Perennial plants may be
deciduousorevergreen; in the former the leaves abscise before winter begins, in the latter the leaves are
functional throughout the year.
Tropical annuals will grow in any climatic zone where the growing season is long enough to allow
them to mature. Thus green beans and marigolds can be cultivated from the equator to the arctic circle. In
contrast, woody perennials will not survive outdoors if grown in an area where winters are too cold. Peach
trees adapted to the temperate zone will grow poorly, or not at all, in the tropics for lack of “chilling” (see
later), whereas mangos will not survive the low winter temperatures characteristic of the temperate zone.
Seed physiology may reflect the environmental conditions in the area of origin of the species. The
seeds of plants native to the humid tropics need no dormancy provided that conditions are favorable for
germination year-round. In contrast, seeds of plants adapted to the temperate zone often exhibit some de-
gree of dormancy. If seeds shed at the end of the growing season were to germinate immediately, they
would not survive the winter. Some species have circumvented this problem by having an abbreviated pe-
riod of fruit development, permitting the shedding of seeds in early summer (silver maple, dandelion). In
others, termed winter annuals, seeds germinate in late summer/early fall, and the seedlings develop suf-
ficient cold hardiness to survive the winter and produce seed early the following year. Such seeds are dor-
mant when shed but become capable of germination in the fall (see later).