Dormancy is a period in an
organism's life cycle when growth, development, and (in animals) physical activity are temporarily stopped. This minimizes
metabolic activity and therefore helps an organism to conserve
energy. Dormancy tends to be closely associated with
environmental conditions. Organisms can synchronize entry to a dormant phase with their
environment
Environment most often refers to:
__NOTOC__
* Natural environment, all living and non-living things occurring naturally
* Biophysical environment, the physical and biological factors along with their chemical interactions that affect an organism or ...
through predictive or consequential means. Predictive dormancy occurs when an organism enters a dormant phase ''before'' the onset of adverse conditions. For example,
photoperiod and decreasing
temperature are used by many
plants to predict the onset of winter. Consequential dormancy occurs when organisms enter a dormant phase ''after'' adverse conditions have arisen. This is commonly found in areas with an unpredictable climate. While very sudden changes in conditions may lead to a high
mortality rate among animals relying on consequential dormancy, its use can be advantageous, as organisms remain active longer and are therefore able to make greater use of available resources.
Animals
Hibernation
Hibernation is a mechanism used by many mammals to reduce energy expenditure and survive food shortages over the winter. Hibernation may be predictive or consequential. An animal prepares for hibernation by building up a thick layer of
body fat during late summer and autumn that will provide it with energy during the dormant period. During hibernation, the animal undergoes many
physiological changes, including decreased
heart rate (by as much as 95%) and decreased
body temperature
Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperatur ...
. In addition to shivering, some hibernating animals also produce body heat by non-shivering thermogenesis to avoid freezing. Non-shivering thermogenesis is a regulated process in which the proton gradient generated by electron transport in mitochondria is used to produce heat instead of ATP in brown adipose tissue. Animals that hibernate include
bats
Bats are mammals of the order Chiroptera.''cheir'', "hand" and πτερόν''pteron'', "wing". With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most bir ...
,
ground squirrels and other rodents, mouse lemurs, the
European hedgehog and other insectivores, monotremes and marsupials. Although hibernation is almost exclusively seen in mammals, some birds, such as the
common poorwill, may hibernate.
Diapause
Diapause is a predictive strategy that is predetermined by an animal's
genotype. Diapause is common in
insects, allowing them to suspend development between autumn and spring, and in
mammals such as the
roe deer (''Capreolus capreolus'', the only
ungulate with
embryonic diapause), in which a delay in attachment of the
embryo to the
uterine lining ensures that
offspring are born in spring, when conditions are most favorable.
Aestivation
Aestivation, also spelled ''estivation'', is an example of consequential dormancy in response to very hot or dry conditions. It is common in
invertebrates such as the
garden snail and
worm but also occurs in other animals such as
lungfish,
salamanders,
desert tortoises, and
crocodiles.
Brumation
While
endotherms and other
heterotherms are described scientifically as
hibernating, the way
ectotherms such as lizards become dormant in cold is very different, and a separate name was invented for it in the 1920s:
brumation
Dormancy is a period in an organism's life cycle when growth, development, and (in animals) physical activity are temporarily stopped. This minimizes metabolic activity and therefore helps an organism to conserve energy. Dormancy tends to be clos ...
. It differs from hibernation in the metabolic processes involved.
Reptiles generally begin brumation in late autumn (more specific times depend on the species). They often wake up to drink water and return to "sleep". They can go for months without food. Reptiles may eat more than usual before the brumation time but eat less or refuse food as the temperature drops. However, they do need to drink water. The brumation period is anywhere from one to eight months depending on the air temperature and the size, age, and health of the reptile. During the first year of life, many small reptiles do not fully brumate, but rather slow down and eat less often. Brumation is triggered by a lack of heat and a decrease in the hours of daylight in winter, similar to hibernation.
Plants
In
plant physiology, dormancy is a period of arrested plant growth. It is a survival strategy exhibited by many plant
species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...
, which enables them to survive in
climates where part of the year is unsuitable for growth, such as winter or
dry seasons.
Many plant species that exhibit dormancy have a
biological clock that tells them when to slow activity and to prepare
soft tissues
Soft tissue is all the tissue in the body that is not hardened by the processes of ossification or calcification such as bones and teeth. Soft tissue connects, surrounds or supports internal organs and bones, and includes muscle, tendons, liga ...
for a period of freezing temperatures or water shortage. On the other hand, dormancy can be triggered after a normal growing season by decreasing temperatures, shortened day length, and/or a reduction in
rainfall. Chemical treatment on dormant plants has been proven to be an effective method to break dormancy, particularly in woody plants such as grapes, berries, apples, peaches, and kiwis. Specifically, hydrogen cyanamide stimulates cell division and growth in dormant plants, causing buds to break when the plant is on the edge of breaking dormancy. Slight injury of cells may play a role in the mechanism of action. The injury is thought to result in increased permeability of cellular membranes. The injury is associated with the inhibition of catalase, which in turn stimulates the pentose phosphate cycle. Hydrogen cyanamide interacts with the cytokinin metabolic cycle, which results in triggering a new growth cycle. The images below show two particularly widespread dormancy patterns amongst
sympodially growing orchids:
Seeds
When a mature and viable
seed under a favorable condition fails to germinate, it is said to be dormant.
Seed dormancy
Seed dormancy is an evolutionary adaptation that prevents seeds from germinating during unsuitable ecological conditions that would typically lead to a low probability of seedling survival. Dormant seeds do not germinate in a specified period of ...
is referred to as embryo dormancy or internal dormancy and is caused by endogenous characteristics of the embryo that prevent
germination (Black M, Butler J, Hughes M. 1987). Dormancy should not be confused with seed coat dormancy, external dormancy, or hardheadedness, which is caused by the presence of a hard seed covering or
seed coat that prevents water and oxygen from reaching and activating the
embryo. It is a physical barrier to germination, not a true form of dormancy (Quinliven, 1971; Quinliven and Nichol, 1971).
Seed dormancy is desired in nature, but the opposite in the agriculture field. This is because agricultural practice desires rapid germination and growth for food whereas in nature, most plants are only capable of germinating once every year, making it favorable for plants to pick a specific time to reproduce. For many plants, it is preferable to reproduce in spring as opposed to fall even when there are similar conditions in terms of light and temperature due to the ensuing winter that follows fall. Many plants and seeds recognize this and enter a dormant period in the fall to stop growing. The grain is a popular example in this aspect, where they would die above ground during the winter, so dormancy is favorable to its seedlings but extensive domestication and crossbreeding has removed most dormancy mechanisms that their ancestors had.
While seed dormancy is linked to many genes, abscisic acid (ABA), a plant hormone, has been linked as a major influencer to seed dormancy. In a study on rice and tobacco plants, plants defective in zeaxanthin epoxidase gene, which are linked to ABA-synthesis pathway. Seeds with higher ABA content, from over-expressing zeaxanthin epoxidase, led to an increased dormancy period while plants with lower numbers of zeaxanthin epoxidase were shown to have a shorter period of dormancy. A simple diagram can be drawn of ABA inhibits seed germination, while gibberellin (GA, also plant hormone) inhibits ABA production and promotes seed germination.
Trees
Typically, temperate woody
perennial plant
A perennial plant or simply perennial is a plant that lives more than two years. The term ('' per-'' + '' -ennial'', "through the years") is often used to differentiate a plant from shorter-lived annuals and biennials. The term is also wide ...
s require chilling temperatures to overcome winter dormancy (rest). The effect of chilling temperatures depends on species and growth stage (Fuchigami et al. 1987).
[Fuchigami, L. H., Nee, C. C., Tanino, K., Chen, T. H. H., Gusta, L. V., and Weiser, C. J. 1987. "Woody Plant Growth in a Changing Chemical and Physical Environment". Proc. Workshop IUFRO Working Party on Shoot Growth Physiology, Vancouver, British Columbia, July 1987, Lavender, D. P. (Compiler & Ed.), University of British Columbia, Forest Science Department, Vancouver, British : 265–282.] In some species, rest can be broken within hours at any stage of dormancy, with either chemicals, heat, or freezing temperatures, effective dosages of which would seem to be a function of sublethal stress, which results in stimulation of
ethylene production and increased cell membrane permeability.
''Dormancy'' is a general term applicable to any instance in which a tissue predisposed to elongate or grow in some other manner does not do so (Nienstaedt 1966).
''Quiescence'' is dormancy imposed by the external environment. ''Correlated inhibition'' is a kind of physiological dormancy maintained by agents or conditions originating within the plant, but not within the dormant tissue itself. ''Rest'' (winter dormancy) is a kind of physiological dormancy maintained by agents or conditions within the organ itself. However, physiological subdivisions of dormancy do not coincide with the morphological dormancy found in white spruce (''Picea glauca'') and other
conifers (Owens et al. 1977).
Physiological dormancy often includes early stages of bud-scale initiation before measurable shoot elongation or before flushing. It may also include late leaf initiation after shoot elongation has been completed. In either of those cases,
buds that appear to be dormant are nevertheless very active morphologically and physiologically.
Dormancy of various kinds is expressed in white spruce (Romberger 1963).
[Romberger, J. A. 1963. "Meristems, Growth, and Development in Woody Plants". USDA, Forestry Service, Washington DC, Technical Bulletin 1293. 214 p.] White spruce, like many woody plants in temperate and cooler regions, requires exposure to low temperature for a period of weeks before it can resume normal growth and development. This "chilling requirement" for white spruce is satisfied by uninterrupted exposure to temperatures below 7 °C for 4 to 8 weeks, depending on physiological condition (Nienstaedt 1966, 1967).
Tree species that have well-developed dormancy needs may be tricked to some degree, but not completely. For instance, if a
Japanese maple
''Acer palmatum'', commonly known as Japanese maple, palmate maple, or smooth Japanese maple (Japanese: ''irohamomiji'', , or ''momiji'', (栴), is a species of woody plant native to Japan, Korea, China, eastern Mongolia, and southeast Rus ...
(''Acer palmatum'') is given an "eternal summer" through exposure to additional daylight, it grows continuously for as long as two years. Eventually, however, a
temperate-climate plant automatically goes dormant, no matter what environmental conditions it experiences.
Deciduous plants lose their leaves;
evergreen
In botany, an evergreen is a plant which has foliage that remains green and functional through more than one growing season. This also pertains to plants that retain their foliage only in warm climates, and contrasts with deciduous plants, whic ...
s curtail all new growth. Going through an "eternal summer" and the resultant automatic dormancy is stressful to the plant and usually fatal. The fatality rate increases to 100% if the plant does not receive the necessary period of cold temperatures required to break the dormancy. Most plants require a certain number of hours of "chilling" at temperatures between about 0 °C and 10 °C to be able to break dormancy (Bewley, Black, K.D 1994).
Short
photoperiods induce dormancy and permit the formation of needle primordia. Primordia formation requires 8 to 10 weeks and must be followed by 6 weeks of chilling at 2 °C. Bud break occurs promptly if seedlings are then exposed to 16-hour photoperiods at the 25 °C/20 °C temperature regime. The free growth mode, a juvenile characteristic that is lost after 5 years or so, ceases in seedlings experiencing environmental stress (Logan and Pollard 1976, Logan 1977).
[Logan, K. T.; Pollard, D. F. W. 1976. "Growth acceleration of tree seedlings in controlled environments at Petawawa". Canadian Forestry Service, Petawawa Forest Experiment Station, Chalk River, Ontario, Information PS-X-62.]
Bacteria
Many
bacteria can survive adverse conditions such as temperature,
desiccation, and
antibiotics by forming
endospores,
cysts, or states of reduced metabolic activity lacking specialized cellular structures. Up to 80% of the bacteria in samples from the wild appear to be metabolically inactive—many of which can be resuscitated. Such dormancy is responsible for the high diversity levels of most natural ecosystems.
Recent research
has characterized the bacterial
cytoplasm as a glass forming fluid approaching the
liquid-glass transition, such that large cytoplasmic components require the aid of
metabolic activity to fluidize the surrounding cytoplasm, allowing them to move through a viscous, glass-like cytoplasm. During dormancy, when such metabolic activities are put on hold, the cytoplasm behaves like a solid
glass, 'freezing' subcellular structures in place and perhaps protecting them, while allowing small molecules like
metabolites to move freely through the cell, which may be helpful in cells transitioning out of dormancy.
Viruses
Dormancy, in its rigid definition, does not apply to
viruses, as they are not metabolically active. However, some viruses such as
poxviruses and
picornaviruses, after entering the host, can become
latent for long periods of time, or even indefinitely until they are externally activated.
Herpesviruses, for example, can become latent after infecting the host, and after years they can activate again if the host is under stress or exposed to ultraviolet radiation.
See also
*
Plant physiology
*
Scotobiology
Scotobiology is the study of biology as directly and specifically affected by darkness, as opposed to photobiology, which describes the biological effects of light.
Overview
The science of scotobiology gathers together under a single descriptive ...
*
Torpor
Notes
References
* Bewley, J. D. and Black, M. (1994). ''Seeds: physiology of development and germination, 2nd end''. New York, London: Plenum Press.
* Black, M.; Butler, J. and Hughes, M. (1987). "Control and development of dormancy in cereals". In: Mares DJ, ed. ''Fourth International Symposium on Pre-Harvest Sprouting in Cereals'', Boulder, Co., USA: Westview Press, 379–92.
*
* {{cite journal , last1 = Quinlivan , first1 = B. J. , year = 1971 , title = Seed coat impermeability in legumes , journal = Journal of the Australian Institute of Agricultural Science , volume = 37 , pages = 283–295
* Scholar team. (2002). "SQA Adv. Higher Biology". ''Environmental Biology''. Heriot-Watt University, 93–95.
Plant physiology
Physiology
Ethology