Chilling requirement
Encyclopedia
The chilling requirement of a fruit is the minimum period of cold weather
after which a fruit-bearing tree will blossom
. It is often expressed in chill hours, which can be calculated in different ways, all of which essentially involve adding up the total amount of time in a winter spent at certain temperatures.
Some bulbs have chilling requirements to bloom, and some seeds have chilling requirements to sprout.
Biologically, the chilling requirement is a way of ensuring that vernalization
occurs.
is a metric of a plant's exposure to chilling temperatures. Chilling temperatures extend from freezing point
to, depending on the model, 45 °F (7.2 °C) or even 60 °F (15.6 °C). Stone fruit
trees and certain other plants of temperate climate develop next year's bud
s in the summer. In the autumn the buds go dormant
, and the switch to proper, healthy dormancy is triggered by a certain minimum exposure to chilling temperatures. Lack of such exposure results in delayed and substandard foliation, flowering and fruiting. One chilling unit, in the simplest models, is equal to one hour's exposure to the chilling temperature; these units are summed up for a whole season. Advanced models assign different weights to different temperature bands.
to dormancy, but the process can be easily reversed with a rise in temperature. After the level of precursor reaches a certain threshold, dormancy becomes irreversible and will not be affected by short-term warm temperature peaks. Apple
s have the highest chilling requirements of all fruit trees, followed by apricot
s and, lastly, peach
es. Apple cultivar
s have a diverse range of permissible minimum chilling: most have been bred for temperate weather, but Gala
and Fuji
can be successfully grown in subtropic Bakersfield, California
.
Peach cultivars in Texas
range in their requirements from 100 chilling units (FlordaGrande cultivar, zoned for low chill regions) to 1,000 units (Surecrop, zoned for high chill regions). Planting a low-chilling cultivar in a high-chill region risks loss of a year's harvest when an early bloom is hit by a spring frost. A high-chilling cultivar planted in a low-chill region will, quite likely, never fruit at all. A four-year study of Ruston Red Alabama
peach, which has a threshold of 850 chilling units, demonstrated that a seasonal chilling deficiency of less than 50 units has no effect on harvest. Deficiency of 50 to 100 units may result in loss of up to 50% of expected harvest. Deficiency of 250 hours and more is a sure loss of practically whole harvest; the few fruit will be of very poor quality and have no market value. Rest-breaking agents (e.g. hydrogen cyanamide
, trade name Dormex), applied in spring, can partially mitigate the effects of insufficient chilling. Dormex can substitute for up to 300 hours of chilling, but an excessive spraying and timing error can easily damage the buds.
Chilling of orange trees
has two effects. First, it increases production of carotenoid
s and decreases chlorophyll
content of the fruit, improving their appearance and, ultimately, their market value. Second, the "quasi-dormancy" experienced by orange trees triggers concentrated flowering in spring, as opposed to more or less uniform round-the-year flowering and fruiting in warmer climates.
Biennial plant
s like cabbage
, sugar beet
, celery
and carrot
s need chilling to develop second-year flowering buds. Excessive chilling in the early stages of a sugar beet seedling, on the contrary, may trigger undesired growth of a flowering stem (bolting) in its first year. This phenomenon has been offset by breeding sugar beet cultivars with a higher minimum chilling threshold. Such cultivars can be seeded earlier than normal without the risk of bolting.
The Utah model assigns different weight to different temperature bands; a full unit per hour is assigned only to temperatures between 37 °F (2.8 °C) and 48 °F (8.9 °C). Maximum effect is achieved at 7 °C (44.6 °F). Temperatures between 55 °F (12.8 °C) and 60 °F (15.6 °C) (the threshold between chilling and warm weather) have zero weight, and higher temperature have negative weights: they reduce the beneficial effects of an already accumulated chilling hours.
Southwick et al. wrote that neither of these models is accurate enough to account for application of rest-breaking agents widely used in modern farming. They advocated the use of a dynamic model taylored to the two-stage explanation of dormancy.
Weather
Weather is the state of the atmosphere, to the degree that it is hot or cold, wet or dry, calm or stormy, clear or cloudy. Most weather phenomena occur in the troposphere, just below the stratosphere. Weather refers, generally, to day-to-day temperature and precipitation activity, whereas climate...
after which a fruit-bearing tree will blossom
Blossom
In botany, blossom is a term given to the flowers of stone fruit trees and of some other plants with a similar appearance that flower profusely for a period of time in spring...
. It is often expressed in chill hours, which can be calculated in different ways, all of which essentially involve adding up the total amount of time in a winter spent at certain temperatures.
Some bulbs have chilling requirements to bloom, and some seeds have chilling requirements to sprout.
Biologically, the chilling requirement is a way of ensuring that vernalization
Vernalization
Vernalization is the acquisition of a plant's ability to flower or germinate in the spring by exposure to the prolonged cold of winter...
occurs.
Chilling units or chilling hours
Chilling unit in agricultureAgriculture
Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...
is a metric of a plant's exposure to chilling temperatures. Chilling temperatures extend from freezing point
Melting point
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...
to, depending on the model, 45 °F (7.2 °C) or even 60 °F (15.6 °C). Stone fruit
Drupe
In botany, a drupe is a fruit in which an outer fleshy part surrounds a shell of hardened endocarp with a seed inside. These fruits develop from a single carpel, and mostly from flowers with superior ovaries...
trees and certain other plants of temperate climate develop next year's bud
Bud
In botany, a bud is an undeveloped or embryonic shoot and normally occurs in the axil of a leaf or at the tip of the stem. Once formed, a bud may remain for some time in a dormant condition, or it may form a shoot immediately. Buds may be specialized to develop flowers or short shoots, or may have...
s in the summer. In the autumn the buds go dormant
Dormancy
Dormancy is a period in an organism's life cycle when growth, development, and 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...
, and the switch to proper, healthy dormancy is triggered by a certain minimum exposure to chilling temperatures. Lack of such exposure results in delayed and substandard foliation, flowering and fruiting. One chilling unit, in the simplest models, is equal to one hour's exposure to the chilling temperature; these units are summed up for a whole season. Advanced models assign different weights to different temperature bands.
Chilling requirements
According to Fishman, chilling in trees acts in two stages. The first is reversible: chilling helps to build up the precursorPrecursor (chemistry)
In chemistry, a precursor is a compound that participates in the chemical reaction that produces another compound. In biochemistry, the term "precursor" is used more specifically to refer to a chemical compound preceding another in a metabolic pathway....
to dormancy, but the process can be easily reversed with a rise in temperature. After the level of precursor reaches a certain threshold, dormancy becomes irreversible and will not be affected by short-term warm temperature peaks. Apple
Apple
The apple is the pomaceous fruit of the apple tree, species Malus domestica in the rose family . It is one of the most widely cultivated tree fruits, and the most widely known of the many members of genus Malus that are used by humans. Apple grow on small, deciduous trees that blossom in the spring...
s have the highest chilling requirements of all fruit trees, followed by apricot
Apricot
The apricot, Prunus armeniaca, is a species of Prunus, classified with the plum in the subgenus Prunus. The native range is somewhat uncertain due to its extensive prehistoric cultivation.- Description :...
s and, lastly, peach
Peach
The peach tree is a deciduous tree growing to tall and 6 in. in diameter, belonging to the subfamily Prunoideae of the family Rosaceae. It bears an edible juicy fruit called a peach...
es. Apple cultivar
Cultivar
A cultivar'Cultivar has two meanings as explained under Formal definition. When used in reference to a taxon, the word does not apply to an individual plant but to all those plants sharing the unique characteristics that define the cultivar. is a plant or group of plants selected for desirable...
s have a diverse range of permissible minimum chilling: most have been bred for temperate weather, but Gala
Gala (apple)
Gala is a clonally propagated apple with a mild and sweet flavor. Gala apples ranked at number 2 in 2006 on the US Apple Association's list of most popular apples, after Red Delicious and before Golden Delicious, Granny Smith, and Fuji ....
and Fuji
Fuji (apple)
The Fuji apple is an apple clone developed by growers at the Tohoku Research Station in Fujisaki, Aomori, Japan, in the late 1930s, and brought to market in 1962...
can be successfully grown in subtropic Bakersfield, California
Bakersfield, California
Bakersfield is a city near the southern end of the San Joaquin Valley in Kern County, California. It is roughly equidistant between Fresno and Los Angeles, to the north and south respectively....
.
Peach cultivars in Texas
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...
range in their requirements from 100 chilling units (FlordaGrande cultivar, zoned for low chill regions) to 1,000 units (Surecrop, zoned for high chill regions). Planting a low-chilling cultivar in a high-chill region risks loss of a year's harvest when an early bloom is hit by a spring frost. A high-chilling cultivar planted in a low-chill region will, quite likely, never fruit at all. A four-year study of Ruston Red Alabama
Alabama
Alabama is a state located in the southeastern region of the United States. It is bordered by Tennessee to the north, Georgia to the east, Florida and the Gulf of Mexico to the south, and Mississippi to the west. Alabama ranks 30th in total land area and ranks second in the size of its inland...
peach, which has a threshold of 850 chilling units, demonstrated that a seasonal chilling deficiency of less than 50 units has no effect on harvest. Deficiency of 50 to 100 units may result in loss of up to 50% of expected harvest. Deficiency of 250 hours and more is a sure loss of practically whole harvest; the few fruit will be of very poor quality and have no market value. Rest-breaking agents (e.g. hydrogen cyanamide
Cyanamide
Cyanamide is an organic compound with the formula CN2H2. This white solid is widely used in agriculture and the production of pharmaceuticals and other organic compounds. It is also used as an alcohol deterrent drug in Canada, Europe and Japan. The molecule features a nitrile group attached to an...
, trade name Dormex), applied in spring, can partially mitigate the effects of insufficient chilling. Dormex can substitute for up to 300 hours of chilling, but an excessive spraying and timing error can easily damage the buds.
Chilling of orange trees
Orange (fruit)
An orange—specifically, the sweet orange—is the citrus Citrus × sinensis and its fruit. It is the most commonly grown tree fruit in the world....
has two effects. First, it increases production of carotenoid
Carotenoid
Carotenoids are tetraterpenoid organic pigments that are naturally occurring in the chloroplasts and chromoplasts of plants and some other photosynthetic organisms like algae, some bacteria, and some types of fungus. Carotenoids can be synthesized fats and other basic organic metabolic building...
s and decreases chlorophyll
Chlorophyll
Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...
content of the fruit, improving their appearance and, ultimately, their market value. Second, the "quasi-dormancy" experienced by orange trees triggers concentrated flowering in spring, as opposed to more or less uniform round-the-year flowering and fruiting in warmer climates.
Biennial plant
Biennial plant
A biennial plant is a flowering plant that takes two years to complete its biological lifecycle. In the first year the plant grows leaves, stems, and roots , then it enters a period of dormancy over the colder months. Usually the stem remains very short and the leaves are low to the ground, forming...
s like cabbage
Cabbage
Cabbage is a popular cultivar of the species Brassica oleracea Linne of the Family Brassicaceae and is a leafy green vegetable...
, sugar beet
Sugar beet
Sugar beet, a cultivated plant of Beta vulgaris, is a plant whose tuber contains a high concentration of sucrose. It is grown commercially for sugar production. Sugar beets and other B...
, celery
Celery
Apium graveolens is a plant species in the family Apiaceae commonly known as celery or celeriac , depending on whether the petioles or roots are eaten: celery refers to the former and celeriac to the latter. Apium graveolens grows to 1 m tall...
and carrot
Carrot
The carrot is a root vegetable, usually orange in colour, though purple, red, white, and yellow varieties exist. It has a crisp texture when fresh...
s need chilling to develop second-year flowering buds. Excessive chilling in the early stages of a sugar beet seedling, on the contrary, may trigger undesired growth of a flowering stem (bolting) in its first year. This phenomenon has been offset by breeding sugar beet cultivars with a higher minimum chilling threshold. Such cultivars can be seeded earlier than normal without the risk of bolting.
Models
All models require hourly recording of temperatures. The simplest model assigns one chilling unit for every full hour at temperatures below 45 °F (7.2 °C). A slightly more sophisticated model excludes freezing temperatures, which do not contribute to proper dormancy cycle, and counts only hours with temperatures between 32 °F (0 °C) and 45 °F (7.2 °C).The Utah model assigns different weight to different temperature bands; a full unit per hour is assigned only to temperatures between 37 °F (2.8 °C) and 48 °F (8.9 °C). Maximum effect is achieved at 7 °C (44.6 °F). Temperatures between 55 °F (12.8 °C) and 60 °F (15.6 °C) (the threshold between chilling and warm weather) have zero weight, and higher temperature have negative weights: they reduce the beneficial effects of an already accumulated chilling hours.
Southwick et al. wrote that neither of these models is accurate enough to account for application of rest-breaking agents widely used in modern farming. They advocated the use of a dynamic model taylored to the two-stage explanation of dormancy.
External links
- Harvest Prediction Model for the counties and towns of California. University of CaliforniaUniversity of CaliforniaThe University of California is a public university system in the U.S. state of California. Under the California Master Plan for Higher Education, the University of California is a part of the state's three-tier public higher education system, which also includes the California State University...
Agricultural and Natural Resources. - gardenweb discussion of chilling requirements