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Various model of calculating of chill units in fruits crops -

Indian Farmer 5(04):439- 442; April -2018 Chhetri et al
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Various models to calculate chill
units in fruit crops
Ashok Chhetri*, Md. Ramjan and Narang Dolley
College of Horticulture and Forestry, Central Agricultural University, Pasighat,
Arunachal Pradesh
*Corresponding author:
Different temperate and subtropical fruit crops grown in different parts of the world requires
chilling temperature for certain period of time to break bud dormancy. Non availability of
favourable chilling temperature results in non flowering. Various models such as chilling hours
model, Utah model and dynamic model have been developed to assess the effective chill
temperature and duration for various fruit crops.
Keywords: Chilling, dynamic model, fruits, Utah model
Chilling requirement is defined as the number of effective chilling hours needed to
restore bud growth potential in spring (Richardson et al., 1974). Chilling refers to the
physiological requirement of low temperature to allow normal spring growth, and
failure to obtain sufficient winter chilling results in a marked decline in both yield and
fruit quality. The chilling requirement is typically measured in terms of numbers of
hours, during which temperature remains at or below 7°C during the winter season.
Winter chill is a necessary factor for deciduous fruits in temperate climates so that
latent buds can break the state of winter recess or endodormancy (ED) and begin
growing during the spring (Saure, 1985; Lang, 1987).
Temperate fruits are mainly produced in the middle latitudes ranging from 30° to 50° N
and S. Their cultivation may extends to lower latitudes (15°-30° N and S) at higher
altitudes and to higher latitudes where large water bodies and congenial climate is
available. In India, the study on seasonal and annual surface air temperature has shown
a significant warming trend of 0.57°C per hundred years (Pant and Kumar, 1997). The
global warming caused loss of vigour, fruit bearing ability, reduction in size of fruits,
less juice content, low colour, reduced shelf-life and increasing attack of pests resulting
in the low production and poor quality apple crop (Jangra and Sharma, 2013). The chill
units critical for apple production have exhibited a decreasing trend. Trend analysis
indicated that snowfall is decreasing at the rate of 82.7 mm/ annum in the entire region
Indian Farmer 5(04):439- 442; April -2018 Chhetri et al
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of Himanchal Pradesh (Gautam et al., 2014), consequently, the apple cultivation area is
moving further up in elevation because of the warmer climate.
1. Chilling hours model
The Chilling Hours Model is the oldest method to quantify winter chill (Chandler, 1942).
According to this model, temperatures between 0°C and 7.2°C are assumed to have a
chilling effect, with each hour at temperatures between these thresholds contributing
one chilling hour. Chilling hours are thus accumulated throughout the dormant season
and then summed up (Luedeling, 2012)
2. Utah model
The Utah Model developed in Utah, USA. It contains a weight function assigning
different chilling efficiencies to different temperature ranges, including negative
contributions by high temperatures. This model of chill units (CU) defines a CU as the
permanence of the buds for a period of 1 hour in a temperature range considered
optimum (2.5-12.5°C) to accumulate chill.
The Utah model is more complex because it introduces the concept of relative chilling
effectiveness and negative chilling accumulation (or chilling negation). According to
Richardson et al. (1974) temperatures between 0 and 16 ˚C promote the breaking of
rest, whereas temperatures > 16ΕC negate such effects. Maximum promotion occurs at
7 ˚C (1 h at 7 ˚C = 1 chill unit); higher and lower temperatures within the 0-16 ˚C range
are less effective.
The model is defined as:
1 hour below 34 ˚F = 0.0 chill unit
1 hour 34.01 36 ˚F = 0.5 chill unit
1 hour 36.01 48 ˚F = 1.0 chill unit
1 hour 48.01 54 ˚F = 0.5 chill unit
1 hour 54.01 60 ˚F = 0.0 chill unit
1 hour 60.01 65 ˚F = -0.5 chill unit
1 hour > 65.01 ˚F = -1.0 chill unit
The model presumes that chill accumulation occurs within a temperature range of 2.5
and 12.5°C, outside of which, the accumulation is nil or negative (Richardson et al.,
1974). This model, although it gives good results in cool and cold temperate climates,
yields a large quantity of negative chill values in sub-tropical climates and because of
this its utilization and applicability have been limited (Dennis, 2003). A modification of
this model consists of not considering the negative values of the Utah model, because of
which it has been termed the model of Positive Chill Units (PCU) and its application in
these sub-tropical zones has improved the results obtained (Linsley-Noakes et al.,
3. Dynamic model
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The Dynamic Model was developed in Israel (Fishman, 1987). It calculates chill in units
known as ‘chill portions’, based on hourly temperatures. According to the Dynamic
model, effective winter chill temperatures follow a bell shape with an optimum chilling
temperature at 6 °C, tapering to zero at 2 °C and 14 °C. High temperatures act to
negate previously accumulated chill and moderate temperatures can enhance chill
Initial temperature
Temperature = cold Temperature = warm
Sum of intermediate product ≥ threshold
Chill portion
(Irreversibly fixed)
Fig 1. Schematic representation of the Dynamic model (Darbyshire et al. 2011)
An intermediate product is produced through exposure to effective winter chill
temperatures. This intermediate product can be destroyed by subsequent exposure to
high temperatures. Once a threshold amount of this intermediate product is amassed it
is irreversibly banked as a chill portion (fig 1). Summing chill portions over autumn and
winter provides an estimate of accumulated winter chill. This complex model adds a
further element of timing of exposure to temperatures in a cycle and appears to be far
more accurate under warm winter conditions.
Several models of winter chill have been developed using the observed effects of
temperature on dormancy breaking. The Chill Hours model was the first to be
developed and estimates winter chill based on hourly temperatures. This is a ‘yes–no’
model with temperatures between 07.2 °C allocated 1 chill hour (yes) and
temperatures outside of that interval allocated a 0 chill hour (no). These chill hours are
summed over autumn and winter to give an estimate of total winter chill. Knowledge of
temperature effects on winter chill has since expanded and the Dynamic chill model is
the current best practice model. It calculates chill in units known as ‘chill portions’,
based on hourly temperatures. The Dynamic model has many features which capture
known temperature-winter chill relationships which are lacking in other models
including the Chill Hours model.
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Chandler, W. H. (1942). Deciduous orchards. Philadelphia, USA: Lea & Febiger, pp: 438.
Darbyshire, R., Webb, L., Goodwin, I. and Barlow, S. (2011). Winter chilling trends for
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Dennis, Jr. F. G. (2003). Problem in standardizing methods for evaluating the chilling
requirements for the breaking of dormancy in buds of wood plants. Hortscience. 38:
Fishman, S., Erez, A., Couvillon, G. A. (1987). The temperature dependence of dormancy
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temperatures. J. Theor. Biol. 126: 309-321.
Gautam, H. R., Sharma, I. M., Kumar, R. (2014). Climate change is affecting apple
cultivation in Himachal Pradesh. Curr. Sci. 106: 498-499.
Jangra, M. S. and Sharma, J. P. (2013). Climate resilient apple production in Kullu valley
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Linsley-Noakes, G., Louw, M. and Allan, P. 1995. Estimating daily positive Utah chill units
using daily maximum and minimum temperatures. J. S. Afr. Soc. Hortic. Sci. 5:19-22.
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production. A review. Sci. Hort. 144: 218-229.
Pant, G. B., Kumar, R. K. (1997). Climates of South Asia. Chichester: John Wiley & Sons
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