Chilling requirement refers to the minimum period of cold weather that fruit-bearing trees need to blossom. It is measured in chilling hours or units. Trees need to accumulate a certain number of chilling units below a threshold temperature, typically between 0-7°C, for dormancy to be broken and blossoming to occur properly. Insufficient chilling can lead to reduced and lower quality yields or even a complete loss of harvest for the year. Different fruits have different chilling requirements that must be considered when selecting varieties for a given region's climate conditions.

1. BY: AYUSH MISHRA

2. The chilling requirement of a fruit crop is the
minimum period of cold weather after which a
fruit-bearing tree will blossom. It is often expressed
in chill hours or chilling unit. The calculation of
chilling hours involve adding of the total amount of
time in a weather spent at certain temperature.
Some buds have chilling requirement to bloom and
some seeds have chilling requirement to sprout.
Biologically, the chilling requirement is a way of
ensuring that vernalisation occurs.

3.  Chilling unit in agriculture is a metric of plants
exposure to chilling temperature. Chilling temperature
extends from 7°C or even 16°C to freezing point
depending upon different models.
 One chilling unit in the simplest model is equal to one
chilling hour exposure to chilling temperature. These
units are summed up for whole season to calculate
total chilling units.

4.  Chilling in trees acts in two stages. The first is
reversible Chilling which helps to build up the
precursor 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 temperature peak.
 Planting of low chilling cultivars in a region of high
chill risks in a loss of a years' harvest while planting a
high chilling cultivar in a region of low chill will quite
likely never give fruits at all.

5.  All model requires hourly requirement of
temperatures.
 The simplest model assign one chilling unit for every
full hour of temperature below 7°C.
 Only hours between 0 to 7°C is counted as
temperature below freezing point do not contribute to
proper dormancy.

6.  Temperature below 1.4 °C has 0 count.
 One hour of Temperature between 1.5 to 2.4 °C is
assigned 0.5units.
 One hour of temperature between 2.5 to 9.1 °C is
assigned 1 unit.
 Temperature above 16°C has been assigned negative
weight.

7.  All fruits that grows in temperate zone e.g. apple,
blueberries, peach, grapes, pears etc.
 Chilling in orange and other citrus produces more
flowers and better tasting fruits.
 Vegetables like cabbage, carrot, sugar beet etc needs
some chill to produce seeds.
 All bulb plants require chilling units.

8. FRUITS NO. OF CHILL HOURS
ALMOND 500-600
APPLE 300-2000
APRICOT 500-600
BLACKBERRY 200-500
BLUEBERRY 800
CHERRY 700-800
GOOSEBERRY 800-1000
GRAPE 100+
KIWI 600-800
PEACH 600-800
PLUM 400
POMEGRANATE 100-200
STRAWBERRY 200-300
WALNUT 600-700

9.  The tree leaf out later (delayed foliation).
 Flower don't develop properly & open over a long period of
time.
 Pollination doesn't happen properly.
 Pollenizers don't overlap sufficient in bloom to provide
right pollen.
 Fruit yield and quality are reduced.
 Deficiency of 50-100 units may result in loss up to 50% of
harvest.
 Deficiency of 250 units or more can lead to complete loss of
harvest. The few fruit that formed will be of poor quality
and no market value.

10.  Some chemical substance like hydrogen cynamide
when applied in spring can partially mitigate the
effects of insufficient chilling.
 BudPro can substitute for up to 300 hours and chilling
but excessive spraying and timing error can also
damage buds.
 TDZ can be applied in apple to prevent bud breaking
by applying before initiation of chilling.
 Gibberellins (50-200ppm) can be used to break
dormancy in cherry and peach.

11.  Apple Anna, Beverly hills, Gordon, sundowner
 Almond All-In-One, Garden Prince
 Apricots Gold Keist, Flora gold
 Blueberry Emerald, Jubilee, Pink lemonade
 Cherry Lapins, Stella
 Grapes All varities
 Pears Hood, orient
 Peaches Babcock, Sweet bagel
 Walnuts Pedro, Placentia

12.  Growing degree unit is a measure of heat accumulation
used to break ectodormency.
 It is used to assess the suitability of a region to production
of a particular crop,estimate growth stages of crops, weeds
& insects and predict timing of fertilizer and pesticides.
 Degree day calculations:
 Degree days= (Max + Min)/2 - base temperature
 10°C is the most common base for GDD calulation but it
depends on like cycle of plant.
 For Wheat, Barley & rye it is 5.5 °C.
 For potato & sunflower it is 8°C.
Growing degree hour requirement or
heat unit requirement