Transcript CALCULATION OF COLD UNITS - Learning

CALCULATION OF
COLD UNITS
by
Linda De Wet
INTRODUCTI0N
 Dormancy
in plants
1. Endodormancy or rest period
Plant will not grow - internal
restraints
Chilling hours
2. Ectodormancy
Conditions (outside the plant) not
right, usually too cold
Dormant period - after the rest
period
INTRODUCTI0N
Chilling hours required varies - 700 to
1300 hours or more
 If warm weather occurs before the
completion of rest no growth occurs
 This allows the plant to track the winter
and know when it is spring
 Chilling and rest normally prevent plants
from beginning growth during warm spells
in the middle of the winter
 This would cause the trees to lose cold
hardiness, i.e. once growth begins the
plant cannot increase its ability to
withstand cold and can be injured by very
cold weather

INTRODUCTI0N


Once the chilling requirement is met, the
plants have completed rest and are
merely dormant because the weather is
too cold for rapid growth
Chilling requirements are usually
completed by midwinter. With the return
of warm weather the plant begins to grow
CHILLING UNITS
Chilling
– Process whereby the plant measures
time during winter. The plant tracks the
amount of time above 0 oC (freezing)
and below 10 oC
– Actually temperatures between 4 oC and
7 oC more practical
 Chill units
– Stimulate growth
– Develop leaves/fruit
– Set fruit

CHILLING UNITS
Chill units vary with species and location
 The accumulated minimum temperature
must be less than the threshold
 Applications of chill units are to

– Predict the end of the rest period
– Determine time for cultivation practices
– ID potential growth locations

Problems include the availablility of
hourly weather data. Generally available
are usually only daily Ta, Tmx and Tmn
CHILLING UNITS
Chilling periods = Vernalization =
Thermoperiodism = “to make springlike”
 Needed in certain plants for flowering to
take place
 Occurs during a 2 - 6 week period with a
temperature < 10 oC
 These values can be nullified if Ta are
high: 30 oC - 35 oC

CALCULATION OF CHILL UNITS




The method to model hourly data is as follows:
Use Tmx and Tmn
Use sine curves from sunrise to sunset
Use log in night when cooling takes place
– In sun:
Tt
=
(Tmx-Tmn).sin[/(D+4)]+Tmn
Tt
=
Tss - [(Tss-Tmn)/{ln(24-D)}]ln(t)
=
temperature at time t after
sunrise or at time t after sunset
maximum temperature
minimum temperature
day length (hours)
temperature at sunset
– In night:
Where Tt
Tmx
Tmn
D
Tss
=
=
=
=
CALCULATION OF CHILL UNITS
Table 1
Use Tt to calculate chill units (CU), which
are measured in hours (Utah model)
o
Tt ( C)
CU
<1.4
0
1.4-2.4
2.4-9.1
0.5
1
9.1-12.4
0.5
12.4-16
0
16-18
-0.5
>18
-1
Table 2
Chill units for fruit
Fruit
Chill units
Apples
800 - 1200 CU
Peaches
500 - 900 CU
Pears
500 - 550 CU
Almonds
o
90 - 427 at < 7 C
Distribution pattern of accumulated positive chill units over South Africa
during winter under baseline climate 1950 – 1999 conditions (observed
climate) (http://rava.qsens.net/themes/agriculture_template/school-of-bio-resources-engineering-and-environmentalhydrology-ukzn/ech_his_pcun_mean%20-map1.jpg/view
Table 3
Place
Year
Chill units for places in South Africa
Robertson Paarl Clanwilliam Villiers
dorp
528.5
Ceres
Bokkeveld
719
1013.2
1988
247.5
355
441.5
1989
369
414
458
1990
468.5
575
632
482
542.5
605.5
Ave
361.6
448
510.3
536
613
771.4
597.5 577.5
695.5
Table 4
Germination temperature for seeds
o
Seed
Temperature ( C)
Minimum
Optimum
Maximum
Maize
Rice
Wheat
8-10
10-12
3-5
32-35
30-37
15-31
40-44
40-42
30-43
Barley
Rye
Oats
3-5
3-5
3-5
19-27
25-31
25-31
30-40
30-40
30-40
Buckwheat
3-5
25-31
35-45
0.5-3
20-35
35-40
10
24
30
Bindweed
Tobacco
SEEDS
 For
seeds
 Germination of seeds is low at low
temperatures.
 Optimal Ta = highest germination in
shortest time.
Table 5
Month
Upper limits for Chill units
Winter + all
year rainfall
Summer
Rainfall
May
300
250
June
400
360
July
450
350
August
450
300
September
350
300
Post categories: climate change, temperature
EXAMPLE OF CLIMATE CHANGE
http://www.bbc.co.uk/blogs/gardening/2010/12/the-impactof-climate-change.shtml
Jim McColl | 16:02 UK time, Thursday, 9 December 2010
Global Warming? Huh! Let’s face a couple of incontrovertible facts:
•We are an off-shore island.
•We have therefore a maritime climate – a lack of extremes, plenty of moisture all
year round but unpredictable because of the all-pervading influence of the sea
Some are sceptical about climate change but I think most people would sign up to
the fact that something is happening. Until last winter (09/10) and the present one
to date it was being argued that winters would be milder but wetter. Gardeners
would be worse off because milder winters would lead to less winterkill of
damaging organisms. Is this just a blip on last year's blip?
Evidence that winters are getting milder in some areas has been shown in a serious
way, and affecting food production. Here is one example.
EXAMPLE OF CLIMATE CHANGE
A very high percentage of the blackcurrant varieties; now grown in Britain
.
for commercial
fruit production, were bred at the Scottish Crop Research
Institute; they carry the ‘Ben’ prefix. To be technical, all temperate fruits –
apples, pears, plums, soft fruit must experience a period of low
temperature to complete their annual physiological life cycle. It is
referred to as the ‘chilling requirement’ and will vary from one fruit species
to another and indeed one cultivar to another.
The use of the word ‘chilling’ may be slightly misleading. When I worked in
this area of endeavour we called them units of ‘coolth’ (as opposed to
warmth!) The plants collect these low temperature ‘units’ cumulatively, they
‘bank’ the low temp degrees, which must then add up to a given figure if the
cycle is to be completed normally.. For example, in the dormant season,
the blackcurrant variety Ben Lomond must collect 2000 hrs below 7.2
°C and if they don’t get them, problems will arise. It has been recorded
that commercial plantations of some of these varieties in the south of
England have reported erratic bud break, leading to a reduction in fruit
quality. This is one of the classic symptoms of not reaching the chill
totals. In layman’s terms, the winters have been too mild
CONCLUSIONS
Chilling units is an important part of many
crops, vegetables and plants and is an
essential part of their normal development
 CHILLING UNITS is an interesting topic
but requires more research before it can
be fully understood and optimized to its
maximum potential
 So, aspiring scientists – there is much
work to be done!
