CALCULATION OF CALCULATION OF COLD UNITSCOLD UNITS

byby

Linda De WetLinda De Wet

INTRODUCTI0NINTRODUCTI0N

Dormancy in plantsDormancy in plants1.1. Endodormancy or rest periodEndodormancy or rest period

Plant will not grow - Plant will not grow - internalinternal restraintsrestraintsChilling hoursChilling hours

2. Ectodormancy2. EctodormancyConditions (Conditions (outside the plantoutside the plant) not ) not right, usually too coldright, usually too coldDormant period - after the rest Dormant period - after the rest periodperiod

Chilling hours required varies - 700 to Chilling hours required varies - 700 to 1300 hours or more1300 hours or more

If warm weather occurs before the If warm weather occurs before the completion of rest no growth occurscompletion of rest no growth occurs

This allows the plant to track the winter This allows the plant to track the winter and know when it is spring and know when it is spring

Chilling and rest normally prevent plants Chilling and rest normally prevent plants from beginning growth during warm spells from beginning growth during warm spells in the middle of the winter in the middle of the winter

This would cause the trees to lose cold This would cause the trees to lose cold hardiness, i.e. once growth begins the hardiness, i.e. once growth begins the plant cannot increase its ability to plant cannot increase its ability to withstand cold and can be injured by very withstand cold and can be injured by very cold weather cold weather

INTRODUCTI0NINTRODUCTI0N

Once the chilling requirement is met, the Once the chilling requirement is met, the plants have completed rest and are merely plants have completed rest and are merely dormant because the weather is too cold dormant because the weather is too cold for rapid growth for rapid growth

Chilling requirements are usually Chilling requirements are usually completed by midwinter. With the return completed by midwinter. With the return of warm weather the plant begins to growof warm weather the plant begins to grow

INTRODUCTI0NINTRODUCTI0N

CHILLING UNITSCHILLING UNITS Chilling Chilling

– Process whereby the plant measures Process whereby the plant measures timetime during winter. during winter. The plant tracks the The plant tracks the amount of time above 0 amount of time above 0 ooC (freezing) C (freezing) and below 10 and below 10 ooC C

– Actually temperatures between 4 Actually temperatures between 4 ooC and C and 7 7 ooC more practicalC more practical

Chill unitsChill units– Stimulate growthStimulate growth– Develop leaves/fruitDevelop leaves/fruit– Set fruitSet fruit

Chill units vary with species and locationChill units vary with species and location The accumulated minimum temperature The accumulated minimum temperature

must be less than the thresholdmust be less than the threshold Applications of chill units are toApplications of chill units are to

– Predict the end of the rest periodPredict the end of the rest period– Determine time for cultivation practicesDetermine time for cultivation practices– ID potential growth locationsID potential growth locations

Problems include the availablility of Problems include the availablility of hourly hourly weather data. Generally available weather data. Generally available are usually only daily Ta, Tmx and Tmnare usually only daily Ta, Tmx and Tmn

CHILLING UNITSCHILLING UNITS

Chilling periods = Vernalization = Chilling periods = Vernalization = Thermoperiodism = “to make springlike” Thermoperiodism = “to make springlike”

Needed in certain plants for flowering to Needed in certain plants for flowering to take placetake place

Occurs during a 2 - 6 week period with a Occurs during a 2 - 6 week period with a temperature < 10 temperature < 10 ooC C

These values can be nullified if Ta are These values can be nullified if Ta are high: 30 high: 30 ooC - 35 C - 35 ooCC

CHILLING UNITSCHILLING UNITS

CALCULATION OF CHILL UNITSCALCULATION OF CHILL UNITS The method to model hourly data is as follows:The method to model hourly data is as follows: Use Tmx and TmnUse Tmx and Tmn Use sine curves from sunrise to sunsetUse sine curves from sunrise to sunset Use log in night when cooling takes placeUse log in night when cooling takes place

– In sun:In sun:

Tt Tt == (Tmx-Tmn).sin[(Tmx-Tmn).sin[/(D+4)]+Tmn/(D+4)]+Tmn– In night:In night:

Tt Tt == Tss - [(Tss-Tmn)/{ln(24-D)}]ln(t)Tss - [(Tss-Tmn)/{ln(24-D)}]ln(t)

Where Tt Where Tt = = temperature at time t after temperature at time t after sunrise or at time t after sunsetsunrise or at time t after sunset

Tmx Tmx = = maximum temperaturemaximum temperatureTmn Tmn = = minimum temperatureminimum temperatureD D = = day length (hours)day length (hours)Tss Tss = = temperature at sunsettemperature at sunset

CALCULATION OF CHILL UNITSCALCULATION OF CHILL UNITS

Table 1 Use Tt to calculate chill units (CU), which are measured in hours (Utah model)

Tt (oC) CU

<1.4 0

1.4-2.4 0.5

2.4-9.1 1

9.1-12.4 0.5

12.4-16 0

16-18 -0.5

>18 -1

Table 2Table 2 Chill units for fruit Chill units for fruit

Fruit Chill units

Apples 800 - 1200 CU

Peaches 500 - 900 CU

Pears 500 - 550 CU

Almonds 90 - 427 at < 7 oC

Distribution pattern of accumulated positive chill units over South Africa Distribution pattern of accumulated positive chill units over South Africa during winter under baseline climate 1950 – 1999 conditions (observed during winter under baseline climate 1950 – 1999 conditions (observed

climate) (climate) (http://rava.qsens.net/themes/agriculture_template/school-of-bio-resources-engineering-and-environmental-hydrology-ukzn/ech_his_pcun_mean%20-map1.jpg/view

Table 3Table 3 Chill units for places in South AfricaChill units for places in South Africa

Place Robertson Paarl Clanwilliam Villiersdorp

Ceres Bokkeveld

Year

1988 247.5 355 441.5 528.5 719 1013.2

1989 369 414 458 597.5 577.5 695.5

1990 468.5 575 632 482 542.5 605.5

Ave 361.6 448 510.3 536 613 771.4

Table 4Table 4 Germination temperature for seedsGermination temperature for seeds

Seed Temperature (oC)

Minimum Optimum Maximum

Maize 8-10 32-35 40-44

Rice 10-12 30-37 40-42

Wheat 3-5 15-31 30-43

Barley 3-5 19-27 30-40

Rye 3-5 25-31 30-40

Oats 3-5 25-31 30-40

Buckwheat 3-5 25-31 35-45

Bindweed 0.5-3 20-35 35-40

Tobacco 10 24 30

SEEDSSEEDS

For seedsFor seeds Germination of seeds is low at low Germination of seeds is low at low

temperatures. temperatures. Optimal Ta = highest germination in Optimal Ta = highest germination in

shortest time.shortest time.

Table 5 Table 5 Upper limits for Chill unitsUpper limits for Chill units

Month Winter + all year rainfall

Summer Rainfall

May 300 250

June 400 360

July 450 350

August 450 300

September 350 300

EXAMPLE OF CLIMATE CHANGEEXAMPLE OF CLIMATE CHANGE

http://www.bbc.co.uk/blogs/gardening/2010/12/the-impact-of-http://www.bbc.co.uk/blogs/gardening/2010/12/the-impact-of-climate-change.shtmlclimate-change.shtml

Post categories: climate change, temperature

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 seaSome 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.

. 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

EXAMPLE OF CLIMATE CHANGEEXAMPLE OF CLIMATE CHANGE

CONCLUSIONSCONCLUSIONS

Chilling units is an important part of many Chilling units is an important part of many crops, vegetables and plants and is an crops, vegetables and plants and is an essential part of their normal developmentessential part of their normal development

CHILLING UNITS is an interesting topic but CHILLING UNITS is an interesting topic but requires more research before it can be requires more research before it can be fully understood and optimized to its fully understood and optimized to its maximum potentialmaximum potential

So, aspiring scientists – there is much work So, aspiring scientists – there is much work to be done!to be done!