CALCULATION OF COLD UNITS

CALCULATION OF COLD UNITS by Linda De Wet

INTRODUCTI0N • Dormancy in plants 1. Endodormancy or rest period Plant will not grow - internalrestraints 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 • In night: Tt = Tss - [(Tss-Tmn)/{ln(24-D)}]ln(t) Where Tt = temperature at time t after sunrise or at time t after sunset Tmx = maximum temperature Tmn = minimum temperature D = day length (hours) Tss = temperature at sunset

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

Table 2 Chill units for fruit

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-environmental-hydrology-ukzn/ech_his_pcun_mean%20-map1.jpg/view

Table 3 Chill units for places in South Africa

Table 4 Germination temperature for seeds

SEEDS • For seeds • Germination of seeds is low at low temperatures. • Optimal Ta = highest germination in shortest time.

Table 5 Upper limits for Chill units

Post categories: climate change,temperature EXAMPLE OF CLIMATE CHANGE http://www.bbc.co.uk/blogs/gardening/2010/12/the-impact-of-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!

THANK YOU

CALCULATION OF COLD UNITS