CONCEPT OF CHILLING REQUIREMENT AND ITS SIGNIFICANCE IN HORTICULTURAL CROPS.Chilling Unit Models.Chilling requirement

Concept of Chilling Requirement and Its Significance in Horticultural Crops
Chilling Requirement in Fruit Crops: The chilling requirement of fruit crops refers to the minimum period of cold weather necessary for fruit-bearing trees to blossom, usually measured in chill hours or chilling units, which are calculated by summing the total time spent at specific temperatures that induce dormancy. Biological Basis: Chilling requirements are essential for vernalization, a biological process that prevents premature flowering or sprouting during brief warm periods in winter. Certain buds require a chilling period to bloom, and some seeds need chilling to sprout properly.

Chilling Units and Chilling Hours
Chilling units measure a plant's exposure to chilling temperatures, which can range from 7°C (or 16°C in some models) down to the freezing point. In the simplest model, one chilling unit equals one hour of exposure to these temperatures. Chilling units accumulate throughout the season and determine whether the plant's chilling requirement has been met. Stages in Chilling Requirement: Chilling occurs in two stages in fruit trees: 1. Reversible Chilling: This stage builds up dormancy precursors and can be reversed with rising temperatures. 2. Irreversible Dormancy: Once the precursor reaches a certain threshold, dormancy becomes irreversible and is unaffected by short-term warm periods. Planting low-chill cultivars in high-chill regions can lead to harvest losses, while planting high-chill cultivars in low-chill areas may result in no fruit production.

Chilling Unit Models
All chilling unit models require hourly temperature data to calculate the total chilling exposure. The simplest model assigns one chilling unit for every hour below 7°C, considering only temperatures between 0°C and 7°C, as temperatures below freezing do not contribute to proper dormancy. Basic Chilling Unit Model: The basic model assigns one chilling unit for every hour within the 0°C to 7°C range. The accumulated chilling units over the dormant season determine if the chilling requirement has been met. Steps to Calculate Chilling Units: 1. Temperature Monitoring: Record hourly temperatures throughout the dormancy period. 2. Temperature Range: Only temperatures between 0°C and 7°C are effective. 3. Accumulation: Each hour within this range counts as one chilling unit. 4. Summation: Total chilling units are summed up over the season. This model assumes a linear relationship between temperature and chilling accumulation but may not fully capture the complexity of the chilling process. Richardson Chilling Unit Model: The Richardson model, also called the Utah Model, provides a more refined approach by assigning different weights to various temperature ranges. It accounts for the fact that some temperatures are more effective at breaking dormancy than others. Temperature Weighting in the Richardson Model: Below 1.4°C: 0 units (no chilling effect); 1.5 to 2.4°C: 0.5 units .

1. Name of student : Akarshit Dhiman (MSc Fruit Science) Submitted to : Dr. F A Khan Sher-e-Kashmir University of Agricultural Sciences and Technology of Kashmir Concept of chilling requirments and its significance in horticultural crops

2. CHILLING REQUIREMENT 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 UNITS or CHILLING HOURS 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. STAGES IN CHILLING REQUIREMENT • 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. YOUR TITLE Lorem ipsum dolor sit amet , consectetur adipiscing elit . YOUR TITLE Lorem ipsum dolor sit amet , consectetur adipiscing elit . YOUR TITLE Lorem ipsum dolor sit amet , consectetur adipiscing elit . 8:00AM 9:00AM 10:00AM CHILL ING UNIT MODEL 2) The simplest model assign one chilling unit for every full hour of temperature below 7°C. 3) Only hours between o to 7°C is counted as temperature below freezing point do not contribute to proper dormancy. 1) All model requires hourly requirement of temperatures.

6. Temperature below 1.4 °C has o 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. CHILLING UNIT MODEL BY RICHARDSON 01 02 03

7. Plants that Require Chilling

8. 1 2 3 4 All fruits that grows in temperate zone e.g. apple, blueberries, peach, grapes, pears etc. Vegetables like cabbage, carrot, sugar beet etc needs some chill to produce seeds. Chilling in orange and other citrus produces more flowers and better tasting fruits. All bulb plants require chilling units Plants that Require Chilling

9. CHILLING UNIT REQUIREMENTS BY FRUIT CROPS

10. APPLE APRICOT CHERRY ALMOND PEAR LOW CHILLING VARITIES

11. Anna, Beverly hills, Gordon, sundowner Gold Keist , Flora gold Lapins , Stella Hood, orient All-In-One, Garden Prince APPLE ALMOND APRICOT PEAR CHERRY LOW CHI LLING VARIETIES

12. Rest breaking Agents • 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 chillingbut 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.

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