Chilling requirement
The chilling requirement refers to the cumulative exposure to low winter temperatures that certain plants, particularly temperate deciduous fruit trees and some perennial species, must accumulate during dormancy to break endodormancy—a physiological state of rest—and enable synchronized bud break, flowering, and fruit development in spring.[1] This process, sometimes associated with vernalization in herbaceous plants, ensures that growth aligns with favorable seasonal conditions, preventing premature or erratic budding that could lead to frost damage.[2] Quantified primarily through chill hours—hours of exposure to temperatures between 0°C and 7.2°C (32°F and 45°F), with optimal chilling around 7°C (45°F)—the requirement varies widely by species and cultivar to match regional climates.[3][4] For example, low-chill peach varieties like 'Florida Prince' need only 150 chill hours, while high-chill apples such as 'Golden Delicious' may require 1,000–1,500 hours for reliable production.[4] Several models refine these calculations: the simple Chilling Hours model counts all qualifying hours equally; the Utah model assigns weighted chill units (e.g., 1 unit for 2.5–9.1°C, with negation by temperatures above 16°C); and the Dynamic model accumulates reversible "chill portions" that become permanent after a threshold, better accounting for warm interruptions in warming climates.[5] In horticulture and agriculture, meeting the chilling requirement is critical for yield and quality, as inadequate accumulation causes delayed, uneven flowering, reduced fruit set, smaller or deformed produce, and long-term tree decline.[4][2] This is especially vital for crops like peaches (400–1,050 hours), pears (800–1,100 hours), and blueberries (900–1,000 hours), where mismatched varieties in warm regions lead to economic losses.[4] Gardeners and growers select cultivars based on local chill hour averages—such as 1,000–1,500 in the U.S. Pacific Northwest—to optimize performance, while tools like online chill calculators aid in forecasting and site selection.[2][5]Biological Basis
Definition and Mechanism
The chilling requirement denotes the minimum duration of exposure to low winter temperatures, typically in the range of 0–7.2°C (32–45°F), that temperate woody perennials, particularly fruit and nut trees, must accumulate to overcome endodormancy and facilitate synchronized bud break, flowering, and fruiting.[6] This process ensures that plants remain dormant during unfavorable winter conditions and resume growth only upon sufficient cold accumulation, preventing premature development in response to sporadic warm spells.[7] Within this range, temperatures between 2–9°C are most effective for accumulating chilling, whereas those below 0°C or above 16°C either fail to contribute or actively counteract the process by negating prior chill gains.[6] The concept of chilling requirement was first formalized in the mid-20th century through studies on peach trees (Prunus persica), stemming from observations in the early 1940s of irregular bud break and flower bud abortion following mild winters in California.[6] Researchers noted that insufficient cold exposure led to delayed or uneven dormancy release, prompting quantitative assessments; for instance, J.H. Weinberger's 1950 work in Georgia established early models linking hours of cold to dormancy breaking in peaches.[6] Physiologically, chilling releases endodormancy by inducing shifts in hormone balances, notably decreasing levels of abscisic acid (ABA)—a promoter of dormancy—and elevating gibberellins (GA), which antagonistically counteract ABA to initiate growth resumption.[8] Concurrently, cold exposure triggers epigenetic modifications in meristematic tissues, including histone alterations such as trimethylation of histone H3 at lysine 27 (H3K27me3), which repress dormancy-associated genes like DORMANCY-ASSOCIATED MADS-BOX (DAM) factors and reprogram chromatin for spring activation.[9] These changes prepare apical and axillary meristems for subsequent forcing by warmer temperatures, culminating in timely blooming.[7] Chilling thus interacts with post-winter heat unit accumulation to precisely time full bloom.[6]Role in Plant Dormancy
Plant dormancy is classified into three sequential phases: paradormancy, endodormancy, and ecodormancy. Paradormancy involves inhibition by signals from other plant parts, such as apical dominance suppressing lateral buds. Endodormancy represents an internal physiological block that prevents growth regardless of environmental conditions, requiring exposure to low temperatures to resolve. Ecodormancy occurs when external factors, like unfavorable temperatures or water availability, inhibit growth after endodormancy has been broken.[10][11] The chilling requirement specifically addresses endodormancy by fulfilling the necessary cold exposure, typically in the range of 0–7°C, to release buds from this internal inhibition and enable subsequent growth phases. This process ensures that plants do not resume growth prematurely during mid-winter warm spells, which could expose tender tissues to subsequent frosts. Without adequate chilling, endodormancy persists, leading to delayed or uneven bud break, reduced flower quality, and lower yields due to asynchronous flowering and poor pollination.[12][13][14] Chilling accumulation occurs progressively, where partial fulfillment yields a partial response, such as limited bud break in terminal positions before lower buds, promoting uneven but initial growth. Full satisfaction of the chilling requirement synchronizes bud break across the plant, optimizing resource allocation and uniform development for reproductive success. This mechanism has evolved in temperate species as an adaptation to align bud burst and flowering with reliable spring conditions, minimizing risks from variable winter thaws and ensuring offspring survival in seasonal climates.[15][16][17]Measurement Methods
Chilling Hours Concept
The chilling hours concept quantifies the exposure of temperate fruit trees to cold temperatures during their winter dormancy period, serving as a basic metric for estimating the fulfillment of chilling requirements needed to break dormancy and resume growth. It counts the total number of hours when air temperatures remain within a narrow optimal window of 0–7.2°C (32–45°F), as defined in the foundational Weinberger model.[18] This accumulation generally occurs from November to February in temperate zones, aligning with the period of endodormancy when trees are most responsive to chilling cues.[19][20] Developed by John H. Weinberger in 1950 specifically for peach cultivars, the chilling hours approach assumes a straightforward, linear buildup of effective cold exposure without any weighting or adjustments for suboptimal temperatures.[21] This model emerged from observations of peach budbreak variability in response to winter cold, establishing a simple framework that counts only full clock hours within the specified range, treating each qualifying hour equally regardless of exact temperature within the window.[22] The simplicity of calculation makes chilling hours a practical tool for growers: temperatures are monitored hourly (often via weather stations), and hours falling between 0°C and 7.2°C are tallied directly, with no corrections for day length, the sequence of cold and warm periods, or partial hours. For instance, a cultivar with a 500 chilling hours requirement would need precisely 500 such clock hours to accumulate sufficient chill for uniform budbreak.[18][23] Despite its foundational role, the chilling hours model has notable limitations, as it tends to overestimate effective chilling in mild winters interrupted by brief warm spells and fails to incorporate the counteractive effects of elevated temperatures above 7.2°C that can negate prior accumulation.[24] These shortcomings become evident in regions with variable or warming winters, where the unweighted linear count does not reflect the nuanced physiological responses to temperature fluctuations.Chilling Units Calculation
Chilling units represent an advanced approach to quantifying winter chilling for temperate fruit trees, assigning differential weights to hours at various temperatures to better reflect the non-linear effectiveness of cold in fulfilling dormancy requirements, unlike the simpler unweighted chilling hours method. This weighting acknowledges that temperatures near 7°C are most effective, while milder or warmer conditions contribute less or even negate accumulation.[25] The seminal Utah model, introduced by Richardson et al. in 1974, calculates chilling units (CU) as the cumulative sum of temperature-specific weights applied to each hour during the dormancy period, typically from November to February in temperate regions. Under this model, CU accumulation proceeds until a cultivar-specific threshold is reached, signaling sufficient chilling for uniform bud break and flowering. The model uses hourly temperature data, though approximations from daily minima and maxima are common for practical applications.[25][20] The core of the Utah model is a step-function weight table, where each hour's contribution is determined as follows:| Temperature Range (°C) | Weight (units per hour) |
|---|---|
| ≤ 1.4 | 0 |
| 1.4–2.4 | 0.5 |
| 2.5–9.1 | 1.0 |
| 9.2–12.5 | 0.5 |
| 12.6–16.0 | 0 |
| 16.1–18.0 | -0.5 |
| > 18.0 | -1.0 |