Chill Hour Research Articles

Pistachio Production Management in Iran by Rest-breaking Treatment and Comparison of Dynamic vs Chill Hour Model for Chill Accumulation

Pistachio Production Management in Iran by Rest-breaking Treatment and Comparison of Dynamic vs Chill Hour Model for Chill Accumulation

County-Specific Chill Hours Accumulation in South Carolina

Winter chilling is critical for most temperate fruits and perennial plants during the winter season. Most fruit and nut trees require a prolonged period of chilling to break their dormant stage and bloom when spring arrives. This research’s primary objective was to calculate the chill hour’s accumulation in each county in South Carolina based on the historical hourly air temperature data for the last ten years (2010-2020). The chilling hours model used to calculate the daily chill hours was based on the number of hours when the air temperature was between 32°F to 45°F (0 to 7.2°C). The total chill hours for each county were then determined by accumulating the daily chill hours from October to June. Our results showed that among the different counties in South Carolina, on average Laurens County had the most prolonged chill hours (1419 hrs). The chill hours were higher between November to March, and counties near the coastal regions had fewer chill hours than the counties in the inland areas. For example, Beaufort, Charleston, Berkeley, Horry, and Dorchester counties that are located near the coastal region had fewer chill hours. In contrast, counties located in the inland areas like Laurens, Chester, Lancaster, and York recorded the most prolonged chill hours. Our results suggest that selecting high chilling requirement crops for the inland areas and low chilling requirement crops for coastal areas would be appropriate. Farmers in South Carolina can use this information to plan their crop selection and management.

Chilling and heat requirement of peach cultivars and changes in chilling accumulation spectrums based on 100-year records in Republic of Korea

Chilling and heat requirements of peach cultivars and spectrums of chilling accumulation over the last 100 years (1919–2018) were evaluated to determine phenological characteristics for dormancy break in Republic of Korea. Chilling requirement was calculated by using Chill Hours, Utah, Dynamic, North Carolina, and Low Chilling models and heat requirement was estimated in growing degree hours (GDH). Chilling requirement of 15 cultivars ranged between 263 and 2123 chill hour (CH), 377 and 1134 chill unit (CU), and 21.3 and 74.8 chilling portion (CP). The Dynamic model showed the highest precision with the smallest variation among years, followed by the Utah model. Heat requirement ranged from 4824 to 5506 GDH and was positively correlated with flowering date. During last 100 years, the initiation date of chilling accumulation had been delayed for 10–12 days in Chill Hours and Utah models. In contrast, the Dynamic model showed no significant changes in chilling accumulation spectrum. Although the chill accumulation was enough to fulfill the chilling requirement so far in Republic of Korea, reduced chills were monitored from 100-year data. Therefore, continuous monitoring or estimation of chilling accumulation in the future is needed.

Implications of climate model selection for projections of decision-relevant metrics: A case study of chill hours in California

Decision-makers today have relatively easy web-based access to climate projections from several different models and downscaled datasets. Yet, there is minimal guidance on the credibility and appropriate use of such models and projections for specific adaptation contexts. The few studies that provide recommendations on model choice are often based on evaluations of broad physical climate metrics (such as temperature averages or extremes) at regional scales, without additional examination of local-scale decision-relevant climatic metrics (such as growing degree days or chill hours) that underpin the adaptation action. While such broad regional skill may be considered necessary for the overall credibility of models, it is not clear whether it is sufficient to ensure good skill for decision applications. This paper evaluates the skill of different Global Circulation Models (GCMs) in predicting the decision-relevant metric of chill hours in Fresno, California, and examines how model selection impacts future projections. We find that good skill in predicting broader physical climate metrics in California does not guarantee skill in prediction of chill hours in Fresno. In fact, the models with good regional climatic skill were mutually exclusive of the ones with good skill for chill hours, which leads to some counterintuitive results for this unique metric. Since many decision-relevant metrics are non-linear derivations of primary physical quantities (like the chill hour metric), more such decision-relevant model evaluations are needed to provide better insights on model credibility and choice for adaptation decisions.

A Web-Based Chill Hours App for Fruit Growers

Many fruit plants have a chill hour requirement for breaking dormancy. Various models estimate chilling, but two models are commonly used for fruit crops. From 2000 to 2016, chill hour data were collected by Mississippi State University experiment station employees as well as by local fruit producers; however, the number of participants in the process dwindled over time. To address this issue, we developed a mobile-friendly web application to interface with Weather Underground data. From these data, fruit growers can assess growing conditions that affect plant physiology and prepare for events in the upcoming season. Extension professionals can introduce the app to fruit growers in their areas.

Estimation of factors associated with the occurrence of white striping in broiler breast fillets

Broiler breast fillets are sometimes characterized grossly by white parallel striations in the direction of the muscle fibers, and the condition is referred to as white striping. Depending on the severity of white striping, fillets can be classified as normal (NORM), moderate (MOD), or severe (SEV). The present study was intended to determine the factors associated with the occurrence of white striping in broiler breast fillets. Broiler birds (59 to 63 d) of 4 different commercial high-yielding strains (both males and females) fed with industrial type or phase-feeding regimens, were processed and ready-to-cook carcass weight was recorded. The carcasses were deboned at either 4 or 6 h postmortem. Fillets were scored for the degree of white striping at 24 h postmortem, and dimensions of fillets (length, width, cranial thickness, and caudal thickness), pH, color (L*, a* and b* values), cook loss, and Meullenet-Owens razor shear energy (MORSE) values were determined. About 55.8% of the birds used in the study showed some degree of white striping with MOD and SEV categories as 47.5 and 8.3%, respectively. Higher degrees of white striping were significantly (P < 0.05) related to higher cranial fillet thickness and ready-to-cook weights. The occurrence of SEV degrees of white striping was accompanied with increased b* values or yellowness of the meat. The growth differences in strains could influence the incidence of this condition, but feeding regimens and chill hour during processing did not. In addition, the degree of white striping did not show any significant (P > 0.05) relationship between various meat quality parameters such as pH, L*, a*, cook loss, and MORSE. In conclusion, the results of this study suggest that there is a greater chance of higher degrees of white striping associated with heavier birds, but the condition is not related to any major changes in cooked meat quality.

ENVIRONMENTAL CHALLENGES OF BREEDING PEACHES FOR LOW CHILL REGIONS

Peach production throughout the world suffers from a multitude of environmental challenges related to both temperature and humidity but it is mainly the stress caused by excessive warmth that makes the low chill regions (<500 chill hour accumulation) of the world unique from those of the high chill production zones. These mild winter environments range from the cool highland mountains of Mexico where peaches can be induced to flower in the summer months so the fruit is harvested in the off season winter months, to hot dry deserts where flower induction is needed to synchronize bloom in the absence of a distinct cool season, to the humid and cool spring lowland environment of southern Brazil, to the humid but warm spring lowland environment of south and central Texas, and to the greenhouse environment of peach protected culture in Asia. The major abiotic environmental challenges are for adaptation to low chilling accumulation, cold damage during bloom, and in many cases also to high heat especially during bloom time but also during subsequent growth of the fruit and the tree. These factors affect productivity as well as fruit ripening time, size and quality.

Accumulated winter chill is decreasing in the fruit growing regions of California

We examined trends in accumulated winter chill across the fruit growing region of central California and its internal coastal valleys. We tested the hypothesis that global warming is in motion in California and is causing accumulated winter chill to decrease across the fruit and nut growing regions of California. The detection of potential trends in accumulated winter chill (between 0 and 7.2°C) was determined using two complementary climate datasets. The California Irrigation Management Information System (CIMIS) contains hourly climate data and is suitable for computing accumulated chill hours and chill degree-hours. But, its longest data records extend back only to the 1980s. The National Weather Service Coop climate record is longer, extending beyond the 1950s at many sites. But its datasets only contain information on daily maximum and minimum temperatures. To assess long term trends in winter chill accumulation, we developed an algorithm that converted information from daily maximum and minimum temperature into accumulated hours of winter chill and summations of chill-degree hours. These inferred calculations of chill hour accumulation were tested with and validated by direct measurements from hourly-based data from the CIMIS network. With the combined climate datasets, we found that the annual accumulation of winter chill hours and chill degree hours is diminishing across the fruit and nut growing regions of California. Observed trends in winter chill range between -50 and -260 chill hours per decade. We also applied our analytical algorithm to project changes in winter chill using regional climate projections of temperature for three regions in the Central Valley. Predicted rates of reduced winter chill, for the period between 1950 and 2100, are on the order of -40 h per decade. By the end of the 21st century, orchards in California are expected to experience less than 500 chill hours per winter. This chronic and steady reduction in winter chill is expected to have deleterious economic and culinary impact on fruit and nut production in California by the end of the 21st Century.

Advanced generation peach-almond hybrids as seedling rootstocks for almond: first year growth and potential pollenizers for hybrid seed production

For decades, seedling peaches have been used as the standard rootstock in California almond orchards. Vigorous, deep rooted trees are needed in almond orchards for maximum yields and to withstand the annual tree-shaking at harvest. Currently, researchers are actively evaluating rootstocks for almonds in field trials and in various screening protocols. In this study, seedling rootstocks, obtained from male-sterile advanced generation peach-almond (PEAL) hybrid mother trees, were compared with ‘Nemaguard’ peach seedlings for emergence in the nursery row, trunk caliper at propagation time, and end of season dormant above ground tree weight. Seedling emergence was affected significantly (P ≤ 0.05) by seed source, as was trunk caliper and end of season dormant above ground tree weight. Trunk caliper and dormant above ground tree weight were also affected significantly (P ≤ 0.05) by planting year. Results obtained in this study demonstrate the enhanced first year growth from seedlings of advanced generation PEAL hybrids, as compared to ‘Nemaguard’ seedlings. Seedling emergence in the rootstock bed was not affected significantly by planting year. Advanced generation PEAL hybrid seedlings were ready for June-budding at an earlier date compared to ‘Nemaguard’ seedlings, providing the potential for larger-sized finished nursery stock by the end of the growing season. Due to the male-sterile status of the advanced generation PEAL mother trees, bloom periods of several root-knot nematode resistant rootstock cultivars were examined for their degree of synchronicity with the mother trees. The examined rootstocks and mother trees varied in both chill hour and post-chill heat requirements necessary to effect bloom. ‘Flordaguard’ peach rootstock began bloom in advance of the male-sterile mother trees, whereas the bloom period of ‘Tsukuba No. 4’ occurred well after, suggesting they would not be effective synchronous pollenizers for consistent hybrid seed production. Based on more limited flowering period data, better bloom synchronicity was achieved with a Tsukuba No. 4 X Flordaguard hybrid.

METHOD TO DETERMINE CHILLING REQUIREMENT IN BLUEBERRIES

A chilling study was initiated in January 1998 on 'Tifblue' rabbiteye (Vaccinium ashei Reade) and blueberry plants that had received > 500 chilling hours (<7'C). The terminal 2 flower buds from both excised stems and paired stems on plants were forced in a greenhouse and rated for floral bud development. After 23 days of forcing, flower buds from all treatments had reached stage 3, an easily recognizable stage of development. Excised stems did not differ from matching intact stems for the first 3.5 weeks of forcing, but after ?4.5 weeks of forcing in the greenhouse floral bud development ratings on excised stems were significantly lower than those on intact stems. Another study was started in October 1999 to determine the accuracy and efficiency of using cuttings from plants to determine the sufficient accumulation of chilling for bloom. In this study 2 rabbitcyc (Tifblue and Climax) and 3 southern highbush (V. corv,nbosun, x) (Jubilee, Pearl River and Biloxi) cultivars were naturally chilled. At 100 chilling hour (<7 C) intervals, plants and cuttings were placed in a greenhouse and floral development was rated weekly. In this study the rate of floral development generally did not significantly differ between buds on excised stems and stems on intact plants for the first 3 weeks of forcing. We have found that for 3.5 weeks, buds on excised stems develop the same as those on intact plants. We therefore propose that chilling requirements of a plant are satisfied if a floral bud reaches stage 3, an easily identifiable stage of flowering in which the floral apices are visible, within a 3 week period of forcing. Using this method to determine chilling hours required to break dormancy, data indicated that 'Biloxi' required 200 hours 7°C, 'Climax' - 300 hours, 'Tifblue', 'Jubilee', and 'Pearl River' - 400 hours. These data approximate field observations on blooming sequence of these eultivars. These studies indicate that floral bud development in excised stems approximate that of intact plants for a period of 3 1/2 weeks and with sufficient chilling reach a stage of at least 3 within this period of forcing. These studies indicate that cuttings from blueberry plants can be used to determine approximate chill hour requirements.

(102) The Dynamic Model and Rest-breaking Agents in California `French' Prune Production

Use of rest-breaking chemicals may partially substitute for chill requirement in “French' prune. Many California prune growers use oil in the dormant season to tighten and advance bloom, with application timing judged by experience and calendar date. Other rest-breaking agents have become commonly used in California cherry production and their application is generally timed by chill portion accumulation, calculated by the Dynamic Model. We evaluated the effects of treatments of dormant oil or CAN17 (calcium ammonium nitrate) + Entry on budbreak and bloom progression in `French' prune with applications timed at regular intervals. While most treatments improved fruit set and reduced reproductive bud death, an optimum range for both types of rest-breaking treatments was found for advancement and compression of bloom. All rest-breaking treatments advanced fruit maturity equally, compared to the untreated control, as measured by fruit firmness. Although chill hour (hours ≤7°C) calculations might also be used for timing these treatments, when chill portion and chill hour accumulations are compared for the 2004–05 dormant season at several different sites, differences from site-to-site are small for chill portions, and much greater for chill hours. This fact supports experimental evidence from numerous California trials in sweet cherry in which rest-breaking treatment timings based on the Dynamic Model tend to be more consistent than the timings based on the “chill hour” model.

Evaluation of Chill Models in `Bing' Sweet Cherry Rest-breaking Trials in California from 1994 to the Present: Validation of the Dynamic Model

Moderate California winters often result in delayed, erratic or extended bloom, inadequate overlap with pollenizers, poor leafing-out, low fruit set, and irregular fruit maturity. In recent years, use of rest-breaking agents has become commonplace in California sweet cherry (Prunus avium L.) culture, mainly to promote earlier bloom and fruit maturity, but also to promote uniform flowering and overcome lack of marginal chilling. Variable responses by different cultivars and in different seasons may be due to different chilling requirements, despite little variation in genetic background for chill requirement in California's commercial cultivars. Other sources of variation include the activity of the rest-breaking agent used, concentration and carrier volume. A minimum amount of effective chill appears to be required for a given cultivar before rest-breaking agents can be effectively applied. This threshold, as exhibited by degree of response to treatment, can be an important indicator of when to spray. Method of measuring chill accumulation, and thus, timing of applications, varies by region and historic acceptance. California's tree fruit industry typically uses the 45 °F “chill hour” model. The Utah Chill Unit Model and the Modified 45 °F Chill Hours Model had not been thoroughly tested under California conditions nor with the rest-breaking chemicals that are in use today in California. We tested our research results against these models and the Dynamic Model developed in Israel and concluded that the Dynamic Model provided the best explanation of responses in our experimental trials. We have been developing recommendations for application of rest-breaking chemicals based on Dynamic Model chill portion accumulation.

`VERNON': A NEW EARLY SEASON RABBITEYE BLUEBERRY CULTIVAR

Released in 2004 by the University of Georgia and U.S. Dept. of Agriculture, `Vernon' is an early season rabbiteye blueberry (Vaccinium ashei Reade), having large fruit size, good yields and excellent plant vigor. `Vernon', tested as T-584, was selected in 1990 at the Coastal Plain Experiment Station in Tifton, Ga. from a cross of T-23 × T-260. `Vernon' fruit ripens early with the cultivar Climax in south Georgia, and few days before `Premier'; however, `Vernon' flowers 5 to 10 days after the standard cultivars. On average over a 6 year period, `Vernon' yielded 5.8 kg/plant per season, compared to 3.1 and 4.5 kg/plant for `Climax' and `Premier', respectively. Berry stem scar, color, firmness, and flavor of the new cultivar are good to excellent. Berry size of `Vernon' is considerably large, averaging 2.05 g/berry over 4 locations in 2003, compared to only an average weight of 1.56 g/berry for `Climax'. `Vernon' berries are firmer than `Premier'. Propagation of the new cultivar is easily accomplished from softwood cuttings. Chill hour requirement is estimated to be in the range of 500 to 550 hours (&lt;7 °C). `Vernon' should be planted with other rabbiteye blueberry cultivars with a similar time of bloom to provide optimum pollination. Propagation rights are controlled by Georgia Seed Development Commission, 2420 S. Milledge Avenue, Athens, GA 30606 (for more information go to www.gsdc.com).

The Effects of Chill Hour Accumulation on Hydrogen Cyanamide Efficacy in Rabbiteye and Southern Highbush Blueberry Cultivars

SUMMARY A controlled environment study was conducted to evaluate the effects of chill hour accumulation on the time of application and the resulting efficacy of the plant growth regulator, hydrogen cyanamide (H2CN2) in both rabbiteye (Vaccinium asheii Reade) and southern highbush (V. corymbosum) blueberry cultivars. Application of H2CN2 at the interval in which accruement of 75% of the individual chill-hour requirements of ‘Bladen’, ‘Jubilee’, ‘Premier’, and Tifblue' blueberry cultivars resulted in greater vegetative bud break than the 50% chill-hour application timing, or than their untreated checks. The 75% timing also resulted in a significant increase in the terminal growth of stems in ‘Bladen’ and ‘Premier’. Consideration of a blueberry cultivar's exposure to chill-hours in application timing decisions should provide a greater degree of precision in optimizing vegetative bud break with H2CN2.

Comparison of Chilling Requirement in `Ta Tao' Budded Peach Trees by Shoot Forcing

Peach trees exposed to `Ta Tao' vegetative chip bud grafts have been shown to have physiological changes that include bloom delay, delayed maturity, reduced shoot vigor, and early autumn defoliation. `Ta Tao' is known to be infected with the graft transmissible agent Peach Latent Mosaic Viroid (PLMVd). PLMVd has been associated with bloom delay and reduced shoot vigor. `Coronet' peach trees planted in a high-density, Y-trained orchard system were treated with vegetative chip buds from `Ta Tao'. Transmission of PLMVd was confirmed in the treated trees by cRNA probe hybridization. A shoot forcing study was set up to determine if exposure to `Ta Tao' would alter the chilling requirement of `Coronet' peach. Terminal fruiting shoots were collected periodically during the dormant season from 8 Jan. 1999 to 19 Feb. 1999, which represented a range of chill hour accumulation from 574 to 927, respectively. Shoots were forced in a greenhouse, and chilling requirement was considered complete when 50% of the flowers opened. Chilling requirement was not changed by exposure to `Ta Tao' chip buds. The number of days required for shoots to bloom was significantly and consistently longer for the `Ta Tao' treated trees. The number of shoots responding to forcing conditions was significantly less in the treated trees. The data suggest that the graft transmissible effects from `Ta Tao' buds increased the growing degree hours required for `Coronet' leaf and flower bud emergence after rest completion under greenhouse forcing conditions.

467 Overcoming Dormancy, Advancing Budbreak, and Advancing Fruit Maturity in `Bing' Sweet Cherry (Prunus avium L.): Surfactants/ Dormant Oils + Calcium Ammonium Nitrate or Hydrogen Cyanamide

In continuing trials (1995-current), we have used a variety of treatments to overcome inadequate chilling, coordinate bloom, improve leaf out and cropping, and advance/coordinate maturity in sweet cherry, cv. Bing. Treatments have included hydrogen cyanamide (HCN, Dormex) and various surfactants or dormant oils combined with calcium ammonium nitrate (CAN17). Chill hour accumulation, (required chilling for `Bing' = 850 to 880 chill hours) has varied greatly in each dormant season from 392 (Hollister, 1995-1996) to adequate, depending both on the season and location (central valley vs. coastal valley). In 1998, 4% HCN advanced budbreak significantly compared to any other treatment, although other chemical treatments also were more advanced than the untreated control. Dormex advanced completion of bloom 11% to 40% more than other treatments, although other dormancy-replacing chemicals were at least 16% more advanced in petal fall than the untreated control. Dormex contributed to slightly elevated truss bud death, as did 2% Armobreak + 25% CAN17. In 1998, fruit set was improved by 2% Armobreak + 25% CAN17 (79%) compared to the untreated control (50%); all other treatments statistically equaled the control. Fruit set was not improved by Dormex, although bloom was advanced by a few days in this treatment. As fruit set was increased by treatments, rowsize decreased (as did fruit weight), as expected, but no treatment resulted in unacceptable size. In 1997, fruit set was also improved by 2% Armobreak + 25% CAN17; however, fruit set was so low overall in that year that no real impact was found. In 1997 and 1998, 4% HCN advanced fruit maturity compared to other treatments, with darker, softer, larger fruit at commercial harvest. These and additional results will be presented.

057 A New Plumcot Adapted to the Southeastern United States

Plumcots are hybrids of plums (usually Japanese-type) and apricots. In recent years, several new plumcots have been released, but most of these have been unreliable croppers and tree health of those tested in the Southeast has been poor. Some do have very high quality fruit, combining the best features of both parents. BY88Z1092 appeared as a chance hybrid in a lot of open-pollinated seedlings from the plum selection BY8111-6, which was a hybrid of BY4-601 (=`Queen Anne'*`Santa Rosa')*`Frontier'. BY8111-6 was a high-quality, midseason plum with black skin and amber flesh. BY88Z1092 blooms about with 750 chill hour peaches, and appears to be somewhat self-fertile. Cropping is heavy at Byron in absence of severe spring frosts. Tree health is good, comparable to local adapted plums such as `Black Ruby'. Trees are upright in growth habit. Fruit of BY88Z11092 ripen in late May, when quality of other adapted plums is insipid. It has firm yellow-orange flesh and a purple-black skin with light pubescence. Flavor is acidic until the fruit begins to soften, at which time it is very good. Fruit size will reach 4 to 5 cm in diameter if properly thinned. BY88Z1092 is in the final stages of testing and will likely be named within the next year.

Moderate-chilling Peach Breeding Project for the Coastal Plain of the Southeastern United States

The purpose of this three-way cooperative project is to develop new fresh-market peach and nectarine varieties in the 400 to 650 chill hour range for the early season shipping market. Since 1990, &gt;3000 seedlings have been evaluated, resulting in 48 selections. Additionally, several hundred selections from other programs have been evaluated. `Sunsplash', an attractive, early season, 400 chill hour nectarine, was released in 1993 as a result of this cooperative effort. A novel aspect of the program has been the use of non-melting flesh parents for the purpose of improving handling characteristics. Selections include both yellow- and white-flesh types, peaches and nectarines. Some may be adapted for use in other production areas and are available for testing under non-propagation agreement. Evaluation summaries of selections and standards will be presented.

Cabium Browning of Cold-damaged Peach Trees in the Nursery

One-year-old peach trees in nurseries at McMinnville, Tenn., were exposed to –11C on 5 Nov. 1991 before digging. The nursery owners were concerned about the relationship of tree cambium browning to potential tree performance after planting. A color scale [0 = nondamage (white) to 6 = severely damaged (brown)] showing discolored cambium of peach nursery trees was developed to rate damage. Browning was rated at 8 cm above graft union. Five trees each of nine cultivars with chill hour requirements ranging from 175 to 1050 were rated. Cultivars with &lt;500 chill hour requirement had higher ratings. Ten `Harbite' trees from each of six size grades were rated. Trees in grades of 30- to 90-cm height had less cambium browning than trees in grades of 90 to 152 cm height. In Dec. 1992, 1-year-old `Red Globe' trees were exposed to –6 (minimum field temperature), –15, –18, –24, –30, or –35C in a programmable freezer. A subsample of five trees per treatment was rated for browning 1 day after treatment and a second subsample rated in mid February. Trees in a third subsample were grown in a nursery the following summer. Slight browning (rating = 1.6) was evident soon after exposure to –24C; however, severe browning was evident on trees exposed to –30 or –35C. Trees exposed to temperature more than –24C did not differ in height, trunk diameter, or dry weight at the end of the growing season, however trees exposed to –30 or –35C did differ. In a similar experiment, `Juneprince' trees exposed to –18C had slight cambium browning (rating = 1.2) but trees died.

DIFFERENCES IN CHILLING AND POST-REST HEAT REQUIREMENT OF VARIOUS PRUNUS SPECIES AND PEACH CULTIVARS (P. PERSICA L. BATSCH)

Differences in chilling and post-rest heat requirements of various stonefruits were investigated through the use of cuttings collected from field grown trees. Materials studied included P. angustifolia Marsh, P. besseyi Bailey, P. maritima Marsh, P. persica (L.) Batsch (`Agua 6-4', `Flordaking', `Pi Tao', `Redhaven', `Redskin', and `Ta Tao'), P. umbellata Ell. and a Japanese type plum (`Byrongold'). Cuttings were collected after natural leaf fall and shortly after the onset of of chill hour accumulation. Cuttings were stored at 4°C. Groups of cuttings were removed from storage after various amounts of chilling and allowed to develop at 16, 21 or 27°C. Cuttings were observed for both vegetative and flower bud break. Magnitude of differences in chilling and post-rest heat requirements and their implications in the breeding of peaches for low and moderate chill areas will be discussed.