Winter Chilling Research Articles

Variability in spruce beetle (Coleoptera: Curculionidae, Scolytinae) adult diapause and evidence for oocyte development prior to winter in a Colorado population.

Diapause regulates seasonal insect life cycles and may be highly variable within and among populations due to genetic and environmental variability. Both types of variation may influence how populations respond plastically or evolutionarily to changing climates. We assessed diapause variability in spruce beetle Dendroctonus rufipennis Kirby (Coleoptera: Curculionidae, Scolytinae), a major forest pest whose life cycle timing is regulated by both prepupal and adult diapauses. Using mating studies and ovary dissections, we tested for variability in adult diapause within and between collection sites in Colorado and Wyoming, USA. Ovary morphology suggested that most females from both sites enter diapause prior to egg formation (oogenesis) when reared at warm temperatures. Though previous studies suggested that adult diapause is obligate, we found that a small proportion of females from both populations terminated diapause without winter chilling in the lab. Moreover, we found that most female beetles sampled at the Colorado field site had mature ovaries relatively early in the fall, suggesting that transient exposure to low temperatures may potentiate pre-winter reproductive development. Adult diapause may act primarily as a block to prevent offspring production late in the season but not necessarily as an overwintering phenotype. Overall, our data do not suggest imminent life cycle shifts mediated by adult diapause, but if the observed variability is heritable, diapause regulation may evolve in response to changing environmental conditions.

Deciphering the genetic mechanisms of chilling requirement for bud endodormancy release in deciduous fruit trees.

Bud endodormancy in deciduous fruit trees is an adaptive trait evolved by selection for the capacity to survive unfavorable environmental conditions. Deciduous trees require a certain amount of winter chill named chilling requirement (CR) to promote bud endodormancy release. In recent decades, global warming has endangered the chill accumulation in deciduous fruit trees. Developing low-CR cultivars is a practical way to neutralize the effect of climate changes on the cultivation and distribution of deciduous fruit trees. In this review, we focus on the effect of chilling accumulation on bud endodormancy release and the genetic mechanisms underlying the chilling requirement in deciduous fruit trees. Additionally, we put forth a regulatory model for bud endodormancy and provide prospective directions for future research in deciduous fruit trees.

Seasonal climate forecasts show skill in predicting winter chill for specialty crops in California

Many fruits and nuts crops in California require sufficient winter chill to break dormancy, and insufficient chill can harm fruit quantity and quality. Early information on winter chill forecast can help growers prepare for a low chill year. Here we evaluate use of dynamic climate models for chill accumulation forecast in California. Using temperature forecasts from seasonal prediction systems, we found that the multimodel forecasts can predict chill. This is evident from the anomaly correlation coefficients exceeding 0.5 between the model-predicted and reference chill values for most California regions. The forecasts correctly identified chill categories in over 50% instances in more than 40% of the Central Valley and southern parts of California. The forecasts also demonstrated skill in capturing the interannual variability of chill, especially during years with substantial decrease in chill. Additionally, the seasonal forecast can provide potentially useful crop specific chill sufficiency prediction. However, forecasts beyond a one-month lead time showed reduced forecast skills.

Chilling or chemical induction of dormancy release in blackcurrant (Ribes nigrum) buds is associated with characteristic shifts in metabolite profiles.

This study reveals striking differences in the content and composition of hydrophilic and lipophilic compounds in blackcurrant buds (Ribes nigrum L., cv. Ben Klibreck) resulting from winter chill or chemical dormancy release following treatment with ERGER, a biostimulant used to promote uniform bud break. Buds exposed to high winter chill exhibited widespread shifts in metabolite profiles relative to buds that experience winter chill by growth under plastic. Specifically, extensive chilling resulted in significant reductions in storage lipids and phospholipids, and increases in galactolipids relative to buds that experienced lower chill. Similarly, buds exposed to greater chill exhibited higher levels of many amino acids and dipeptides, and nucleotides and nucleotide phosphates than those exposed to lower chilling hours. Low chill buds (IN) subjected to ERGER treatment exhibited shifts in metabolite profiles similar to those resembling high chill buds that were evident as soon as 3 days after treatment. We hypothesise that chilling induces a metabolic shift which primes bud outgrowth by mobilising lipophilic energy reserves, enhancing phosphate availability by switching from membrane phospholipids to galactolipids and enhancing the availability of free amino acids for de novo protein synthesis by increasing protein turnover. Our results additionally suggest that ERGER acts at least in part by priming metabolism for bud outgrowth. Finally, the metabolic differences presented highlight the potential for developing biochemical markers for dormancy status providing an alternative to time-consuming forcing experiments.

A comparison of interpolation methods to predict chill accumulation in a Mediterranean stone fruit production area (Región de Murcia, SE Spain)

One of the most important agroclimatic variables for stone fruit production is winter chill accumulation. To estimate chill accumulation in locations where climatic data is not recorded, spatial interpolation is necessary. In this study, we compare different interpolation methods for mean and Safe Winter Chill (SWC) in a Mediterranean stone fruit production area (Region of Murcia, SE Spain) using data from 49 climatic stations. To choose the most accurate interpolation method, as its choice may substantially influence the prediction accuracy, the predictive capability of several interpolation methods with different parameterizations (for a total number of 32 instances) was compared through out-of-bag bootstrap cross-validation, concluding that the best ones were Radial Basis Functions applied on the altitude-dependent regression residuals for mean winter chill and the altitude+latitude linear regression for SWC. The incorporation of altitude in the interpolation increased greatly the accuracy of the estimation. In fact, most of the chill accumulation spatial dependency was explained through altitude. The accuracy of the interpolation was not homogeneous across the study area. Chill accumulation in warmer coastal localities was overestimated by all the methods, possibly due to the proximity to the sea, highlighting the importance of microclimatic variables at higher-resolution spatial interpolations. Differences between methods were more notable in higher locations, where distance-only based methods underestimated chill accumulation and methods that consider altitude slightly overestimated it. This study demonstrates the importance of comparing the performance of multiple spatial interpolation methods before applying any for chill accumulation data.

Effect of Warm Temperature Interruption on the Accumulation of Winter Chilling in Kiwifruit (Actinidia chinensis Planch. and A. deliciosa A. Chev.)

Warm temperature exposure during winter has reportedly resulted in the apparent negation of chilling in several fruit species. This study was conducted to investigate the floral and vegetative response of two pistillate kiwifruit cultivars to intermittent warm temperature interruption during chilling accumulation. Dormant 1-year-old canes of Actinidia chinensis ‘AU Golden Dragon’ and Actinidia deliciosa ‘AU Fitzgerald’ were collected in December 2018 and 2019 (334 and 360 chilling units, respectively), shortly after leaf abscission. Canes were cut to 10 nodes after removing the first six basal nodes, placed in jars filled with distilled water, and transferred to respective chilling treatments. Treatments included continuous chilling (CC) (in addition to base chilling) at 1-week (168 chilling units) increments (0–5 weeks) and chilling exposure at the same increments with intermittent warm temperature (WT). For the WT treatments, each week of chilling was followed by 3 days of exposure to warm conditions. Chilling and warm temperature exposure were simulated by 7/4 °C and 25/17.2 °C (day/night) air temperatures, respectively, using separate climate-controlled growth chambers. After treatments, canes were forced in a third chamber at 21.1 to 25.0 °C with light-emitting diode lighting. Vegetative budbreak, floral bud number (from here on defined as floral response), and floral development stage were recorded for each cane at 2-day intervals. For ‘AU Golden Dragon’, WT did not result in any reduced floral response at any of the observed chilling levels. However, lower mean floral response was observed with WT, as compared with CC for ‘AU Fitzgerald’ at 5 weeks of chilling over the 2 years (P = 0.05). WT also lessened the effect of apical dominance with respect to vegetative/floral response to node position for both cultivars. Chilling type had no significant effect on vegetative response in either cultivar. Estimated chilling requirements (CC) in this experiment were similar to those reported previously for these cultivars. Results suggest that A. chinensis cultivars may respond more favorably than A. deliciosa to the erratic winter temperature patterns experienced in the southeastern United States.

Perspectives on the adaptation of Japanese plum-type cultivars to reduced winter chilling in two regions of Spain.

Japanese plum, like other temperate fruit tree species, has cultivar-specific temperature requirements during dormancy for proper flowering. Knowing the temperature requirements of this species is of increasing interest due to the great genetic variability that exists among the available Japanese plum-type cultivars, since most of them are interspecific hybrids. The reduction of winter chilling caused by climate change is threatening their cultivation in many regions. In this work, the adaptation perspectives of 21 Japanese plum-type cultivars were analyzed in two of the main plum-growing regions in Spain, Badajoz and Zaragoza, to future climate conditions. Endodormancy release for subsequent estimation of chilling and heat requirements was determined through empirical experiments conducted during dormancy at least over two years. Chill requirements were calculated using three models [chilling hours (CH), chilling units (CU) and chilling portions (CP)] and heat requirements using growing degree hours (GDH). Chilling requirements ranged 277-851 CH, 412-1,030 CU and 26-51 CP, and heat requirements ranged from 4,343 to 9,525 GDH. The potential adaption of the cultivars to future warmer conditions in both regions was assessed using climate projections under two Representative Concentration Pathways (RCP), RCP4.5 (effective reduction of greenhouse gas emissions) and RCP8.5 (continuous increase in greenhouse gas emissions), in two time horizons, from the middle to the end of 21st century, with temperature projections from 15 Global Climate Models. The probability of satisfying the estimated cultivar-specific chilling requirements in Badajoz was lower than in Zaragoza, because of the lower chill availability predicted. In this region, the cultivars analyzed herein may have limited cultivation because the predicted reduction in winter chill may result in the chilling requirements not being successfully fulfilled.

Estimating the temperature limitations for growth cessasion and floral initiation in red rasberry (Rubus idaeus L.) and the floral-inducing effect of winter chilling

ABSTRACT Floral initiation in biennial-fruiting red raspberry is controlled by the interaction of temperature and photoperiod. To determine the threshold temperatures for short day (SD) floral initiation in early- and late-flowering cultivars, we exposed plants of ‘Glen Ample’, ‘Glen Mor’ and ‘Duo’ to 12°, 16° and 20°C in a daylight phytotron under naturally decreasing autumn daylength at Ås, Norway (59°40’N). While none of the cultivars ceased growing or initiated floral primordia at 20°C, ‘Glen Ample’ and ‘Glen Mor’ initiated buds at 12° and 16°C, whereas ‘Duo’ formed flower buds at 12°C only. Surprisingly, however, all plants flowered abundantly in spring after winter chilling in the dark at −1.5 ± 0.5°C for 7 months. We discuss two possible explanations for this unusual and novel flowering response. Fractional induction is well known in raspberry, and we visualise that in SD at 20°C, the SD requirement is fulfilled, while floral induction is still blocked by inappropriate temperature. A vernalisation-like response is alternatively suggested as this can take place at near-freezing temperatures in the dark. A combination of the two mechanisms is also possible and likely. We conclude, however, that the two floral induction processes are fundamentally different and controlled by different physiological mechanisms.

Overcoming Dormancy in Prunus Species under Conditions of Insufficient Winter Chilling in Israel.

The phenomenon of dormancy and the evolutionary causes for its development are presented together with the effects of the climatic factors: temperature and light. Shade and darkness have been found to enhance bud breaking in peach. The effects of various temperatures on chilling accumulation, chilling negation and chilling enhancement are described. The way these are computed in the face of global warming is explained, using the dynamic model. When natural chilling is less than that required, there are ways of compensation, up to a certain level. Various horticultural, physical and chemical means to achieve this are described, including bending branches, reducing vegetative vigor, shading the orchard, sprinkling to reduce daytime temperature and the application of various chemicals to break dormancy. When winter chilling is markedly reduced and temperatures increase considerably, the use of dormancy avoidance is suggested in frost-free places. This technique can induce a new growing cycle by avoiding dormancy altogether. However, the best approach is to breed high-quality cultivars requiring much less chilling. Another aspect discussed in this work, independent of the chilling requirement, is the negative effect of heat spells in winter and spring on the abnormal development of flower buds, leading to a low level of the stone fruit set and a reduced yield.

Enhancing Uniform Sprouting, Yield, and Quality in Grapevines: The Impact of Hydrogen Cyanamide (50% S.L) Treatment

Grape (Vitis vinifera L.) is considered as one of the most important commercial fruit crops in temperate, tropical and subtropical regions of the world. In India grape cultivation in tropical and subtropical regions assumes great significance, but one of the biggest issues preventing grapes from being produced in warm climates has been the lack of or insufficient winter chilling. Under these circumstances, there is uneven bud bursting, flowering, and delayed fruit development. The most crucial component of grape development is, consistent bud breaking is required to increase grape quality and maturity. Many plant growth regulators are used to promote bud burst and increase the proportion of bud break. Several locations across the world utilize plant growth regulators to intentionally cause bud break. The application of Hydrogen Cyanamide (H2CN2) during pruning time of grapes, helps to enhance growth uniformity, encourage earlier and more even bud break, raises the proportion of fruit buds, berry set, number of clusters per vine, weight of clusters, and dimensions of clusters. It also gives positive response to various metabolic activities; maximize yield and Maturity, Fruit quality, as well as production efficiency. In grape orchards, it is commonly used to promote consistent and improved bud burst, shortened flowering period, and boost output.

Temperature and Daylength Effects on Growth and Floral Initiation in Biennial-Fruiting Blackberry

Little is known about the environmental control of growth and flower bud initiation (FBI) in commercial blackberries. We studied the processes in the cultivars ‘Lock Ness’, ’Ouachita’ and ‘Sweet Royalla’ at 12, 16 and 20 °C in a daylight phytotron under naturally decreasing autumn daylength at Ås, Norway (59°40′ N). Growth rate increased with increasing temperature but was much lower at all temperatures in the erect ‘Ouachita’ than in the trailing cultivars ‘Lock Ness’ and ‘Sweet Royalla’. In all cultivars, FBI occurred earliest at 16 °C, whereas little or no FBI took place in ‘Ouachita’ and ‘Lock Ness’ at 12 °C. Growth cessation was earliest at 16 °C where it occurred in early September in all cultivars, suggesting a critical daylength of approximately 14 h. At variance from earlier statements, FBI started in lateral buds situated several nodes below the apex and progressed in both acropetal and basipetal directions as previously reported for red raspberry. Winter chill at 0 °C enhanced flowering in spring in marginally induced plants of all cultivars except ‘Ouachita’ grown at 12 °C, which remained vegetative in spring. The results suggest that temperature is as important as daylength for FBI in biennial-fruiting blackberry, and that winter chilling may enhance flowering and yield potential in partially induced plants.

Temporal transcriptome and metabolite analyses provide insights into the biochemical and physiological processes underlying endodormancy release in pistachio (Pistacia vera L.) flower buds.

Pistachio (Pistacia vera L.), an economically and nutritionally important tree crop, relies on winter chill for bud endodormancy break and subsequent blooming and nut production. However, insufficient winter chill poses an increasing challenge in pistachio growing regions. To gain a better understanding of the physiological and biochemical responses of endodormant pistachio buds to chilling accumulation, we investigated the global gene expression changes in flower buds of pistachio cv. Kerman that were cultivated at three different orchard locations and exposed to increasing durations of winter chill. The expression of genes encoding β-1,3-glucanase and β-amylase, enzymes responsible for breaking down callose (β-1,3-glucan) and starch (α-1,4-glucan), respectively, increased during the endodormancy break of pistachio buds. This result suggested that the breakdown of callose obstructing stomata as well as the release of glucose from starch enables symplasmic trafficking and provides energy for bud endodormancy break and growth. Interestingly, as chilling accumulation increased, there was a decrease in the expression of nine-cis-epoxycarotenoid dioxygenase (NCED), encoding an enzyme that uses carotenoids as substrates and catalyzes the rate-limiting step in abscisic acid (ABA) biosynthesis. The decrease in NCED expression suggests ABA biosynthesis is suppressed, thus reducing inhibition of endodormancy break. The higher levels of carotenoid precursors and a decrease in ABA content in buds undergoing endodormancy break supports this suggestion. Collectively, the temporal transcriptome and biochemical analyses revealed that the degradation of structural (callose) and non-structural (starch) carbohydrates, along with the attenuation of ABA biosynthesis, are critical processes driving endodormancy break in pistachio buds.

Effect of chilling on the spring emergence of overwintering Eurytoma maslovskii (Eurytomidae: Hymenoptera).

This study investigated the effect of chilling temperature and duration on diapause termination in Eurytoma maslovskii, a major apricot pest in Korea and China. The experimental insects were sampled in mid-autumn. Overwintering larvae were subjected to a temperature range at -16, -4, 2, 9, 13 °C for 12 wk (a temperature of 19 °C was used for nonchilled control), and then exposed to cold temperature (2 °C) for different durations (from 1 to 16, 19, and 22 wk) at different times (20 September and 30 November) to determine adult emergence, prolonged larval diapause, and development rate postchilling. The results demonstrated a strong association between chilling temperature and duration with the emergence of E. maslovskii. It was observed that diapause may be terminated after at least 6 wk in the cold. However, the rate only reached a higher significance after 10 wk or longer duration of chilling. The optimal chilling temperature ranged from 2 to 9 °C. Late autumn chilling (mid-September) did not differ from winter chilling (late November). The postdiapause development rate of E. maslovskii was determined to be affected by the chilling duration and could be described by a 3-parameter probability density Weibull function. These results suggest that winter conditions play a crucial role in the phenology of E. maslovskii in spring.

Poleward shifts in the maximum of spring phenological responsiveness of Ginkgo biloba to temperature in China.

Global warming is advancing the timing of spring leaf-out in temperate and boreal plants, affecting biological interactions and global biogeochemical cycles. However, spatial variation in spring phenological responsiveness to climate change within species remains poorly understood. Here, we investigated variation in the responsiveness of spring phenology to temperature (RSP; days to leaf-out at a given temperature) in 2754 Ginkgo biloba twigs of trees distributed across subtropical and temperate regions in China from 24°N to 44°N. We found a nonlinear effect of mean annual temperature on spatial variation in RSP, with the highest response rate at c. 12°C and lower response rates at warmer or colder temperatures due to declines in winter chilling accumulation. We then predicted the spatial maxima in RSP under current and future climate scenarios, and found that trees are currently most responsive in central China, which corresponds to the species' main distribution area. Under a high-emission scenario, we predict a 4-degree latitude shift in the responsiveness maximum toward higher latitudes over the rest of the century. The identification of the nonlinear responsiveness of spring phenology to climate gradients and the spatial shifts in phenological responsiveness expected under climate change represent new mechanistic insights that can inform models of spring phenology and ecosystem functioning.

Chilling Requirements of Apricot (Prunus armeniaca L.) Cultivars Using Male Meiosis as a Dormancy Biomarker.

Apricot has undergone an important cultivar renewal during the last years in response to productive and commercial changes in the crop. The impact of the sharka disease (plum pox virus) prompted the release of cultivars resistant/tolerant to this virus, leading to a major cultivar renewal worldwide. This has caused high variability in chilling requirements on new releases that remain unknown in many cases. In many apricot-growing areas, the lack of winter chilling is becoming a limiting factor in recent years. To deal with this situation, growers must choose cultivars well adapted to their areas. However, the information available on the agroclimatic requirements of the cultivars is very limited. To fill this gap, in this work, we have characterized the chilling requirements of 13 new apricot cultivars from Europe (France, Greece and Spain) and North America (USA) in two experimental collections in Aragón (Spain). We established the chilling period using male meiosis as a biomarker for endodormancy release over two years. Chilling requirements ranged from 51.9 Chill Portions (CP) to 70.9 CP. Knowing the chilling requirements of cultivars will help growers to select suitable cultivars adapted to the chill availability of their region.

Reducing vulnerability of fruit orchards to climate change

CONTEXTFarmers are forced to undertake adaptive measures to protect their crops against climate change impacts. Fruit trees present farmers with unique adaptive challenges. In pursuit to expand the knowledge base on climate change effects on crops in underreported regions, we present herein data from Chile and Tunisia on the vulnerability of cherry and peach orchards to climate change and on possible measures to counter its impact. OBJECTIVESWe aimed to ascertain a) the extent to which fruit farming biophysical operations in Chile and Tunisia are vulnerable to climate change impacts, b) whether the vulnerability can be reduced through agronomic, technological or financial adaptive measures and, c) how to overcome challenges that may prevent implementation of these measures. METHODSThe study is based on face-to-face interviews with 801 peach and cherry farmers in Tunisia and Chile and three focus meetings with farm representatives. RESULTS AND CONCLUSIONSFruit farmers have used and are planning to use a number of agricultural, technological, and financial tools to reduce the vulnerability of their crops to climate change. Agronomical measures are focused on planting varieties with higher drought and disease resistance as well as lower winter chill requirements. Technological measures focus on installing nets reflects farmer regional experiences with crop hail damage and rain at the fruit maturing stage. Planned improvements in irrigation reflect anticipated future problems with water availability and drought. The decision not to implement adaptive measures is mainly linked to economic barriers. Specific actions are proposed to overcome these and other measure implementation barriers. SIGNIFICANCEThe self-reported experiences of fruit farmers provide valuable insight into the effectiveness of various climate change adaptive measures, which often differ from scientific expectations or recommendations. For example, farmers are not overly concerned about fulfilling winter chill requirements, they are also not well informed about all available climate change adaptive tools. In addition, many adaptive measures are not implemented due to administrative or financial barriers. To help address this, regionally-led cooperation between farmers, policy makers, industry, and scientists is needed to identify and overcome these barriers, allowing for the successful implementation of appropriate adaptive measures.

Is winter coming? Impact of the changing climate on plant responses to cold temperature.

Climate change is causing alterations in annual temperature regimes worldwide. Important aspects of this include the reduction of winter chilling temperatures as well as the occurrence of unpredicted frosts, both significantly affecting plant growth and yields. Recent studies advanced the knowledge of the mechanisms underlying cold responses and tolerance in the model plant Arabidopsis thaliana. However, how these cold-responsive pathways will readjust to ongoing seasonal temperature variation caused by global warming remains an open question. In this review, we highlight the plant developmental programmes that depend on cold temperature. We focus on the molecular mechanisms that plants have evolved to adjust their development and stress responses upon exposure to cold. Covering both genetic and epigenetic aspects, we present the latest insights into how alternative splicing, noncoding RNAs and the formation of biomolecular condensates play key roles in the regulation of cold responses. We conclude by commenting on attractive targets to accelerate the breeding of increased cold tolerance, bringing up biotechnological tools that might assist in overcoming current limitations. Our aim is to guide the reflection on the current agricultural challenges imposed by a changing climate and to provide useful information for improving plant resilience to unpredictable cold regimes.

Using long-term data from a whole ecosystem warming experiment to identify best spring and autumn phenology models.

Predicting vegetation phenology in response to changing environmental factors is key in understanding feedbacks between the biosphere and the climate system. Experimental approaches extending the temperature range beyond historic climate variability provide a unique opportunity to identify model structures that are best suited to predicting phenological changes under future climate scenarios. Here, we model spring and autumn phenological transition dates obtained from digital repeat photography in a boreal Picea-Sphagnum bog in response to a gradient of whole ecosystem warming manipulations of up to +9°C, using five years of observational data. In spring, seven equally best-performing models for Larix utilized the accumulation of growing degree days as a common driver for temperature forcing. For Picea, the best two models were sequential models requiring winter chilling before spring forcing temperature is accumulated. In shrub, parallel models with chilling and forcing requirements occurring simultaneously were identified as the best models. Autumn models were substantially improved when a CO2 parameter was included. Overall, the combination of experimental manipulations and multiple years of observations combined with variation in weather provided the framework to rule out a large number of candidate models and to identify best spring and autumn models for each plant functional type.

Flowering Phenology of Olive Cultivars in Two Climate Zones with Contrasting Temperatures (Subtropical and Mediterranean)

The large amount of olive cultivars conserved in germplasm banks can be used to overcome some of the challenges faced by the olive growing industry, including climate warming. One effect of climate warming in olive is the difficulty to fulfill the chilling requirements for flowering due to mild winter temperatures. In the present work, we evaluate seven olive cultivars for their adaptation to high winter temperatures by comparing their flowering phenology in the standard Mediterranean climate of Cordoba, Southern Iberian Peninsula, with the subtropical climate of Tenerife, Canary Islands. Flowering phenology in Tenerife was significantly earlier and longer than in Cordoba. However, genotype seems to have little influence on the effects of the lack of winter chilling temperatures, as in Tenerife. This was found even though the cultivars studied had a high genetic distance between them. In fact, all the cultivars tested in Tenerife flowered during the three-year study but showed asynchronous flowering bud burst. ‘Arbequina’ showed an earlier day of full flowering compared with the rest of the cultivars. The results observed here could be of interest to refine the phenological simulation models, including the length of the flowering period. More genetic variability should be evaluated in warm winter conditions to look for adaptation to climate warming.

What Is Wrong with Winter Chill Models in Warm Climates?

Bud dormancy is an evolutionary strategy that protects temperate fruit trees from low temperatures during winter. With adequate winter chilling, it is a process consisting of a rapid entrance phase where growth inhibition increases to a maximum and is then released via an exit phase. This progression is, however, different in areas with insufficient winter chill. In such areas, the dormancy entrance phase is protracted, and the exit phase is often incomplete. Several chill models have been designed to describe the plants’ response to chill accumulation to match cultivar and site. Although it is known that these models are less accurate in warm winter areas, their accuracy during the two dormancy phases has always been assumed equal. The aim of this study was to investigate the accuracy of four chill models during the entrance and exit phases of bud dormancy of ‘Royal Gala’ buds from two contrasting climatic areas. One-year-old shoots were collected from commercial orchards and forced to determine their bud dormancy level. Before forcing, the shoots received varied amounts of chill additional to the field chill. The bud dormancy progression was represented as joint two- or three-line models to determine the entrance and exit phases. For each phase, the dormancy level of every sample was plotted against its chill accumulation based on four chill models (Chilling Hours, Utah, Positive Utah, and Dynamic Model). Results were compared to determine the ability of each model to predict the linearity of the dormancy progression. The results indicated that although all the models were able to describe the exit phase of bud dormancy successfully, none could describe a linear entrance phase. This suggests that during the onset of dormancy, buds respond to temperature differently to what is measured by the chill models making the models unreliable when winter chill is inadequate.