Chilling Accumulation Research Articles

Impact of chilling accumulation and hydrogen cyanamide on floral organ development of sweet cherry in a warm region

The microscopic investigation of the floral development of sweet cherry (Prunus avium L. cv. Hongdeng) from a warm winter climate (Shanghai) and cold winter climate (Tai’an, Shandong Province, China) was conducted to explore the reason of low fruit set. The effect of hydrogen cyanamide (HCN) on floral development under warm winter conditions was also investigated. Trees grown in Shanghai with insufficient chilling accumulation exhibited little difference in the progression of microspore development compared to trees in Tai’an that accumulated adequate chilling, but showed substantial delays in ovule and embryo sac development. The growth of nucelli did not proceed beyond the macrospore mother cell and macrospore stages with abortion rates of 13, 15 and 45% by 6, 3 and 0 d before full bloom, respectively. These abnormalities in the ovule and embryo sac in the Shanghai-grown trees were eliminated by HCN application. These results suggest that chilling regulates the development of female floral organs in winter dormancy; therefore, insufficient chilling accumulation, causing abnormality of the female floral organs, restricts the cultivation of sweet cherry in warm winter regions. Interestingly, HCN application, which decreased the chilling requirements for Hongdeng, may be a potential strategy for sweet cherry cultivation in warm winter regions.

Climate change impacts on thermal growing conditions of main fruit species in Portugal

Thermal growing conditions of temperate fruit and nut species in Portugal are assessed by two indices: growing degree hours (GDH) and chilling portions (CP). The first evaluates growing season heat accumulation (February–October), while the second determines chill accumulation during dormancy (October–February). These two indices are estimated based on gridded daily minimum and maximum temperatures from a gridded observation-based dataset (E-OBS). Both indices are statistically downscaled to a 1 km grid over mainland Portugal for 1981–2015 (35 years). Furthermore, multi-model climate change projections are provided using four EURO-CORDEX global-regional climate model chains under two future emission scenarios (RCP4.5 and RCP8.5, 2041–2070). Overall, increases of heat accumulation and decreases of chilling accumulation are projected over most of Portugal. However, owing to frequent above-optimum temperatures for temperate fruit trees, decreases of heat accumulation are expected over inner southern Portugal, which combined with significant reductions of winter chill make this region the most affected by climate change. Crop-specific GDH/CP diagrams for eight fruit classes (carob tree, almond tree, chestnut tree, citrus fruits, fresh fruits trees, olive trees, pine nut trees and vines) are analysed taking into account their current spatial distributions. Shifts in their thermal conditions under future scenarios are discussed. Thermal growing conditions of fruit species are innovatively assessed using suitable heat and chilling accumulation measures at very-high spatial resolution and under current and future climates in Portugal. These results may support the Portuguese fruit production sector in planning future strategies to cope with climate change.

Performance of three numerical models to assess winter chill for fruit trees—a case study using cherry as model crop in Germany

The Meckenheim fruit growing region in Western Germany may be affected as a consequence of climate change by lack of chilling, i.e. a cold period in the winter required for tree species to flower in the subsequent spring. As sweet cherry requires high chill, 160 potted trees of three sweet cherry cultivars with threefold different chilling requirements were cultivated in a large experiment over 4 years, either in the orchard or in an unheated greenhouse (simulated climate change). The orchard temperatures in the warmer winter (6.0 °C) exceeded those in the unheated greenhouse (4.7 °C) in the cold winter. Chilling computations for this region showed that cherry trees of low chill cultivar ‘6000CZ’ required 22.3–26.6 chill portions (CP), medium chill cv. ‘Brooks’ about 37.9–54.4 CP and the high chill cv. ‘Schneiders spate Knorpelkirsche’ about 54.4–79.3 CP for a natural flowering. Cherry trees cv. ‘6000CZ’ receiving <300 CH, cv. ‘Brooks’ <500 CH and cv. ‘Schneiders spate Knorpelkirsche’ <700 CH were unable to flower, equivalent of 50 % of the assumed chilling optimum of the respective cultivar. Maximum chill accumulation in this region is currently achieved, so that warmer winters may reduce the available chill. The beginning of leaf drop was identified as a suitable initiation point for computing chill accumulation. Overall, the work has shown that environmental change has an effect on chilling availability in Meckenheim and possibly other fruit growing regions along 50°N. The insights gained ultimately highlight the need for cross-sectoral, adaptive management practices that jointly target a sustainable regional development.

Chilling requirements for hatching of a New Zealand isolate of Nematodirus filicollis

The eggs of some species of the parasitic nematode Nematodirus require a period of chilling before they can hatch; N. filicollis is one such species. This study investigated this requirement for chilling in a New Zealand strain of this species. Eggs of N. filicollis were extracted from lamb’s faeces and incubated at 20°C to allow development to the third stage larvae within the egg. These eggs were then placed into tissue culture plates and incubated at: 2.7°C (±0.99), 3.6°C (±0.90), 4.7°C (±0.35), 6.4°C (±0.37), 8.0°C (±1.54) or 9.9°C (±0.14) for up to 224 days. At 14day intervals until day 84, then every 28 days, one plate was removed from each temperature and placed at 13.1°C (±0.44) for 14 days. Eggs were then assessed for hatching. From this data, chill units were calculated by subtracting the culture temperature from a constant threshold of 11°C and multiplying by the number of days for which the sample was cultured; then the Gompertz model fitted. Even though hatching overall was low, a greater proportion of eggs hatched with chill accumulation. Maximum hatching of eggs required 800–1000 chill units. Consequently in the field, more than one season of chilling would be required before hatching. As such a generation time could take more than one year to complete. This is different to the hatching dynamics of N. spathiger, the other main species found in New Zealand sheep, which does not display this requirement for chilling and hatches immediately once the third stage larvae are developed.

A crop and cultivar-specific approach to assess future winter chill risk for fruit and nut trees

Anthropogenic climate change will influence winter chill accumulation, with future declines likely in temperate locations. However, these declines only translate as impacts when cultivar winter chilling requirements are not satisfied. This study presents a methodology to evaluate future impacts of declining winter chill through a cultivarspecific approach which is useful for growers, industry and policy-makers to develop adaptation strategies. A risk based system was applied to represent the likelihood of meeting cultivar chilling requirements using low, medium, medium-high and high risk ratings based on percentiles. This was combined with climate projection uncertainty graphically at 16 Australian growing districts historically (1981–2010) and for 2030, 2050 and 2090. The results demonstrated that impacts and likely adaptation options differed between cultivars, some recording limited risk at all sites out to 2090 ('Nonpareil' almond) whilst others recorded greater risk both historically and into the future ('Chandler' walnut). Notably, risk differed across sites and with the future time period. These results highlight which cultivars are susceptible to low winter chill conditions, where this risk does and does not manifest and the different time horizons at which the risk will materialise across Australia's main growing districts. Using this approach, changes in winter chill conditions are presented in a useable form which allows for appropriate climate adaptation strategies to be developed, securing the industries into the future.

Terral de Vicuña, a Foehnlike Wind in Semiarid Northern Chile: Meteorological Aspects and Implications for the Fulfillment of Chill Requirements in Deciduous Fruit Trees

AbstractTheterral de Vicuñais a warm and dry wind that flows down the Elqui Valley in north-central Chile typically at dawn and early morning. Given that most terral episodes occur in austral winter when chill accumulation by deciduous fruit trees proceeds, negative effects on agriculture may be expected. During 11 (2004–14) winters a meteorological characterization of terral winds and the assessment of their impact on chill accumulation, by the modified Utah Model and the Dynamic Model, were performed. Within this period, 67 terral days (TD) were identified as those in which nighttime to early morning wind direction and speed, air temperature, and relative humidity reached defined thresholds on an hourly basis (terral hours). Most frequent TD featured 6–9 consecutive terral hours; duration is considered here as a proxy for their intensity. Synoptic-scale meteorological analysis shows that 65% of moderate and strong terral events develop as a cold, migratory anticyclone drifts poleward of the study area, coinciding with the onset of a midtropospheric ridge over central Chile, bringing southwest winds on top of the Andes (~500-hPa level). The remaining 35% are either associated with 500-hPa easterlies (foehn like), with prefrontal conditions ahead of a trough driving northwest 500-hPa winds, or with transitional 500-hPa westerlies. Assessments of chill accumulation during TD show that, although present average and cold winter conditions do not represent a major TD hazard to local agriculture, lower chill accumulation associated with anomalously high nocturnal temperatures could be significantly more important during present and future warmer winters.

Agro-metrological investigation of bumper apple productivity in Himachal Pradesh, India

The study aimed to investigate the productivity and weather relationship for the apple growing areas of Himachal Pradesh viz., Kalpa, Bhuntar and Shimla in district Kinnaur, Kullu and Shimla, respectively. The results revealed that pre bloom period (November to February) in the year 2009-10 remained cooler. The minimum temperature of 0.4 to 0.9, 1.0 to 1.1°C and 1.9 to 2.2°C and maximum temperature of 6.7, 1.0 to 1.1 and 1.7°C were lower in Shimla, Bhuntar and Kalpa region, respectively compared to 1995-2009.. The maximum temperature for the chill accumulation months of November, December, January and February during 2009-10 showed 13 to 19 per cent lower compared to 1995-2009. The average pre bloom rainfall during 2010 was 39 to 57 per cent higher than 1995-2009 indicating sustainable bloom period. The 3 to 4°C temperature rise during March 2010 (19 to 24°C) as compared to 1995-2009 (16 to 21.4°C) coupled with 52 per cent higher precipitation benefited the crop in profuse flowering and hence good fruit set. The average maximum temperature during the post bloom period (May-June) in 2009-10 was 1°C higher compared to the previous years coupled with 23 per cent higher rainfall resulting in an highest productivity. The highest productivity (8.57 MT/ha) during 2010 which was 58 per cent higher than the previous years can be ascribed due to the favorable low temperature in pre bloom period and increase in the temperature inthe month of March along with adequate rainfall in the bloom and post bloom period.

Differentiated Responses of Apple Tree Floral Phenology to Global Warming in Contrasting Climatic Regions.

The responses of flowering phenology to temperature increases in temperate fruit trees have rarely been investigated in contrasting climatic regions. This is an appropriate framework for highlighting varying responses to diverse warming contexts, which would potentially combine chill accumulation (CA) declines and heat accumulation (HA) increases. To examine this issue, a data set was constituted in apple tree from flowering dates collected for two phenological stages of three cultivars in seven climate-contrasting temperate regions of Western Europe and in three mild regions, one in Northern Morocco and two in Southern Brazil. Multiple change-point models were applied to flowering date series, as well as to corresponding series of mean temperature during two successive periods, respectively determining for the fulfillment of chill and heat requirements. A new overview in space and time of flowering date changes was provided in apple tree highlighting not only flowering date advances as in previous studies but also stationary flowering date series. At global scale, differentiated flowering time patterns result from varying interactions between contrasting thermal determinisms of flowering dates and contrasting warming contexts. This may explain flowering date advances in most of European regions and in Morocco vs. stationary flowering date series in the Brazilian regions. A notable exception in Europe was found in the French Mediterranean region where the flowering date series was stationary. While the flowering duration series were stationary whatever the region, the flowering durations were far longer in mild regions compared to temperate regions. Our findings suggest a new warming vulnerability in temperate Mediterranean regions, which could shift toward responding more to chill decline and consequently experience late and extended flowering under future warming scenarios.

The Physiology of Chilling Temperature Requirements for Dormancy Release and Bud-break in Temperate Fruit Trees Grown at Mild Winter Tropical Climate

It is studied that inadequate winter chilling may interfere with the normal processes of plant growth, reproductive development and subsequent yield. As much of the evidences behind these studies are subjective and region based, the available information was collated and evaluated to further investigate the impacts of winter chill that is currently an issue of fruit growers in mild winter areas. Though, the period of adequate low temperatures is insufficient in warmer regions to satisfy the chilling requirements of temperate fruit trees, this call up on many option for chill compensation; like planting low chill cultivars, use of dormancy breaking chemicals, forced defoliation, pruning and some other techniques as an alternative strategies. However, the diverse agro-climatic conditions due to major differences in altitude, rainfall as well as in slope characteristics showed the existence of different chilling temperature requirements across locations that favor many temperate fruit trees to grow: including apple, pear, grape, peach, nectarine, plum, cherry, walnut, almond and other fruit tree species. Temperate fruit production in mild winter areas now days increased rapidly and even more new industries are being developed in regions where none previously existed. To date, in tropical highlands, fruit productivity and quality have been gradually improved through introduction and selection of better adaptable varieties based on their chilling requirements. To supplement the present existing knowledge gap in relation to the cultivar-environment interactions, the use of chill models to quantify chill accumulation during winter months would help in classifying the environment (potential growing areas) according to the amount of chill hours existed in that location. Up to date, a number of valid methods were applied for quantification of chill accumulation in tropical and sub-tropical conditions showed varying results depending on the types of species, existing warm temperature and other climatic variables. The limitation in their predictive performance from region to region is due to their designing approaches that were primarily for temperate climate. This requires a model comparison for specific location, i.e. by using more than one model to avoid the potential mis­leading in calculation and chill estimation. Of the chill models tested to quantify the chill accumulation in the mild winter areas, the use of dynamic model gave good estimation that it nullifies the chilling reversal by high temperature. Also, the model of Positive Chill Units (PCU, or Positive Utah) is a competent under warm climate, next to the dynamic model. The Positive Utah model, an iteration of the original Utah, excludes the negation influence of high temperatures. The procedure for PCU is the same as for the original Utah model except that, when negative, the chill unit value is set equal to zero. Therefore, the accumulated chill units are equal to zero until the temperatures drop into the effective zone and positive chill units begin to accumulate. Though, for these areas with warm climate, using the 0 - 7.2 °C model is not recommended, because of its sensitivity to changes in temperatures that represent different weighing factors recorded for other models. Other important alternatives to these classical models include, the Growing Degree Hour Model (GDH), the Mean Temperature Model, Exponential temperature response functions and others are applied as independently, or in combination with classical chill models for a better chill estimation. Winter chill should be studied like other weather dependent processes because the present trends in chill decline across locations significantly affect fruit culture in areas with mild winter. Therefore, identifying the problems related with lack of insufficient winter chilling would help in designing possible strategies for the changing scenarios and understanding the current physiological responses of the plant against these changes.

Changes in Dormant Phase and Bud Development of ‘Fuji’ Apple Trees in the Chungju Area of Korea

Abstract: In this study, we investigated the onset and release of endo-dormancy under natural conditions by observing bud break characteristics in ‘Fuji’ apple trees using water cuttings. Through examinations of bud break rate and days to bud break, we found that the endo-dormancy of ‘Fuji’ apple tree continues for 70 d from 165 to 255 d after full bloom (DAFB), from late October to early January of the following year. In addition, within 20 d of first bud break, based on a final bud break rate of 60% or more, we able to identify the timing of the changeover from para-dormancy to endo-dormancy, and endo-dormancy to eco-dormancy. Analysis of the chilling requirement during the endo-dormancy period revealed that chilling accumulation up to 255 DAFB to release endo-dormancy amounted to 666 and 517 h based on the CH and Utah models, respectively. Observation of internal changes in the bud during endo-dormancy showed that flower bud differentiation begins from mid-July a, nd time of inflorescence of the disk flower is available to find. The flower buds subsequently developed slowl y but steadily during endo-dormancy and in the following year in February, the developmental stage of each organ had pro gressed. Moreover, the flower buds of ‘Fuji’ apples were mostly healthy during the dormancy period, but some exhibited necrosis of flower primordium, due partial cell damage from the formation of ice crystals rather than a direct effect of the low tempera ture. Flower buds were formed in both the axillary buds of bourse shoots and terminal buds of spurs, but lower bud differentiation was observed for the terminal buds of spurs at rate of about 65% of total buds, which was directly related to the bud size and shoot diameter.Additional key words:

UNIDADES DE FRIO PARA MACIEIRAS NA REGIÃO DE VACARIA – RS, BRASIL

As macieiras necessitam de um período de acumulação de frio para a quebra da dormência das gemas. Neste estudo, as unidades de frioforam quantificadas por diferentes métodos de cálculo para a região de Vacaria-RS, Brasil, a fim de compará-las com as necessidades de macieiras ‘Gala’ e ‘Fuji’. As unidades de frio foram calculadas para o período de2000/2009, através dos seguintes métodos: Horas de Frio Ponderadas, Utah, Carolina do Norte, Utah Modificado e Carolina do Norte Modificado. Na região estudada, o número de unidades de frio apresenta altavariabilidade entre os anos, independentemente dos métodos de cálculo. Mesmo em anos com alta disponibilidade de horas de frio, o número de unidades de frio necessárias para a quebra de dormência em macieiras ‘Gala’ e ‘Fuji’ não é alcançado, naturalmente. Maio, junho e julho é o principal trimestre para acumular unidades de frio, durante o período de dormência de gemas de macieira.

Functional and expression analyses of kiwifruit SOC1-like genes suggest that they may not have a role in the transition to flowering but may affect the duration of dormancy.

The MADS-domain transcription factor SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1) is one of the key integrators of endogenous and environmental signals that promote flowering in the annual species Arabidopsis thaliana. In the deciduous woody perennial vine kiwifruit (Actinidia spp.), environmental signals are integrated to regulate annual cycles of growth and dormancy. Accumulation of chilling during winter is required for dormancy break and flowering in spring. In order to understand the regulation of dormancy and flowering in kiwifruit, nine kiwifruit SOC1-like genes were identified and characterized. All genes affected flowering time of A. thaliana Col-0 and were able to rescue the late flowering phenotype of the soc1-2 mutant when ectopically expressed. A differential capacity for homodimerization was observed, but all proteins were capable of strong interactions with SHORT VEGETATIVE PHASE (SVP) MADS-domain proteins. Largely overlapping spatial domains but distinct expression profiles in buds were identified between the SOC1-like gene family members. Ectopic expression of AcSOC1e, AcSOC1i, and AcSOC1f in Actinidia chinensis had no impact on establishment of winter dormancy and failed to induce precocious flowering, but AcSOC1i reduced the duration of dormancy in the absence of winter chilling. These findings add to our understanding of the SOC1-like gene family and the potential diversification of SOC1 function in woody perennials.

Increased heat requirement for leaf flushing in temperate woody species over 1980-2012: effects of chilling, precipitation and insolation.

Recent studies have revealed large unexplained variation in heat requirement-based phenology models, resulting in large uncertainty when predicting ecosystem carbon and water balance responses to climate variability. Improving our understanding of the heat requirement for spring phenology is thus urgently needed. In this study, we estimated the species-specific heat requirement for leaf flushing of 13 temperate woody species using long-term phenological observations from Europe and North America. The species were defined as early and late flushing species according to the mean date of leaf flushing across all sites. Partial correlation analyses were applied to determine the temporal correlations between heat requirement and chilling accumulation, precipitation and insolation sum during dormancy. We found that the heat requirement for leaf flushing increased by almost 50% over the study period 1980-2012, with an average of 30 heat units per decade. This temporal increase in heat requirement was observed in all species, but was much larger for late than for early flushing species. Consistent with previous studies, we found that the heat requirement negatively correlates with chilling accumulation. Interestingly, after removing the variation induced by chilling accumulation, a predominantly positive partial correlation exists between heat requirement and precipitation sum, and a predominantly negative correlation between heat requirement and insolation sum. This suggests that besides the well-known effect of chilling, the heat requirement for leaf flushing is also influenced by precipitation and insolation sum during dormancy. However, we hypothesize that the observed precipitation and insolation effects might be artefacts attributable to the inappropriate use of air temperature in the heat requirement quantification. Rather than air temperature, meristem temperature is probably the prominent driver of the leaf flushing process, but these data are not available. Further experimental research is thus needed to verify whether insolation and precipitation sums directly affect the heat requirement for leaf flushing.

Transcription profiling of the chilling requirement for bud break in apples: a putative role for FLC-like genes.

Apple production depends on the fulfilment of a chilling requirement for bud dormancy release. Insufficient winter chilling results in irregular and suboptimal bud break in the spring, with negative impacts on apple yield. Trees from apple cultivars with contrasting chilling requirements for bud break were used to investigate the expression of the entire set of apple genes in response to chilling accumulation in the field and controlled conditions. Total RNA was analysed on the AryANE v.1.0 oligonucleotide microarray chip representing 57,000 apple genes. The data were tested for functional enrichment, and differential expression was confirmed by real-time PCR. The largest number of differentially expressed genes was found in samples treated with cold temperatures. Cold exposure mostly repressed expression of transcripts related to photosynthesis, and long-term cold exposure repressed flavonoid biosynthesis genes. Among the differentially expressed selected candidates, we identified genes whose annotations were related to the circadian clock, hormonal signalling, regulation of growth, and flower development. Two genes, annotated as FLOWERING LOCUS C-like and MADS AFFECTING FLOWERING, showed strong differential expression in several comparisons. One of these two genes was upregulated in most comparisons involving dormancy release, and this gene's chromosomal position co-localized with the confidence interval of a major quantitative trait locus for the timing of bud break. These results indicate that photosynthesis and auxin transport are major regulatory nodes of apple dormancy and unveil strong candidates for the control of bud dormancy.

USING NON-PARAMETRIC REGRESSION TO MODEL DORMANCY REQUIREMENTS IN ALMONDS

Bud dormancy, by which buds must experience a certain amount of chill and heat before blooming or leafing out, affects orchard management and cultivar planting choices, and will be an important component to the effects of climate change on tree crops. Previous work has found that there is a partially compensatory relationship between chill and heat requirements in bud dormancy, as opposed to the common conception of one numeric combination of chill and heat alone resulting in bud-break. To date, this relationship has not been satisfactorily modeled for horticultural crops in a Mediterranean climate. This work aimed to address one of the first questions in defining this relationship – when do chill and heat accumulations start? Previous models have assumed starting dates based on historic conditions, or fitted these dates as model parameters. We have instead worked with a number of non-parametric approaches not previously utilized. This study used bloom timing data for Prunus dulcis ‘Nonpareil’, from three locations spanning the Central Valley of California, over ten years (1996-2005). We first used a fitted spline to help identify the most promising starting dates of chill and heat accumulations. Then, we used LOESS to fit polynomial regression functions based on local neighborhoods of data points. Using the Dynamic Model for chill accumulation starting in October, and the ASYMCUR GDH heat accumulation model starting in January, bloom timing was well modeled in 90% of the site-years. These dates differ from many found in the literature, suggesting pomologists and growers may be miscalculating when dormancy requirements have been met. Our results further support the theoretical model of a partially compensatory relationship between chill and heat towards stimulating bud-break.

Chilling requirement of Ribes cultivars.

It is usually thought that adequate winter chill is required for the full flowering of many temperate woody species. This paper investigates the sensitivity of blackcurrant bud burst and flowering to natural weather fluctuations in a temperate maritime climate, and compares a range of chill models that have been proposed for assessing the accumulation of winter chill. Bud break for four contrasting cultivars are compared in an exceptionally cold and in a mild winter in Eastern Scotland. The results confirm the importance of chilling at temperatures lower than 0°C and demonstrate that no single chilling function applies equally to all blackcurrant cultivars. There is a pressing need for further model development to take into account the relationship between chilling temperatures and warming temperatures occurring both during and after the chill accumulation period.

Breaking Dormancy of “Tupy” Blackberry in Subtropical Conditions

In subtropical or tropical conditions, the insufficient winter chill accumulation is often a limiting factor to break the dormancy of temperate-climate species such as the blackberry, which requires using products to help break dormancy. This study evaluates the efficacy of compounds in breaking dormancy of blackberry and its consequent influence on phenology and crop yields. The experiment was conducted in S?o Manuel, State of S?o Paulo, Brazil, in the 2011/2012 production cycle. The plants used were two-year-old “Tupy” blackberry (Rubus spp.), with 0.6 × 4.0 m spacing (4.166 plants·ha-1). Pruning was performed in August, followed by the application of these treatments: control (water); hydrogen cyanamide (Dormex?); nitrogen fertilizer (Erger?) and mineral oil (Assist?). The concentrations of each compound used were: 2.0%, 4.0%, 6.0% and 8.0%. The compounds used influenced the budding, flowering and fruit harvest stages, in addition to providing increased yields depending on the concentration used. For hydrogen cyanamide the recommended concentration is of 4.2% and 5.4% for nitrogen fertilizer; doses above these concentrations may cause phytotoxic effects. For mineral oil the dose recommended is of 8.0%.

Green-up of deciduous forest communities of northeastern North America in response to climate variation and climate change

Temporal shifts in phenology are important biotic indicators of climate change. Satellite-derived Land Surface Phenology (LSP) offers data for the study of vegetation phenology at landscape to global spatial scales. However, the mechanisms of plant phenological responses to temperature are rarely considered at broad spatial scales, despite the potential improvements to spatiotemporal predictions. Geographical gradients in community species composition may also affect LSP spatially and temporally. Using a modified survival analysis, we reveal how weather and climate relate to physiological chilling and heating requirements and affect deciduous forest green-up in New England, USA over 9 years (2001–2009). While warm daily temperatures lead to earlier green-up of deciduous forests, chilling temperatures had a larger influence on green-up. We also found that the effects of community composition across the landscape were as important as the effects of weather. Greater oak dominance led to later green-up, while sites with more birch tended to have earlier green-up dates. Projection into the future (2046–2065) with statistically downscaled, bias corrected climate model output suggested advanced green-up (8–48 days) driven by higher heating and chilling accumulations, but green-up in coastal areas may be delayed due to reduced chilling accumulation. This study provides an innovative statistical method combining plant physiological mechanisms, topographic spatial heterogeneity, and species composition to predict how LSP responds to climate and weather variation and makes future projections.

Comparison of methods for estimation of chilling and heat requirements of nectarine and peach genotypes for flowering

Chilling and heat requirements for breaking dormancy and flowering were studied in 63 nectarine and 118 peach genotypes (Prunus persica (L.) Batsch) for seven years. We compared two methods for estimation of requirements and four models for calculation of chill accumulation: Chilling Hours (CH), Chilling Units (CU), Positive Chilling Units (PCU) and Dynamic, while heat accumulations were estimated in growing degree hours (GDH). Additionally, correlations between chilling requirements, heat requirements and flowering date were established. Our results suggest that the Luedeling model is better than the Alonso model to estimate chilling and heat requirements in our field conditions. Moreover, it is recommendable to calculate the chill accumulation in accordance with the Chilling Hours or Positive Utah models. Additionally, since chilling requirements were positively correlated with flowering dates, they seem to be important to regulate blooming in nectarine and peach genotypes.

Chilling trends in a warm production area and their impact on flowering and fruiting of peach trees

Biological effects of contrasting chill accumulation on scion-rootstock combinations of peach were investigated using field observations during 2006–2010. Early and mid-season ripening peach cultivars (Early May Crest and Royal Glory, respectively) grafted on GF677 and Cadaman rootstocks were monitored over a five year period in the region of Mornag in northern Tunisia (36°41′N, 10°15′E). Flowering date, flowering duration and double fruits rate were observed over the entire experimental period. The phenological behavior, expressed as fruit set and flower buds abscission, was also surveyed. At harvest, yield, fruit size and number of commercial fruits were determined. Trends of chilling accumulation were characterized over twenty years using two chilling hours models (0–7.2°C model, CH and Crossa-Raynaud model, CHCR), Utah and Dynamic models. Correlations among chilling models showed that the Dynamic model is more appropriate to estimate chilling accumulation in our warm production area. Climatic conditions during the five experimental years were variable, with chill accumulation in the range 127–421 CH, 82–294 CHCR, 298–860 chill units (CU) and 19–50 chilling portions (CP). Exceptionally warm conditions were observed in winter 2006–2007 and 2009–2010 with significant drop of chill accumulation to levels less than 50% of average chill accumulation. Flowering and fruiting of both early and mid-season maturing commercial cultivars were affected by chill accumulation variability. Warmer winter delayed flowering, extended flowering duration, increased bud abscission and affected fruit set. A chill accumulation less than threshold values of 25 CP, 200 CH, 100 CHCR, and 350 CU resulted in substantial decrease of yield and fruit quality. Cultivar responses to chilling conditions in term of flowering date and yield seemed to be rootstocks dependant. Adoption of appropriate scion-rootstock combination and cultural practices based on chill accumulation could be used in peach industry as means to adapt to increasing frequency of warm winters.