Dynamic of chill accumulation in Australian pistachios

Insufficient winter chill accumulation can detrimentally impact agriculture. Understanding the changing risk of insufficient chill accumulation can guide orchard management and cultivar selection for long-lived perennial crops including peaches. This study quantifies the influence of modeled anthropogenic climate change on observed chill accumulation since 1981 and projected chill accumulation through the mid-21st century, with a focus on principal peach-growing regions in the southeastern United States, and commonly grown peach cultivars with low, moderate, and high chill accumulation requirements. Anthropogenic climate change has reduced winter chill accumulation, increased the probability of winters with low chill accumulation, and increased the likelihood of winters with insufficient chill for commonly grown peach cultivars in the southeastern United States. Climate projections show a continuation of reduced chill accumulation and increased probability of winters with insufficient chill accumulation for cultivars with high chill requirements, with approximately 40% of years by mid-century having insufficient chill in Georgia. The results highlight the importance of inter-annual variability in agro-climate risk assessments and suggest that adaptive measures may be necessary in order to maintain current peach production practices in the region in the coming decades.

Previous studies have shown a negative correlation between chilling accumulation (CA) and heat requirements (HRs) in peaches (Prunus persica (L.) Batsch), consistent with findings in other plants in spring events. However, there is a lack of comparative research on the CA–HR relationship in peach varieties with different chilling requirements (CRs), and the specific impact of CA on HR reduction remains poorly described. To address this, we investigated the effects of CA on the days and HR for flowering in 54 peach varieties of differing CRs. Scanning electron microscopy was used to observe the effects of CA on the phenology of floral organ development in a selected peach variety. Our results indicate that, in general, peaches exhibit a reduced HR and accelerated flowering as the CA increases, but that the strength and trend of the CA–HR relationship is influenced by the CR and the variety. Low-CR varieties showed less sensitivity to CA increments, requiring higher relative increases in CA to significantly lower the HR, whereas high-CR varieties appeared to be more sensitive, with even modest changes leading to substantial reductions in HR. However, variations from this generality exist, even within varieties displaying the same rCA (the ratio of CA to CR). Additionally, we provide a summary of the relationship between the rCA and drHR in peaches of differing CRs, and identify several varieties exhibiting a strong response in the CA–HR relationship. This study also highlights the impact of CA on flower bud development, revealing slower progression under lower CA levels and accelerated growth with an increased CA. In particular, we identified the critical period of the enlargement and initiation of green scales as indicative of successful pollen grain formation. Finally, we present a schematic of the CA–HR relationship for flowering in peaches.

Fundamental questions in bud dormancy remain, including what temperatures fulfill dormancy requirements (i.e. chill accumulation). Recent studies demonstrate freezing temperatures promote chill accumulation and cold hardiness influences time to budbreak-the phenotype used for dormancy evaluations. Here we evaluated bud cold hardiness and budbreak responses of grapevines (Vitis hybrids) throughout chill accumulation under three treatments: constant (5 °C), fluctuating (-3.5 to 6.5 °C daily), and field conditions (Madison, WI, USA). Chill treatments experiencing lower temperatures promoted greater gains in cold hardiness (field>fluctuating>constant). All treatments decreased observed time to budbreak with increased chill accumulation. However, perceived treatment effectiveness changed when time to budbreak was adjusted to remove cold acclimation effects. Among three classic chill models (North Carolina, Utah, and Dynamic), none was able to correctly describe adjusted time to budbreak responses to chill accumulation. Thus, a new model is proposed that expands the range of chill accumulation temperatures to include freezing temperatures and enhances chill accumulation under fluctuating temperature conditions. Most importantly, our analysis demonstrates that adjustments for uneven acclimation change the perceived effectiveness of chill treatments. Therefore, future work in bud dormancy would benefit from simultaneously evaluating cold hardiness.

Although the impacts of teleconnection indices on climate metrics such as precipitation and temperature in California have been widely studied, less attention has been given to the impact on integrated climate indices such as chill accumulation. This study investigates the linkages between large-scale teleconnections and winter chill accumulation for specialty crops in California, which may enable more effective and dynamic adaptation to in-season climate variability. Three large-scale teleconnection indices were selected: Oceanic Nino Index (ONI), Pacific-North American teleconnection pattern (PNA), and Pacific Decadal Oscillation (PDO) index to assess their effects on chill accumulation. The Chill Hours Model and Dynamic Model are adopted to calculate chill accumulation in Chill Hours (CH) and Chill Portions (CP) from November to January. Three major crop-producing regions, including the Central Coast, Sacramento Valley, and San Joaquin Valley, are used as the focused regions. Our results suggest CP generally has a stronger response to teleconnection patterns than CH in California. The correlations between chill accumulation and teleconnections are generally weaker during the summer than other seasons, and significant correlation can be observed 2–10 months before the start of the chill accumulation period. Among the three teleconnection indices, ONI is most weakly correlated to chill accumulation in focused regions, while PDO shows the strongest positive correlation and explains up to 39% variability of CP. PNA presents the most widespread negative correlation with chill accumulation. When aggregated to different teleconnection modes, +3.6 above-average CP is expected during ONI positive mode; +2.3 above-average CP is expected during PDO positive mode, while +2.1 above-average CP is expected during PNA negative mode. This study provides insights on early-season chill prediction and feasible management and adaptation strategies, and the methodology presented here can be used to develop decision support tools of risk control for agricultural producers and policymakers.

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

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

Chilling accumulation and photoperiod regulate rest break and bud burst in five subtropical tree species

Abstract. Growing trees are quite vulnerable to cold temperatures. To minimise the effect of these cold temperatures, they stop their growth over the coldest months of the year, a state called dormancy. In particular, endodormancy requires accumulating chilling temperatures to finish this sort of dormancy. The accumulation of cool temperatures according to specific rules is called chilling accumulation, and each tree species and variety has specific chilling requirements for correct plant development. Under global warming, it is expected that the fulfilment of the chilling requirements to break dormancy in fruit trees could be compromised. In this study, the impact of climate change on the chilling accumulation over peninsular Spain and the Balearic Islands was assessed. For this purpose, bias-adjusted results of 10 regional climate models (RCMs) under Representative Concentration Pathways (RCPs) 4.5 and 8.5 were used as inputs of four different models for calculating chilling accumulation, and the results for each model were individually compared for the 2021–2050 and 2071–2100 future periods under both RCPs. These results project a generalised reduction in chilling accumulation regardless of the RCP, future period or chilling calculation model used, with higher reductions for the 2071–2100 period and the RCP8.5 scenario. The projected winter chill decrease may threaten the viability of some tree crops and varieties in some areas where the crop is currently grown, but also shows scope for varieties with lower chilling requirements. The results are relevant for planning future tree plantations under climate change, supporting adaptation of spatial distribution of tree crops and varieties in Spain.

The objective of this study was to develop a model to estimate the chilling requirements of 10 oriental pear cultivars by standardizing dormancy depth. The dormancy depth of these cultivars was standardized to a range of 0–100 by setting the lowest bud burst rate during endodormancy as 0 and the bud burst rate immediately after the completion of endodormancy as 100. The endodormancy period was divided into periods. Stage I represented continuously increasing dormancy depth until the deepest dormancy was reached. Stage II represented the period of decreasing dormancy depth following the deepest dormancy; this stage included the breaking of dormancy. In addition, the 10 studied cultivars were classified into three types according to their dormancy depth changing pattern: stage I drop type, stage II leap-up type, and stage I·II symmetric type. A chilling accumulation model was derived based on both the endodormancy period and the dormancy depth of each cultivar. The results showed that − 1.9 to 12.0 °C was an effective range for chilling accumulation and that 2.1–4.0 °C was the most effective range for chilling accumulation. The results also revealed that negative accumulation, the reverse reaction of chilling accumulation, occurred under high-temperature conditions (above 14 °C). The results of the model verification process confirmed that the high-temperature conditions (above 14 °C) exhibited negative accumulation values due to an inverse relationship with dormancy depth during endodormancy stage I and that the chilling accumulation for the actual dormancy break mostly occurred during endodormancy stage II.

Graft-transmissible agents found in `Ta Tao 5' peach have been associated with phenological changes, including delay in bloom, reduced shoot vigor, and early autumn defoliation. Peach Latent Mosaic Viroid (PLMVd) is present as a graft-transmissible agent in `Ta Tao 5'. In order to further characterize the changes occurring in trees exposed to PLMVd from `Ta Tao 5' grafts, total fatty acid content was measured for peach buds during chilling accumulation and release from dormancy in `Coronet' peach trees and `Coronet' trees treated with `Ta Tao 5' bud grafts. Palmitic (16:0), stearic (18:0), oleic (18:1), linoleic (18:2), and linolenic (18:3) acids were the major fatty acids in dormant and releasing peach buds of both the controls and treated trees. The degree of unsaturation increased immediately following completion of chilling requirement in both the untreated controls and in the treated trees. However, the desaturation of linoleic acid to linolenic acid was significantly inhibited in the trees treated with `Ta Tao 5' bud grafts, which was accompanied by a concomitant delay in the resumption of growth. The disparity between the control and treated trees in the trend toward increased fatty acid unsaturation continued through the resumption of growth. The changes in degree of fatty acid saturation correlated with a response to forcing conditions and the release from dormancy. The presence of PLMVd in `Coronet' peach trees affects membrane fatty acid saturation during chilling accumulation and dormancy release. These findings suggest that metabolic pathways involving fatty acid desaturation are linked to the phenotypic variation in trees exposed to PLMVd.

Warming winters due to climate change may critically affect temperate tree species. Insufficiently cold winters are thought to result in fewer viable flower buds and the subsequent development of fewer fruits or nuts, decreasing the yield of an orchard or fecundity of a species. The best existing approximation for a threshold of sufficient cold accumulation, the "chilling requirement" of a species or variety, has been quantified by manipulating or modeling the conditions that result in dormant bud breaking. However, the physiological processes that affect budbreak are not the same as those that determine yield. This study sought to test whether budbreak-based chilling thresholds can reasonably approximate the thresholds that affect yield, particularly regarding the potential impacts of climate change on temperate tree crop yields. County-wide yield records for almond (Prunus dulcis), pistachio (Pistacia vera), and walnut (Juglans regia) in the Central Valley of California were compared with 50 years of weather records. Bayesian nonparametric function estimation was used to model yield potentials at varying amounts of chill accumulation. In almonds, average yields occurred when chill accumulation was close to the budbreak-based chilling requirement. However, in the other two crops, pistachios and walnuts, the best previous estimate of the budbreak-based chilling requirements was 19-32 % higher than the chilling accumulations associated with average or above average yields. This research indicates that physiological processes beyond requirements for budbreak should be considered when estimating chill accumulation thresholds of yield decline and potential impacts of climate change.

Temperate woody perennials survive to low temperatures in winter entering a dormant stage. Dormancy is not just a survival strategy, since chilling accumulation is required for proper flowering and arbitrates species adaptation to different latitudes. In spite of the fact that chilling requirements have been known for two centuries, the biological basis behind remain elusive. Since chilling accumulation is required for the normal growth of flower buds, it is tempting to hypothesize that something might be going on at this particular stage during winter dormancy. Here, we characterized flower bud development in relation to dormancy, quantifying changes in starch in the flower primordia in two sweet cherry cultivars over a cold and a mild year. Results show that, along the winter, flower buds remain at the same phenological stage with flower primordia at the very same developmental stage. But, surprisingly, important variation in the starch content of the ovary primordia cells occurs. Starch accumulated following the same pattern than chilling accumulation and reaching a maximum at chilling fulfillment. This starch subsequently vanished during ecodormancy concomitantly with ovary development before budbreak. These results showed that, along the apparent inactivity during endodormancy, flower primordia were physiologically active accumulating starch, providing a biological basis to understand chilling requirements.

Theterral 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.

The Western Cape region of South Africa, a major producer of deciduous fruit, has been identified as highly vulnerable to climate change. Climatic trends were analysed on a monthly basis for 12 rural meteorological stations across the region for the period 1967 to 2007. Significant warming trends were found for midto late-summer and end-winter to spring for daily minimum temperature (Tmin), and for mid-summer, autumn and spring for daily maximum temperature (Tmax). Chill unit accumulation has decreased significantly, particularly in autumn. Air temperatures are predicted to increase by a further 1-2°C within the next 30 years, together with decreasing rainfall especially in autumn/winter. An analysis of possible impacts of regional projections of climate change on deciduous fruit production was conducted, with the emphasis on chilling accumulation and dormancy in apples. The warmer or otherwise more marginal production areas, and high chill cultivars are likely to be the first to experience negative impacts of warming. INTRODUCTION Global climate change is occurring as evidenced by warming of average air and ocean temperatures, melting of snow and ice, and rising sea level (IPCC, 2007). Mediterranean-climate regions worldwide have been identified as highly vulnerable to climate change (Baldocchi and Wong, 2006), including the Western Cape region of South Africa (Midgley et al., 2005). Climate projections for fruit regions for 2030-2040 include warming by 1-2°C, reductions in frontal autumn and winter rainfall, and a contraction of the winter season (Hewitson et al., 2005). Apples are generally grown under conditions of insufficient winter chilling; this necessitates the use of chemical rest-breaking treatments to achieve satisfactory budburst, fruit set, yield, and fruit quality (Cook and Jacobs, 2000). In the event of climate warming, we expect that accumulation of chill hours will decrease, eventually reaching a critical threshold at which apple production would no longer be commercially sustainable in currently marginal areas. However, the rate of decline in chill hours could vary intra-seasonally, with different phenological results, and between colder and warmer production areas. We set out to analyze recent historical regional-scale changes in temperature and chill accumulation, predicted reductions in chill accumulation with future warming, and implications for fruit production, particularly apples. MATERIALS AND METHODS Recent Historical Temperature and Chill Unit Trends in the Western Cape Quality-controlled data for twelve meteorological stations throughout the Western Cape of South Africa (Fig. 1) were obtained from the division AgroMet of the Institute for Soil, Climate and Water, Agricultural Research Council, South Africa, for the period 1967-2007. The stations are spread throughout the climatically divergent fruit-growing regions. Daily minimum (Tmin) and maximum (Tmax) air temperatures, and monthly means of Tmin and Tmax were used. Langkloof Experimental Farm was excluded due to closure in Proc. IX IS on Orchard Systems

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