Chilling Accumulation Research Articles

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.

Dynamic of chill accumulation in Australian pistachios

Dynamic of chill accumulation in Australian pistachios

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.

The potential for an increasing threat of unseasonal temperature cycles to dormant plants.

Two functional responses largely guide woody plants' survival to winter conditions: cold hardiness and dormancy. Dormancy affects budbreak timing based on chill accumulation. Effects of warming on dormancy may appear time-shifted: fall and winter warming events decrease chill accumulation, delaying budbreak observed in spring. The same warming events also affect cold hardiness dynamics, having immediate implications. As cold deacclimation rates increase with dormancy progression, the same amount of warming has greater damage risk the later it occurs in the season, depending on return of low temperatures. Should frequency of erratic weather increase with climate change, more instances of risk are expected. However, understanding how plants fare through seasons now and in future climates still requires better knowledge of winter physiology.

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.

Effectiveness of freezing temperatures on dormancy release of temperate woody species.

Spring phenological change of plants in response to global warming may affect many ecological processes and functions. Chilling temperature regulates budburst date by releasing dormancy. However, whether freezing temperature (<0°C) contributes to dormancy release is still debated. Our poor understanding of the role of chilling makes estimating shifts in budburst date difficult. A two-year chilling-forcing experiment was explicitly designed to test the effects of chilling temperatures on dormancy release of 9 temperate woody species in Beijing, China. A total of 1620 twigs were first exposed to a wide range of temperatures (-10 to 10 °C) with different durations and then moved to growth chambers. Based on budburst data in experimental conditions, we examined whether freezing temperatures are effective on dormancy release. We also developed a new framework for constructing chilling functions based on the curve between chilling duration and forcing requirement (FR) of budburst. The chilling function derived from this framework was not affected by experimental forcing conditions. We demonstrated that freezing temperatures down to -10°C were effective in dormancy release. The rate of dormancy release, indicated by the rate of decay in chilling duration-FR curve, did not differ significantly between chilling temperatures in most cases, although it exhibited a maximum value at 0 or 5°C. The chilling function-associated phenological models could simulate budburst date from independent experimental and observational data with a mean RMSE of 7.07 days. The effective freezing temperatures found here are contrary to the well-known assumption of <0°C temperature generally not contributing to accumulated chilling in many previous chilling functions. A chilling function assuming that temperature below an upper-temperature threshold has the same effects on dormancy release could be adopted to calculate chilling accumulation when using experiments to develop spring phenological models based on the chilling-forcing relationship.

No slowdown of growing season extension with warming in a permafrost‐affected meadow on the Tibetan Plateau

Abstract The Tibetan Plateau holds the world's largest alpine permafrost and is undergoing an acceleration of warming. Phenological shifts over alpine permafrost in a warmer world have been little studied and are greatly underrepresented in current syntheses. Here, we conducted seasonal and gradient temperature‐controlled experiments in a permafrost‐affected meadow to evaluate how warming drives shifts in spring and autumn phenology, and associated growing‐season length at both community and species levels. We found that there is no sign of slowdown in spring advance with warming under a higher year‐around warming treatment, aligning with a future medium warming scenario. Although spring advance led to an advance in autumn senescence according to spring‐only warming experiments, the advance could not offset the delay due to concurrent warming. As a result, year‐around warming significantly delayed autumn senescence, although there was a deceleration in delay with warming under high temperature treatment than under the low one. This finding can be attributed to the possibility that winter warming is insufficient to reduce chilling accumulation, which would not delay spring phenology and then lead to a non‐slowdown in spring phenological advancement. Taken together, there is no slowdown in an extension of growing season length with warming under a higher year‐around warming treatment, with an increase of length by 9 and 21 days at the end of this century under a CO2 stabilization and medium warming scenarios, respectively. Synthesis. Our results suggest that a continued growing season extension at least under the medium warming scenario would help permafrost‐affected meadow ecosystems to mitigate permafrost carbon release on the Tibetan Plateau.

Cold hardiness-informed budbreak reveals role of freezing temperatures and daily fluctuation in a chill accumulation model.

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.

Controlled experiments fail to capture plant phenological response to chilling temperature

AbstractAimControlled experiments are increasingly important for investigating how and to what degree plant phenology responds to global climate change. Current experiments underline that chilling and forcing temperatures are two major environmental cues shaping the budburst date of temperate species, but whether experiments could reflect the observed responses to chilling has rarely been examined.LocationEurope and North America.Time periods1951–2021.Major taxa studiedTemperate trees and shrubs.MethodsUsing an experimental database of budburst dates for 50 species derived from previous literature and observational data of the same species at 12,579 stations in Europe and 1469 stations in the USA, we compared the response of forcing requirement (FR) of the budburst date to chilling accumulation (CA) between observations and experiments using a common measure of FR and CA.ResultsThe median, variance and probability distribution of CA‐FR curves differed significantly between experiments and observations in most cases. The distinction in chilling effects between experiments and observations could be attributed to the difference in thermal space, heat stress, genetic variation among provenances, different forcing treatments adopted and plant materials used in the experiments.Main conclusionsOur results suggest that the uncertainty of phenological models based solely on the experimental data needs to be re‐evaluated when predicting future spring phenological responses across broad spatial scales.

Investigating the effect of plant growth regulators in the endodormancy release in apricot by a metabolomic approach

Endodormancy is one of the most studied physiological processes in perennial plants like apricot. This period is vital both for the tree survival against the adverse climatic conditions of winter and for obtaining a proper flowering and fruit set. Many studies have remarked the importance of chill accumulation as the limiting factor for endodormancy release. The increase of mean temperatures caused by climate change has been seriously endangering this process during the last decades. Because of this, plant growth regulators for promoting endodormancy release have spread worldwide. However, due to the toxicity and the irregular efficiency, there is a great necessity of developing new environment-friendly regulators for promoting endodormancy release. In this 3-year study, we applied four different commercial plant growth regulators to the Flopría apricot cultivar. Two of them, Broston® and Erger® were the most effective ones to advance endodormancy release. The physiology and biochemistry behind these treatments were studied by a non-target metabolomic and expression analysis in flower buds. Metabolic groups, like phospholipids, only varied in treated samples, whereas others like by-products of L-Phe metabolism, or ABA significantly varied in both types of samples throughout endodormancy release. Finally, to validate these results, solutions of phospholipids, phenylpropanoids, or ABA, among others, were applied for the first time to apricot trees, showing, i.e., that phospholipids treated-trees released from endodormancy two weeks earlier than control. This study aims to be an initial stage for the elaboration of environmentally safe regulators in apricot, with a potential in other Prunus and temperate fruit tree species.

Dormancy dynamics and cold hardiness of apple trees grown in a mild winter region

In temperate fruit plants, climate is the main factor correlating changes in plant physiology, mainly those related to entry and exit from dormancy and cold hardiness. Acclimation to cold and its relationship with dormancy are still unknown in areas with a subtropical climate. Therefore, this study aimed to investigate the dormancy dynamics and their interaction with the cold hardiness of different tissues of apple cultivars with different chilling requirements in two cultivation sites in a mild winter region in southern Brazil. The analyses were conducted from 2018 to 2022. The cultivars Gala, Fuji, and Eva were used in two environments with different altitudes. The dormancy dynamics were analyzed using the single-node cutting test, and the cold hardiness of the different tissues of each genotype was evaluated through electrolyte leakage in laboratory freezing tests. Cultivars Gala and Fuji had a greater depth of dormancy than Eva. Plants grown in orchards with higher chill accumulation had a greater depth of dormancy. Cold hardiness varied among cultivars, with the Eva cultivar having the lowest cold hardiness in mid-winter. The tissues (bud, bark, and wood) had different cold hardiness in early winter, with wood having the greatest cold hardiness, followed by bud and bark. As Eva did not go into deep dormancy, a superficial and short acclimation to cold was observed, in addition to rapid sprouting, even in winter. This condition predisposed it to cold damage to tissues in the pre-sprouting period. In this study, we found that maximum cold hardiness occurred after the end of the deep-endodormancy phase. Deacclimation of all cultivars occurred very quickly.

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.

Establishment of a Homologous Silencing System with Intact-Plant Infiltration and Minimized Operation for Studying Gene Function in Herbaceous Peonies.

Gene function verification is a crucial step in studying the molecular mechanisms regulating various plant life activities. However, a stable and efficient homologous genetic transgenic system for herbaceous peonies has not been established. In this study, using virus-induced gene silencing technology (VIGS), a highly efficient homologous transient verification system with distinctive advantages was proposed, which not only achieves true "intact-plant" infiltration but also minimizes the operation. One-year-old roots of the representative species, Paeonia lactiflora Pall., were used as the materials; prechilling (4 °C) treatment for 3-5 weeks was applied as a critical precondition for P. lactiflora to acquire a certain chilling accumulation. A dormancy-related gene named HOMEOBOX PROTEIN 31 (PlHB31), believed to negatively regulate bud endodormancy release (BER), was chosen as the target gene in this study. GFP fluorescence was detected in directly infiltrated and newly developed roots and buds; the transgenic plantlets exhibited remarkably earlier budbreak, and PlHB31 was significantly downregulated in silenced plantlets. This study established a homologous transient silencing system featuring intact-plant infiltration and minimized manipulation for gene function research, and also offers technical support and serves as a theoretical basis for gene function discovery in numerous other geophytes.

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.

Alternative products in the budbreak of ‘eva rubi’ apple trees under subtropical climate conditions

The apple tree requires chilling accumulation for budbreak. In locals where the chill accumalition is insufficient, chemical promoters are used to induce uniform budburst. However, alternatives that minimize damage to the environment and applicator have been sought. Thus, the aim of this work was to evaluate the efficiency of hydrogen peroxide, pau-d'alho hydrolate, lime sulphur and neem oil in inducing budbreak of ‘Eva Rubi’ apple tree in agroecological cultivation. Different concentrations (0%, 5%, 10%, 15% and 20%) of pau-d'alho hydrolate and hydrogen peroxide were tested in Palmas and Pato Branco. Three additionals treatments were added in the Lapa region, where mineral oil 3%, Dormex 0.5%, and Dormex 0.5% + 3% mineral oil were applied. In 2023, a new experiment was carried out only in Pato Branco, repeating the same concentrations of pau d'alho hydrolate and including new treatments with lime sulphur (1, 2, 3 and 4%) associated with 3% Neem oil. Percentage of budbreak, fruit set, productivity, and fruit mass were evaluated. The treatments showed no difference in the analyzed variables for Pato Branco and Palmas. In Lapa, there was an improvement in the percentage of budbreak in the plants treated with 15% ‘pau d'alho’ hydrolate, but still insufficient to state that this product can be used as budbreak inducer, however it suggests potential use, requiring, for that, the continuity of research in other species and experimental conditions. No phytotoxic effect was observed for hydrogen peroxide and ‘Pau d’alho’ hydrolate at the studied dosages.

Suitable areas for temperate fruit trees in a Brazilian hotspot area: Changes driven by new IPCC scenarios

Chilling accumulation and sufficient water availability are mandatory requirements for most temperate fruit trees (TFT) productivity. Given the strong dependence of these crops on air temperature and water availability, it is imperative to analyze the impacts of climate change on TFT, especially in areas considered persistent climate change hotspots. This study analyzed the impacts of climate change on suitable areas for planting TFT in a Brazilian state considered a hotspot for future climate changes - Minas Gerais, using an Agroclimatic Zoning (AZ) method, and outlined adaptation measures for the sustainable cultivation of the main TFT under future climate scenarios. Daily near-surface air temperature (minimum and maximum) and precipitation data were used from 10 Earth System Models (ESMs) from the NASA Earth Exchange Global Daily Downscaled Projections (NEX-GDDP), which are derived from the Coupled Model Intercomparison Project Phase 6 (CMIP6), in two Shared Socioeconomic Pathways scenarios (SSP2–4.5 and SSP5–8.5). The AZ method was applied for the current climate simulations (1995–2014), as well as for projections by the near (2041–2060) and far future (2081 – 2100) climate conditions considering two base temperature values (Tb = 7ºC and 13ºC). Given intense and persistent increases in air temperature, water deficit, and reduced chilling accumulation, TFT plantations will be restricted and economically unfeasible for about 69% (SSP2.4–5) to 86% (SSP5.8–5) of Minas Gerais in the far future for both Tb values. Only ∼6% (near future) and ∼2% (far future) of the study area located in a small portion of the Southern region of Minas Gerais will be suitable for TFT plantations. However, the future of TFT plantations in places classified as regular will depend on adopting adaptation measures to reduce vulnerability to these crops, including low-chill genotypes selection, improvements in breeding programs, using strategies to break dormancy artificially, and intensifying irrigation with less water-demanding techniques.

Daylength predominates the bud growth initiation of winter deciduous forest trees in the monsoon region of China.

Climate warming has induced significant shifts in spring phenology over both temperate and boreal forests. The timing of bud growth resuming from dormancy is crucial for predicting spring phenology. However, the mechanisms by which environmental cues, other than chilling accumulation, initiate bud growth remains unclear. By constructing a revised process-based spring phenology model incorporating photoperiod and temperature triggers of bud growth, we simulated the first leaf unfolding and first flowering dates of four deciduous forest trees during 1981-2014 at 102 stations across China's monsoon regions. Then, we revealed spatial patterns of the two triggers. Moreover, we compared fitting precision and robustness of the revised model with three mainstream models. Results show that the revised models can effectively simulate all spring phenology time series. Growth initiation of foliar and floral buds was induced by photoperiod lengthening in 80.8% and 77.7% of time series, and by temperature increasing in remaining 19.2% and 22.3% of time series, respectively. The proportions of time series with photoperiod- and temperature-initiated bud growth significantly increase and decrease from northern to southern climatic zones, respectively. Chilling exposure controls the predominant bud growth triggers in different climate zones. Specifically, in regions with long and severe winters where chilling requirement is easily fulfilled, rising temperature in spring alleviates the cold constraint and initiate bud growth. Conversely, in regions with short and mild winters, prolonged daylength in spring compensates the lack of chilling exposure to initiate bud growth. These findings suggest that photoperiod may limit spring phenology response to temperature in low-latitudes. Overall, our model slightly outperforms other models in terms of efficiency, accuracy, and robustness in modeling leaf unfolding and flowering dates. Therefore, this study deepens our understanding of the mechanisms of spring phenology, and improves the predicting capability of spring phenology models in the face of ongoing global warming.

Spring wood phenology responds more strongly to chilling temperatures than bud phenology in European conifers.

A comparative assessment of bud and wood phenology could aid a better understanding of tree growth dynamics. However, the reason for asynchronism or synchronism in leaf and cambial phenology remains unclear. To test the assumption that the temporal relationship between the budburst date and the onset date of wood formation is due to their common or different responses to environmental factors, we constructed a wood phenology dataset from previous literature, and compared it with an existing bud phenology dataset in Europe. We selected three common conifers (Larix decidua Mill., Picea abies (L.) H. Karst. and Pinus sylvestris L.) in both datasets and analyzed 909 records of the onset of wood formation at 47 sites and 238,720 records of budburst date at 3051 sites. We quantified chilling accumulation (CA) and forcing requirement (FR) of budburst and onset of wood formation based on common measures of CA and FR. We then constructed negative exponential CA-FR curves for bud and wood phenology separately. The results showed that the median, variance and probability distribution of CA-FR curves varied significantly between bud and wood phenology for three conifers. The different FR under the same chilling condition caused asynchronous bud and wood phenology. Furthermore, the CA-FR curves manifested that wood phenology was more sensitive to chilling than bud phenology. Thus, the FR of the onset of wood formation increases more than that of budburst under the same warming scenarios, explaining the stronger earlier trends in the budburst date than the onset date of woody formation simulated by the process-based model. Our work not only provides a possible explanation for asynchronous bud and wood phenology from the perspective of organ-specific responses to chilling and forcing, but also develops a phenological model for predicting both bud and wood phenology with acceptable uncertainties.

Transcription factors BZR2/MYC2 modulate brassinosteroid and jasmonic acid crosstalk during pear dormancy.

Bud dormancy is an important physiological process during winter. Its release requires a certain period of chilling. In pear (Pyrus pyrifolia), the abscisic acid (ABA)-induced expression of DORMANCY-ASSOCIATED MADS-box (DAM) genes represses bud break, whereas exogenous gibberellin (GA) promotes dormancy release. However, with the exception of ABA and GA, the regulatory effects of phytohormones on dormancy remain largely uncharacterized. In this study, we confirmed brassinosteroids (BRs) and jasmonic acid (JA) contribute to pear bud dormancy release. If chilling accumulation is insufficient, both 24-epibrassinolide (EBR) and methyl jasmonic acid (MeJA) can promote pear bud break, implying that they positively regulate dormancy release. BRASSINAZOLE RESISTANT 2 (BZR2), which is a BR-responsive transcription factor, inhibited PpyDAM3 expression and accelerated pear bud break. The transient overexpression of PpyBZR2 increased endogenous GA, JA, and JA-Ile levels. In addition, the direct interaction between PpyBZR2 and MYELOCYTOMATOSIS 2 (PpyMYC2) enhanced the PpyMYC2-mediated activation of Gibberellin 20-oxidase genes PpyGA20OX1L1 and PpyGA20OX2L2 transcription, thereby increasing GA3 contents and accelerating pear bud dormancy release. Interestingly, treatment with 5 μm MeJA increased the bud break rate, while also enhancing PpyMYC2-activated PpyGA20OX expression and increasing GA3,4 contents. The 100 μm MeJA treatment decreased the PpyMYC2-mediated activation of the PpyGA20OX1L1 and PpyGA20OX2L2 promoters and suppressed the inhibitory effect of PpyBZR2 on PpyDAM3 transcription, ultimately inhibiting pear bud break. In summary, our data provide insights into the crosstalk between the BR and JA signaling pathways that regulate the BZR2/MYC2-mediated pathway in the pear dormancy release process.

Time to budbreak is not enough: cold hardiness evaluation is necessary in dormancy and spring phenology studies.

Dormancy of buds is an important phase in the life cycle of perennial plants growing in environments where unsuitable growth conditions occur seasonally. In regions where low temperature defines these unsuitable conditions, the attainment of cold hardiness is also required for survival. The end of the dormant period culminates in budbreak and flower emergence, or spring phenology, one of the most appreciated and studied phenological events - a time also understood to be most sensitive to low-temperature damage. Despite this, we have a limited physiological and molecular understanding of dormancy, which has negatively affected our ability to model budbreak. This is also true for cold hardiness. Here we highlight the importance of including cold hardiness in dormancy studies that typically only characterize time to budbreak. We show how different temperature treatments may lead to increases in cold hardiness, and by doing so also (potentially inadvertently) increase time to budbreak. We present a theory that describes evaluation of cold hardiness as being key to clarifying physiological changes throughout the dormant period, delineating dormancy statuses, and improving both chill and phenology models. Erroneous interpretations of budbreak datasets are possible by not phenotyping cold hardiness. Changes in cold hardiness were very probably present in previous experiments that studied dormancy, especially when those included below-freezing temperature treatments. Separating the effects between chilling accumulation and cold acclimation in future studies will be essential for increasing our understanding of dormancy and spring phenology in plants.