Climatic requirements during dormancy in apple trees from northwestern Spain – Global warming may threaten the cultivation of high-chill cultivars

Highlights

• Chill requirements (CR) of apple cultivars ranged from 59 to 90 Chill Portions (CP).

• Decreasing winter chill is projected in northwestern Spain under all scenarios.

• Chill-related problems are likely to arise on farms growing high-chill cultivars.

• Cultivars with low CR are expected to be viable under pessimistic climate scenarios.

• Adaptation strategies are feasible since 72 CP are expected for the worst scenario.

Abstract

Winter chill is expected to decrease in many mild-winter regions under future climatic conditions. Reliable estimates of the chill requirements (CR) of fruit trees are essential for assessing the current suitability of cultivars and potential climate change impacts on fruit production. We determined chill and heat requirements of ten apple cultivars in northwestern Spain using a bud-forcing method. CR ranged from 59 (‘Granny Smith’) to 90 (‘Regona’) Chill Portions (CP) according to the Dynamic Model. These results indicate that international dessert apple cultivars such as ‘Elstar’ and ‘Granny Smith’ have clearly lower CR than the studied local cultivars. The agro-climatic needs of the traditional apple cultivars are aligned with the historical climate conditions in the region. To assess future apple cultivation in northwestern Spain, we evaluated winter chill availability over the course of the twenty-first century by applying an ensemble of future climate scenarios. Relative to the past, projected winter chill might decline by between 9 and 12 CP under an intermediate global warming scenario and by between 9 and 24 CP under a pessimistic scenario. Despite relatively minor changes, the viability of some local apple cultivars may be jeopardized by their high CR. Results suggest that even a moderate decline in future winter chill, relative to fairly high levels observed in the past, can threaten the economic sustainability of fruit tree orchards composed of high-chill genotypes. Strategies such as growing low- to moderate-chill cultivars may be critical for sustaining future apple production in the region. Our findings can help guide new breeding strategies aiming to develop climate-resilient cultivars adapted to future environmental conditions.

Introduction

Dormancy is a physiological mechanism that allows temperate trees to cease growth and remain inactive during unfavourable climatic conditions in order to protect sensitive tissue (Faust et al., 1997). From dormancy onset to flowering time, trees experience complex physiological modifications triggered by cold and warm temperatures (Luedeling et al., 2013; Malagi et al., 2015). According to Lang et al. (1987), the dormant period can be divided into two sub-stages: endodormancy, usually known as the true dormant state and regulated by cold temperatures; and ecodormancy, which is mainly modulated by favourable environmental conditions that eventually evoke bud burst. To overcome dormancy and resume growth in the following season, temperate fruit trees must fulfil a genotype-specific chill requirement (CR) between late autumn and early spring (Campoy et al., 2011a; Erez, 2000; Luedeling, 2012). In mild winter climates, insufficient chill accumulation during dormancy can lead to numerous phenological disorders such as uneven vegetative development, bud abortion, a delayed and extended flowering period, poor fruit set and, ultimately, a significant reduction in final yield (Atkinson et al., 2013; Petri and Leite, 2003; Sunley et al., 2006). In order to explain the process of winter chill accumulation in deciduous fruit trees, various models have been developed. Traditionally, the Chilling Hours Model (Hutchins, 1932, as cited by Weinberger, 1950) has been used to quantify chill accumulation. The Utah Model (Richardson et al., 1974) provides good results in cold climates but has not performed well in mild-winter regions (Dennis, 2003). Finally, the Dynamic Model (Erez et al., 1990; Fishman et al., 1987a, 1987b) is recognized to date as the most adequate model for mild-winter areas (e.g. Campoy et al., 2013; Luedeling et al., 2009; Parkes et al., 2020; Ruiz et al., 2007). Regarding the quantification of heat, Growing Degree Hours (GDH) are estimated using the Growing Degree Hours Model proposed by Anderson et al. (1986). Large genotypic variation in chill requirements has been reported among temperate fruit trees. Knowing the chilling requirement of a specific cultivar has important practical and economic implications (Fennell, 1999).

The European Apple Inventory (Watkins, 1984) lists around ten thousand cultivars of apple (Malus domestica Borkh.). In comparison with other fruit species, most apple cultivars show high to medium chilling requirements (El Yaacoubi et al., 2016; Hauagge and Cummins, 1991; Parkes et al., 2020), mostly because the wild relatives of the domesticated apple originated from central and inner Asia where winters are very cold (Forsline et al., 2003; Ignatov and Bodishevskaya, 2011). However, several breeding programs have released cultivars with lower CR in order to extend the cultivation of this species to warmer climates. Although few studies have reported on the CR of different apple cultivars, the methodologies employed for the determination of the trees’ climatic needs vary widely, hampering the comparability of results among studies (El Yaacoubi et al., 2016; Funes et al., 2016; Guak and Neilsen, 2013; Hauagge and Cummins, 1991; Parkes et al., 2020). Two main tests are often used to experimentally determine the chilling requirements in temperate fruit trees after a certain period under controlled conditions: measurement of the weight of floral primordia (Tabuenca, 1964) and forcing of detached shoots under warm conditions (e.g. Cook and Jacobs, 2000; Cook et al., 2017; Malagi et al., 2015; Parkes et al., 2020; Prudencio et al., 2018). Forcing long cuttings with numerous buds from the previous year and monitoring their phenological evolution has been the preferred choice in many studies (Dennis, 2003). On the other hand, statistical approaches based on long-term phenological datasets have also been used to determine the length of the endodormancy phase (Alonso et al., 2005; Darbyshire et al., 2016; Funes et al., 2016; Luedeling et al., 2021, 2013; Luedeling and Gassner, 2012). The dynamics of dormancy release in apple trees cultivated in Oceanic climate regions have rarely been studied. Asturias, a coastal location in North-West Spain, has an Oceanic climate, subtype Atlantic, according to Martín-Vide and Olcina (2001), with mild winters in the fruit production areas (Delgado et al., 2021). Cider apples have a high economic value in Asturias representing approximately 20% of the total production of Spain (MAPA, 2019). Apart from the direct impact on the growers’ economy, the cider industry also has industrial, cultural and social relevance in the region. The Servicio Regional de Investigación y Desarrollo Agroalimentario de Asturias (SERIDA) maintains an apple germplasm bank with 800 accessions, out of which 525 are Asturian apple accessions. Most of the cultivars grown in the region during the past 30 years were selected by the institute and included under a “Protected Designation of Origin” quality label (Dapena and Blázquez, 2009). To avoid early flowering after warm spells during winter, approximately 86% of the locally maintained apple cultivars exhibit intermediate to very late flowering times (Dapena, 1996).

Significant changes in mean temperature are predicted for the 21st century due to climate change (IPCC, 2014). For future scenarios, the Intergovernmental Panel on Climate Change (IPCC) describes four Representative Concentration Pathways (RCPs) of atmospheric greenhouse gas concentrations (IPCC, 2014). These pathways include an intermediate scenario (RCP4.5) as well as a pessimistic scenario with very high greenhouse gas concentrations (RCP8.5; IPCC, 2014). The pace of global warming depends on the mitigation measures implemented to tackle climate change. Without significant action, annual mean global temperature increase is likely to exceed 1.5 °C between 2030 and 2052 compared to pre-industrial levels (IPCC, 2018). Global changes in environmental conditions will potentially affect phenology trends in temperate fruit trees (Menzel et al., 2006), and higher winter temperatures are expected to compromise the fulfilment of CR necessary to break endodormancy (Campoy et al., 2011a; Darbyshire et al., 2011; Luedeling et al., 2009, 2011; Luedeling, 2012). The impacts of increasing temperatures on winter chill can vary across different growing regions (Fernandez et al., 2020c; Luedeling et al., 2011). In some regions, the impacts of climate change on temperate fruit production may be severe, and adaptation measures will be required in order to ensure the economic viability of farms (Benmoussa et al., 2017; Campoy et al., 2011a; Luedeling et al., 2011). Projected climate change impacts on winter chill accumulation in apple trees have been studied in warm-winter climates such as Australia (Darbyshire et al., 2014; Parkes et al., 2020) and northeastern Spain (Funes et al., 2016; Rodríguez et al., 2021). In such locations, some of the widely grown apple varieties are expected to be unsuitable due to both the medium/high chill needs of most of the commercial varieties and the severe decline in winter chill expected for the second part of the 21st century.

In a recent study, Delgado et al. (2021) analysed phenological and temperature trends in Asturias over a 41-year period (1978–2019). This work showed that a temperature rise of 0.30 °C/decade did not imply a significant reduction in winter chill accumulation (in Chill Portions), indicating that cultivar-specific CR have been easily satisfied in the past. However, the rate of greenhouse gas emissions can change over the coming decades, affecting the rate of temperature increases, which may lead to conditions that fall outside the effective range defined for chill accumulation according to different chill models (Fernandez et al., 2020c). The local cultivars traditionally planted in the Asturias region are phenotypically adapted to their area of origin, which may explain observations of weak phenological responses to warming conditions (Delgado et al., 2021). However, the phenotypic plasticity to respond to the increase in global temperatures inevitably has limits (Donnelly et al., 2012), and some consequences of insufficient winter chill, such as delayed and irregular bud burst, were observed in a small number of cultivars in recent growing seasons (Consejo Regulador de la Denominación de Origen Protegida Sidra de Asturias, personal communication). Climatic conditions projected for the future may threaten the viability of the local cultivars, making adaptation strategies essential for ensuring orchard viability in a warming future. For this reason, precise characterization of the chill and heat requirements of each individual genotype is needed to support farmers in identifying adequate sets of cultivars that allow them to adapt their orchards to future climate conditions.

The main goals of this study were (1) to calculate the agro-climatic requirements of ten apple cultivars by implementing a reliable method for forcing shoots under environmentally controlled conditions and compare the precision and reliability of chill models under the mild-winter conditions of northwestern Spain; (2) to evaluate the impacts of climate change on winter chill for the 21st century for an ensemble of future climate scenarios; and (3) to generate a portfolio of the most suitable apple cultivars to be cultivated in the Asturias region under future climate scenarios.