Flowering biology and agroclimatic requirements of cider apple (Malus domestica Borkh.) cultivars from Asturias in a context of climate change

Abstract

Like most woody perennial species that evolved in cold climates, apple (Malus domestica Borkh.) trees spend the winter months in a dormant state that allows them to survive unfavourable weather conditions (Campoy et al., 2011; Faust et al., 1997; Saure, 1985). To break dormancy, cultivar-specific requirements must be fulfilled. Inadequate dormancy release results in poor budbreak and delayed flowering, which negatively impact fruit set (Erez, 2000; Sunley et al., 2006). Since the dormancy phase is characterized by the absence of any easily observable changes in buds or other structures of the plant, the determination of chill and heat accumulation phases remains difficult to address in a satisfactory way (Fadón et al., 2020a). Asturias (north-western Spain) has a long tradition of apple growing, with local production largely based on traditional cider cultivars. To date, negative consequences of insufficient winter chill accumulation have rarely been observed in commercial apple orchards in the region. However, there is overwhelming scientific evidence that temperatures are rising worldwide (Cook et al., 2016), and these changes are expected to have severe environmental and biological implications (IPCC, 2014). One of the main aims of this thesis is to ascertain if the cultivars that originated and are currently well-adapted to the environmental conditions of Asturias will be still suitable in the future. In this sense, knowledge of the temperature requirements of the cultivars is an issue of practical concern (Fennell, 1999) for scientists and growers, since winter chill may decrease as a consequence of global warming in many areas (Fernandez et al., 2020c; Luedeling et al., 2011). The present PhD thesis aims to gain insights into the agroclimatic requirements of local apple cultivars and the historic and projected future changes in winter chill in Asturias. For this purpose, first, the thesis focuses on how changes in chill and heat accumulation have affected the phenology of apple cultivars in the region. Second, this research has evaluated the impacts of climate change on the availability of winter chill for the 21st century with the objective of choosing the most suitable apple cultivars to be grown in Asturias. Third, the thesis aims to improve the knowledge of several flowering-related traits of an extensive collection of apple accessions selected by the SERIDA Fruit Research Group in order to better manage interannual variation in pollination outcomes. This thesis is organized into four chapters that correspond to four scientific articles. Over a period of 41 years, winter chill accumulation did not show a significant decrease despite temperature increases by 0.30 °C per decade. The bloom-advancing effect of a warmer forcing phase appears to have partially overcompensated the bloom-delaying effect of a later onset of the endodormancy phase. In this thesis, chill and heat requirements from a set of apple cultivars were determined experimentally and statistically. The studies suggest that the methodology used for estimating the agroclimatic needs have a significant impact on the results. Nevertheless, both approaches showed that the majority of the Asturian apple cultivars have medium to high chill requirements. Regarding future scenarios, Asturias will not be severely affected by chill losses through the 21st century. However, the projections for a pessimistic setting show a notable risk of failing to fulfil the chill requirements of high-chill cultivars after 2070. Since high chill accumulation of 72 Chill Portions is still expected even for the worst-case scenario, developing adaptation strategies for the fruit industry should be quite feasible. Matching cultivars with the regional climate conditions appears to be the most promising adaptation option. Since most apple varieties are self-incompatible, the flowering period, genetic compatibility and pollen viability must be considered in attempts to optimize fruit yields. The characterization of floral and pollen traits demonstrated that the SERIDA apple collection presents large phenotypic variability in floral biology traits. Several cultivars have a long flowering period, an optimal level of return bloom and produce copious amounts of viable pollen. Some of the best pollen donor cultivars are the new releases `X9406-11, `Perurico and `Raxila Dulce, `Raxona Dulce and `Raxona Acida. Overall, the results and methods developed in this thesis are expected to be helpful to the local apple breeding program in the development of cultivars that remain viable in a warmer future. The methodologies presented in this work can be easily implemented by tree fruit breeding institutes worldwide