South European countries

Apple skin colour contributes to the appeal of the fruit, and as each cultivar has a characteristic colouration, it aids consumer recognition. A lack of red skin colouration can result in a reduced commercial value associated with poor consumer acceptance (Baugher et al., 1990). Along with fruit size, the intensity and quality of red skin colour provide the basis for the grading standards used by European Union (EU) countries. Red skin colouration in apple is due to the presence of anthocyanins, primarily cyanidin-3-galactoside or idaein (Lancaster, 1992). The development of anthocyanin is partly regulated by temperature (Lin-Wang et al., 2011; Palmer et al., 2012; Iglesias et al., 2016). The skin of red apple cultivars grown in warm climates often has poor reddening because of lower anthocyanin production under warm summer temperatures. One practice for optimising apple fruit colouration in commercial orchards is to increase light exposure of the fruit by using reflective films (Iglesias and Alegre, 2009), while another involves the indirect modification of temperature by the use of over-tree sprinkler irrigation (Williams, 1993; Iglesias et al., 2000). However, these techniques are expensive and alternatives are thus required. Planting new cultivars and/or strains with enhanced fruit colouration is a more cost-effective strategy. In the last decades, red sports of traditional bicoloured cultivars such as ‘Gala’, ‘Red Delicious’, ‘Jonagold’ and ‘Fuji’ have been widely planted in the main apple-producing areas of the world as they can achieve a more characteristic colouration in regions with hot summers (Iglesias et al., 1999, 2008, 2012; Iglesias and Echeverría, 2009). In addition, apple breeding programmes worldwide focus on developing new cultivars with superior fruit characteristics (including red colouration and eating quality) that are adapted to a range of environments (Janick et al., 1996; Fellman et al., 2000; Sansavini et al., 2005, 2012; Batlle et al., 2008; Cantin et al., 2015). The use of genetic markers linked to red colouration in marker-assisted breeding (MAB), and a better understanding of the molecular and physiological control of red skin colouration will contribute to developing new apple cultivars with high colouration that are adapted to a range of environments (Espley et al., 2007; Lin-Wang et al., 2011; Iglesias et al., 2016).