Influence of Gene x Environment
Whilst the general biosynthesis and regulation of colour pathways has been elucidated, the final colour of the fruit is determined by both genetic and environmental factors. The genetic background determines the ability of fruit to produce certain pigments, whilst environmental factors affect the final composition and amounts of pigments in a ripe fruit. The fruit skin, which acts as a barrier protecting the internal tissues of developing fruit, including the photosynthetic machinery, is where the greatest effects of environmental factors on fruit pigment production are normally detected. Both controlled and field studies with cultivated and wild ecotypes have provided a better understanding of gene x environment (GxE) interaction in fruit pigment production (Jaakola & Hohtola 2010). Such studies have identified variation in anthocyanin content between fruit ecotypes or cultivars growing in different geographical locations (Fredes et al. 2014; González-de-Peredo et al. 2019; Sikuten et al. 2021). With wild Vaccinium berries, field studies have often shown high variation in the anthocyanin composition between growth location and seasons (Connor et al. 2002; Rohloff et al. 2015). However, specific geographical trends also have been observed, such as higher total anthocyanin contents and a higher proportion of delphinidin branch anthocyanins towards northern latitudes (Åkerström et al.2010; Latti et al. 2010; Lätti, Riihinen & Kainulainen 2008) and at higher altitudes (Zoratti et al. 2015). A controlled study with southern and northern bilberry ecotypes indicated that ecotypes from higher altitudes produced a higher concentration of anthocyanins irrespective of day length and temperature conditions (Uleberg et al. 2012). Geographical variation has also been detected in the carotenoid contents of different fruit species and cultivars (Diaset al. 2021). In some cases, climatic factors have been found to have greater impacts on carotenoid biosynthesis than cultivar differences, for instance in papaya (Kimura, Rodriguez-Amaya & Yokoyama 1991), mango (Mangifera indica ) (Mercadante & Rodriguez-Amaya 1998), and West Indian cherry (Malpighia glabra ) (Rodriguez-Amaya 2001). A comprehensive analysis of the carotenoid content in Portuguese fruit showed variation between the same apple and orange (Citrus sinensis ) cultivars grown in different geographical regions (Dias, Camões & Oliveira 2009). High variation in carotenoid content was detected between wild Seabuckthorn (Hippophae rhamnoides ) populations growing at different altitudes trans-Himalaya in India (Korekar et al. 2014). The major stressors are considered in the following sections, with the proviso that GxE can have additional and compounding effects.