Non-destructive chlorophyll fluorescence and colour measurements of 'Braeburn' and 'Royal Gala' apple (Malus domestica) fruit development over the growing season

Fruit size, skin colour determinations, and chlorophyll fluorescence of two apple (Malus domestica) cultivars, 'Braeburn' and 'Royal Gala', were followed throughout the growing season to assess fruit development. Chlorophyll fluorescence was measured with a pulse-amplitude modulated fluorometer on the blush and back sides of the fruit and skin colour with a tristimulus colorimeter giving colour coordinates as hue angle, chroma, and lightness coefficient. Fruit of both cultivars followed a single sigmoid growth pattern with 'Royal Gala' apples attaining full size at c. 140 days after full bloom (DAFB) and 'Braeburn' fruit at c. 168 DAFB. Fluorescence peaked at 80 DAFB then declined in all instances. Hue angle varied consistently throughout the growing season in both cultivars, indicative of shifts in colour from yellow-green to red. Background and blush colours of 'Braeburn' remained distinct throughout the growing season whereas for 'Royal Gala' the two sides merged to the same colour. Skin colour intensity also differed between the two sides initially but merged to a uniform intensity at the end of the growing season. Hue angle was best able to discriminate between the two cultivars in ripening patterns and thus would be a useful technique to quantify fruit development in an objective manner.