The red colour of apples is an important selection criterion for consumers and, therefore, also for retailers.
The colour is produced by compounds called anthocyanins and flavonols, and the intensity of colour is controlled by various factors. Light is probably the most important external factor and both its spectral quality and its daily integral have some effect.
The apple cultivar 'Fuji' was chosen for the experiments reported here because it is harvested late in autumn when poor colour development can be a problem, perhaps because of low light. Trees of 'Fuji' grafted on M9 were grown either under hail nets or in the open and either with or without a reflective foil spread over the orchard floor beneath the trees. The hail nets reduced average light levels but the reflective foil considerably increased the amount of light reaching the fruit, especially on their lower side.
The fruit with the most intense red colour generally came from trees that had reflective foil covering the orchard floor beneath them. This intense colour was associated with increases in certain anthocyanins. The poorest colour and the lowest concentrations of these anthocyanins occurred in fruit grown without any reflective floor covering and under hail nets.
The Effect of Reflective Foil and Hail Nets on the Lighting, Color and Anthocyanins of 'Fuji' Apple by Jakopic, Veberic and Stampar (2007). Scientia Horticulturae 115: 40-46. The contents of issues of Scientia Horticulturae and abstracts of papers are provided at www.elsevier.com/locate/scihorti.