the reds, purples, and their blended combinations responsible for autumn foliage are derived from anthocyanins. unlike carotenoids, anthocyanins are not present in the leaf throughout the growing season, but are produced actively, toward the end of summer. they develop in late summer in the sap of leaf cells, resulting from complex interactions of factors inside and outside the plant. their formation depends on the breakdown of sugars in the presence of light as the level of phosphate in the leaf is reduced. orange leaves in autumn result from a combination of anthocyanins and carotenoids.