Flower colour is an important feature for the aesthetic appeal of any ornamental crop. Information about the pigments present in a particular plant is necessary for the development of new colour varieties, even if different factors can influence the colour expression of the same pigments. Flower color is primarily due to four types of natural pigments: flavonoids, carotenoids, betalains and chlorophylls. Among these classes, flavonoids are the most common and responsible for an enormous range of flower colors including strong yellow, creamy yellow, ivory, pink, scarlet, blue and violet. The carotenoids produce the yellow, orange and red colours, the betalains produce the yellow, orange, purple and red colours only in the floral tissues of a few species (Caryophyllales) and the chlorophylls are responsible for a very few cases of green flowers. Anthocyanins are the most prevalent type of flavonoids and are generally considered responsible for most pink through blue flower colours. As anthocyanins alone are virtually colorless within the pH range of 4 to 6, other flavonoids and related compounds (but also metals such as iron and aluminum) can associate with anthocyanins and form colored anthocyanin-copigment complexes that are stable at pH values where anthocyanins alone are virtually colorless. The colors and color enhancement caused by the formation of anthocyanin-copigment complexes are greatly influenced by pH, concentration of anthocyanins and molar ratio of copigment to anthocyanin. External factors that can influence flower color, reviewed in this paper, are temperature, light, fertilization, addition of sugar or metals in the conservation water. Even if a significant amount of information is available on the genetics of flower colour, the new colour combinations that can be obtained by classical breeding are limited, the cross and selection programs are often too long and, moreover, some colours are absent in the genome of many important flowers (for example, the blue colour in rose, carnation, lily, gerbera). The transgene technology can be, and has been, extensively used to create new flower colours in many ornamental species. In this review, the factors that determine flower colour, the techniques that can be applied for modification, improvement or conservation of colour quality, the genetic control of the character, the biosynthetic pathway of the main pigments and the results obtained through the transgene technology to modify flower colour are reported.
Keywords: ornamentals, quality, postharvest, pigments, anthocyanins