Aluminum ions are involved in purple flower coloration in Camellia japonica 'Sennen-fujimurasaki'

Flowers of wild Camellia japonica L. are usually red, but infrequently the flowering trees of this species may have purple flowers. Such purple flowers are a highly desired horticultural property, but the color expression is not fixed. Even if a tree has splendid purple flowers in the spring, they may revert back to the red color of a wild C. japonica flower the next year. We investigated the factors responsible for the purple coloration using red, purplish-red, and purple flowers of the cultivar 'Sennen-fujimurasaki'. The epidermal cells of purplish-red and purple petals were composed of both red and purple colored cells, whereas those of the red petals were uniformly red. Many of the purple cells contained blue-black granules. Cyanidin 3-glucoside and cyanidin 3-p-coumaroylglucoside, major pigments of red-flowered C. japonica, were the major anthocyanins of 'Sennen-fujimurasaki'. The anthocyanin contents were not noticeably different among flowers of these different colors. Potential co-pigments such as flavones, flavonols, and cinnamic acid derivatives were negligibly detected. No significant differences were found in the Ca, Mg, Mn, Fe, Cu, and Zn ion contents or in the pH of petal homogenates; however, a significant difference was found in the Al ion content. The Al content of the purplish-red and the purple petals was 4-10 times higher and 14-21 times higher than that of red petals, respectively. A cyanidin 3-glucoside solution prepared at pH 4.8 was pale red with no precipitates. When Al ions were added to the cyanidin 3-glucoside solution, the solution became purple and produced blue-black precipitates similar to the blue-black granules observed in the purple colored cells. Differences in the spectral properties of the petals from those of the prepared solution could be caused by the co-occurrence of red and purple cells and may be influenced by other Al-chelating compounds and/or substantial Al concentrations in the vacuoles. We conclude that the purple flower color of 'Sennen-fujimurasaki' is generated by chelation of Al ions by anthocyanins. In other purple-flowered C. japonica exhibiting unstable flower coloration similar to that of 'Sennen-fujimurasaki', Al-anthocyanin chelation is also likely associated with the purple flower color.