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The sepals of many hydrangea cultivars change color from red in basic/neutral soil to blue in acidic soil. This change is generally attributed to Al(III) becoming mobile in acidic soils, allowing it to be absorbed through the roots as a citric acid complex; the ion of Al(III) then forms a blue complex in the sepals with an anthocyanin that is red in the absence of Al(III). This study investigated selected metal ions that might also result in similar color changes in hydrangea sepals. Model anthocyanins such as cyanidin and delphinidin glucoside readily formed blue complexes with metal ions with a high charge/size ratio [that is: Mo(VI), U(VI), and Zr(IV), in addition to Al(III)]. The anthocyanins only formed weak complexes with Fe(III) and Ga(III), and no complexes with Mg(II) and Mn(II). In order for the color change to occur in the sepals, though, the hydrangea must first be able to selectively concentrate the metal ion in the plant from the soil as a complex with citric or oxalic acid. The complexation of Al(III) with the organic acid is shown by the measurement of the heat of solution of citric and oxalic acid in Al(III) solutions as half that of the acids in just water. The presence of Al(III) also enhanced the solubility of oxalic acid in water. Mo(VI) likewise enhanced the organic acid's solubility, while Fe(III), Fe(II), and U(VI) did not appreciably affect the solubility. Mo(VI) and similar ions may be candidates to artificially induce bluing of hydrangea sepals, instead of the current use of Al(III).

(96) Color Generation in Hydrangea Sepals as Controlled by Metal Ion Complexes