Role of aluminum in red-to-blue color changes in Hydrangea macrophylla sepals

Abstract

Red, purple, and blue sepals on selected cultivars of Hydrangea macrophylla were analyzed for their aluminum content. This content was determined to be a function of the sepal color with red sepals possessing 0-10μg Al/g fresh sepal, purple sepals having 10-40μg Al/g fresh sepal, and blue sepals containing greater than 40μg Al/g fresh sepal. Accordingly, the threshold aluminum content needed to change H. macrophylla sepals from red to blue was about 40μg Al/g fresh sepal. Higher aluminum concentrations were incorporated into the sepals, but this additional aluminum did not affect the intensity or hue of the blue color. These observations agreed with a chemical model proposing that the concentration of the blue Al(3+)-anthocyanin complex reached a maximum when a sufficient excess of aluminum was present. In addition, the visible absorbance spectra of harvested red, purple, and blue sepals were duplicated by Al(3+) and anthocyanin (delphinidin-3-glucoside) mixtures in this model chemical system.