Many cultivars of Hydrangea macrophylla produce inflorescences bearing sepal colors that reflect the soil pH; such sepals are pink-red when the shrub is grown in neutral to basic soils and blue when grown in acid soils. The primary pigment in the sepals is an anthocyanin, delphinidin-o-glucoside (Takeda et al., 1985), whose flavylium cation provides the characteristic pink-red color to hydrangea sepals (Moncada et al., 2003). Sepals turn blue because this anthocyanin forms a complex with aluminum as Al3+ (Toyama-Kato et al., 2003), with the coloration intensified and stabilized by other copigments (Kondo et al., 2005). Accordingly, because Al3+ is mobile only in acidic soils, the bluing of the hydrangea sepals is thought to follow the availability of Al3+ to the shrub.
Prior studies (Asen et al., 1960; Robinson, 1939) provided evidence that the concentration of delphinidin-o-glucoside in blue sepals differed from that in red sepals, consequently contributing to the color change in hydrangea inflorescences. Although the role of Al3+ in the bluing of the sepals has been firmly ascertained (Kondo et al., 2005), it is still not certain whether this is the sole mechanism for the bluing (Pharr et al., 2006). Thus, to establish a comprehensive chemical basis for the unique color change in hydrangea sepals, the anthocyanin concentrations in sepals need to be determined as a function of cultivar, color, and growing conditions. Such studies may lead to the development of alternative mechanisms for coloration and perhaps to methods for the bluing of sepals when hydrangeas are grown in neutral or basic soils or for generating novel sepal colors, such as yellow or orange.
Furthermore, there is a need for rapid field-portable measurements of anthocyanin content in hydrangea sepals, as determinations of extractable anthocyanin concentration are typically lengthy and destroy the inflorescence. Such measurements will allow the simultaneous analyses of other components (for example, Al3+) in the same sepals. The method would also allow quick and quantitative measurements of the effect of such variables as climate, bloom stage, fertilizer, chemical treatments, and sunlight on the anthocyanin concentration and, thus, the color intensity of the sepals.
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