Analyses of Coloration-related Components in Hydrangea Sepals Causing Color Variability According to Soil Conditions

摘要

In many Hydrangea cultivars, sepal color depends on soil conditions. The traditional concept is that different levels of absorption of aluminum ions from soil and its accumulation in sepal vacuoles changes Hydrangea sepal color. To investigate how sepal coloration can be stabilized, we examined the components that may contribute to color variability according to the traditional concept. Using 10 cultivars and lines with sepals of stable red or stable blue color plants or with sepals of variable color (red or purple) plants grown in acid soils and alkaline soils, we analyzed sepal pH and sepal contents of anthocyanin, aluminum ion, 5-O-caffeoylquinic acid, and 3-O-caffeoylquinic acid. Sepals of all cultivars became bluer when plants were grown in acid soil than when they were grown in alkaline soil, even if the change in stable color plants was milder than that of variable color plants. The same component changes probably happen in sepals of both stable and variable color plants in response to different soil conditions to cause the coloration change. When the two soil conditions were compared, a statistically significant difference was detected for delphinidin 3-glucoside, which is a major anthocyanin of Hydrangea, in the variable-color line 'HH2' and for 3-O-caffeoylquinic acid in the stable red line 'HH19', but not for any other compound examined, including aluminum ions. Although there is possibility that localization of aluminum ions in vacuoles of the colored cells changes, it is assumed that changes in contents of aluminum ion chelaters such as phosphoric acid affect the sepal color change in response to different soil conditions, as well as the coloration stability or variability. When cultivars were compared in terms of properties of sepal coloration, although contents of aluminum ions and 5-O-caffeoylquinic acid tended to be higher in stable blue cultivars than in other cultivars, these differences were not statistically significant. In agreement with previous reports, our data indicate that a lower content of 3-O-caffeoylquinic acid is essential for blue Hydrangea sepals.