Wednesday, July 25, 2012

The bigleaf of the Hydrangea world

Species name: Hydrangea macrophylla

Common name: Bigleaf hydrangea, hortensia

Location: Ontario

This species of Hydrangea (there are somewhere between 70 and 75 species depending on which authority you consult; even DNA evidence gives two different stories depending on which gene is used to determine species limits) is native to Japan, and there are over 15 common cultivars (over 600 named cultivars in total, although many of these are not common outside of Japan) used in landscaping.

The most unique aspect of this plant (and one of the reasons why it is so widely planted outside of Japan) is the variable flower colour depending on which pH the soil is. It's not the pH directly that contributes to flower colour; it's actually the ability for the plant to take up aluminum ions. There are pigments in the flowers that automatically produce a deep pink or magenta (in some plants this can even be red), regardless of availability of aluminum. If this pigment is not present (and in some cultivars, mutations have been bred into the plant that prevent this pigment from being made), the flowers will be white. At lower pH in the soil, aluminum is more available for the plant to take up into its tissues. When aluminum and the pigment are both present in the petals of the flower, the flower will take on a blue colour. The intensity of this colour is directly proportional to the availability of aluminum to associate with the pigment; the lower the soil pH, the bluer the flower will be. The reverse can be said about higher pH: the higher the soil pH the less available aluminum will be to the plant, and so the pinker the flower colour will be. Towards the middle of the range, around pH 7.0, there is an equal availability of regular non-associated pink pigment, and aluminum-associated blue pigment so the flower appears purple. This concept of "litmus colour change" (where the idea for the litmus paper came from) is backwards from what you would usually think; usually red is a strong acid and blue is a strong base. That's one of the great things about chemistry! Depending on what causes the colour change, the same colour can mean different things under different chemical conditions.

All parts of the Hydrangea macrophylla plant contain trace amounts of cyanogenic glycosides, so consumption of this plant should be done with extreme caution. I would advise against all consumption, but many Asian countries use this plant as a medicinal herb. In Japan it is used to make a tea that is consumed at all times of the year, but in very high amounts around April 8th when it is believed to be Buddha's birthday. In Korea and China, the leaves are also used to make an herbal tea called sugukcha or ilsulcha.