What causes color variation in hydrangeas?
Brilliant colors adorn gardens and roadsides as summer flowers come into bloom. I’m always amazed by the display of nuances in hydrangeas, a spectrum between red and blue. What causes this color variation?
The shades of hydrangeas depend on the acidity of the soil. Basic or neutral soil results in red sepals, while acidic soil causes bluing. A wide range of colors exists in between, such as purple, lavender, and violet. Chemically speaking, aluminum in the soil causes sepals to turn blue. Aluminum is normally extremely insoluble, but the acidity increases its mobility in the soil. Acidic soil, thus, enables the uptake of aluminum by the plant roots.
(What is a sepal? Read the previous post to find out!)
Hydrangeas are red in the absence of aluminum. Addition of aluminum sulfate (sources of both aluminum and acidity) to the soil will change the sepals from red to blue (or a gradient of purple). This color change can be reversed by making the soil more basic. This is usually achieved by adding lime (the white powder, calcium oxide; not to be confused with the citrus fruit!). In the picture below, a research team led by Judith Cain from Virginia Military Institute achieved the color change by soaking hydrangea in aluminum for 2 days. (Their work is published in Biometals.) A note to a reader who wants to try this at home: putting aluminum foil in the vase wouldn’t work because aluminum is very insoluble in water, unless in acidic form. The scientists used a chemical, aluminum citrate, which is soluble in water.
As mentioned above, the default color of hydrangea is red. This is due to the presence of a red molecule called anthocyanin (specifically, delphinidin-3-glucoside or Myrtillin) in the sepals. This molecule can bind aluminum to form a blue complex of aluminum and anthocyanin.
According to Cain and her colleagues, the “rule-of-thumb” is that the blue sepals contain about 5 times more aluminum than the red sepals. For those who love numbers, Cain’s team measured the exact contents of aluminum in sepals with different colors. According to their findings, the minimum amount of aluminum needed for bluing to occur is about 40mg per kg of sepals. The range between 0-10mg of aluminum per kg of sepals gives red hue, above which the sepals begin red-to-purple transformation.
Next time you spot blue hydrangeas, you know that the soil is acidic. (Impress your friends with this snippet of horticultural knowledge!) As much as it is pleasing to the eye, blue sepals is not only for aesthetic reasons, but a survival mechanism. Not all plants are able to grow in acidic soil, primarily due to aluminum toxicity; the metal interferes with the nutrient uptake. Some plants, including hydrangea, have adapted to grow in acidic soils by either excluding aluminum or inactivating its toxicity. Hydrangea is the latter, “aluminum accumulator” type. By incorporating the metal as an aluminum-anthocyanin complex, the harmful effect of aluminum is held back, while giving the sepals a blue hue. Biochemistry is amazing!