Curious Chemistry Behind Hydrangea Colors!

Hydrangeas’ varied hues secret is in the soil pH and the right additives!

Hydrangea macrophylla

One of the world’s most popular ornamental flowers conceals a bouquet of biological and biochemical surprises. The iconic “snowball” shaped blooms of Hydrangea macrophylla are popular for both landscaping and the cut flower market. And their popularity continues to grow!

Hydrangeas are ubiquitous—but they are not what they seem. The bloom of the hydrangea is not a true flower, but an inflorescence: Sepals, or modified leaves, make up most of the bloom and overshadow the small, almost unnoticeable fertile floral portions at the center.

The bloom colors are what really make the hydrangea stand out: They range from pink to blue, including all shades of lavender to violet to purple, as well as green and white. Color intensities run the gamut from vibrant to pastel. Noticeably absent from the kaleidoscope of possible hydrangea colors are yellows and oranges.

Hydrangea colors are not what they seem, either; they are not the result of a variety of different pigments, as is the case for flowers such as roses or tulips. They are more akin to the colors seen in litmus paper, the chemically treated strips classically used to determine whether solutions are acidic or basic. At the molecular level, acids are proton (or hydrogen ion) donators and bases are proton acceptors in chemical reactions. When one dips blue litmus paper into an acidic solution (pH < 7, where pH is a measure of the concentration of hydrogen ions), the paper turns red, whereas red litmus paper changes to blue in the presence of a basic solution (pH > 7).

In a similar fashion, the color of many hydrangea blooms acts as a natural pH indicator for the soil in which the plant grows. Such blooms have blue sepals when the shrub grows in acidic soil, but develop red or pink sepals when grown in neutral to basic soils. The hydrangea’s bloom color reveals the pH of the soil, but with its distinguishing colors being the reverse of those for litmus paper. The hydrangea is unique among plants in this ability to indicate soil acidity.

Because of this trait, gardeners can chemically manipulate hydrangea bloom colors using soil additives. In fact, a hydrangea can have different bloom colors on the same bush if the roots of the plant sample soils of differing pH. 

Homespun recipes abound for changing the pink blooms of a hydrangea to blue: pouring vinegar or lemon juice on the soil; mulching the plant with coffee grounds, citrus fruit rinds, or pine tree needles; or burying rusty nails, old tin cans, or copper pennies next to the bush. All these strategies tend to turn soil more acidic, and eventually transform the bloom color to blue.

Soil acidity actually is not the underlying chemical mechanism behind the color change. The answer goes even deeper into the connection between soil composition and sepal color— a connection that has inspired our ongoing research into the biochemistry of these flowering plants.

A METAL KEY-

Hydrangea colors ultimately depend on the availability of aluminum ions (Al3+) within the soil. The role of aluminum has been known since the 1940s, but it did not reach the mainstream horticultural literature until about the past two decades, and the exact mechanism was only recently defined. Aluminum ions are mobile in acidic soil because of the ready availability of other ions they can react with, which can be taken up into the hydrangea to the bloom where they interact with the normally red pigment. But in neutral to basic soil, the ions combine with hydroxide ions (OH-) to form immobile aluminum hydroxide, Al(OH)3. Consequently, for the bluing of hydrangea blooms, one needs both aluminum ions and acidic soil. The best soil additive for bluing is one that contributes both, such as commercially available aluminum sulfate, Al2(SO4)3. Conversely, if one wishes to change blue-blooming hydrangea to red-blooming, adding lime (calcium hydroxide, Ca(OH)2) results in basic soil and the desired color transition.

The chemistry of aluminum in soil establishes its different properties under acidic and basic conditions. In acidic soils, aluminum occurs in what are called coordination complexes, with Al3+ ions at the center, surrounded by bonded strings of other molecules. These aluminum ions can travel from soil into the plant. But at neutral to basic pH, aluminum precipitates as aluminum hydroxide, making it unavailable for incorporation into the shrub. Lavenders, magentas, violets, and purples appear as bloom colors in transitional soil pHs, with aluminum ions only somewhat available to the hydrangea roots.

Data on the sepal’s aluminum content (see figure above) show that red sepals possess essentially no aluminum. But a little aluminum goes a long way toward bluing the bloom. At a threshold of only about 40 micrograms of aluminum per gram of fresh sepal, hydrangea sepals turn blue, but they don’t become bluer with yet more aluminum. Intermediate sepal colors of lavenders to purples have aluminum contents lower than this threshold.

Thus, it’s all about the availability of aluminum ions in the soil for the generation of the blue sepal color in hydrangea blooms, with the soil pH just being a necessary facilitator of this aluminum mobility and availability.

A SINGLE PIGMENT-

In other cases where a plant has a flower that can be different colors, it is usually because the underlying pigments are likewise different, or the proportion of its pigments changes. However, the hydrangea is additionally unique because the color comes from only a sole pigment, delphinidin-3-glucoside (which is in the anthocyanin family, the same group that turns leaves red in autumn and gives berries their color). The underlying chemical system is thus, in a sense, relatively simple.

The color of the delphinidin-3- glucoside, as well as other anthocyanins, is a function of its molecular structure, which determines what wavelengths of light it absorbs. These molecules consist of a central three-ring carbon chain with one oxygen substitution, called a flavylium cation at low pH, to which various sugars are connected. The anthocyanin loses one or more hydrogen ions as the pH environment changes, which alters its absorbance spectra.

What goes on at the pigment level inside the cell is actually further proof that the soil pH is not directly responsible for the color switch, but rather mostly an indicator of aluminum ion availability. The internal cell pH remains constant for both red and blue sepals. The flavylium cation is red and stable at low pH, the opposite of the overall bloom color under acidic conditions. But under neutral conditions it transforms to the purple form of what is called a quinoidal base, meaning the molecule has lost a hydrogen ion and rearranged its double bonds. At basic pHs, the quinoidal base anion forms with a blue structure upon loss of another hydrogen ion and further rearrangement of the double bonds in the core delphinidin component of the pigment.

On the other hand, studies have shown that there is a way to stabilize this blue quinoidal base anion in an acidic cell medium. Aluminum ions will complex with the normally red pigment, as also shown in the figure above, for delphindin-3-glucoside, and result in additional bluing. Once again, the presence of Al3+ becomes the key for the bluing of hydrangea sepals—both at the molecular level and in the field. Its presence circumvents the need for a high pH inside the cells to create the blue structure.

ALUMINIUM'S PATH-

A key step in the bluing of hydrangea sepals relies on getting Al3+ into the plant and transporting it to the sepals, but as seems to be a theme with hydrangeas, it turns out that there’s another step in the process of aluminum transport. Al3+ is mobile under acidic soil conditions and, in response to its stimulus, the roots of the hydrangea exude citric acid (C6H8O7). Consequently, a solution of citrate ions (C6H5O73-) and citric acid forms around the roots at relative concentrations that are specific to the soil pH. Al3+ then establishes a stable complex with the citrate ions, which is absorbable into the roots of the hydrangea. The plant transports Al3+ throughout as this citrate complex. Other Al3+-tolerant plants, such as buckwheat and rye, likewise exude simple organic acids to detoxify aluminum. In fact, such strategies are becoming quite important in cultivating crops that are being both bred and genetically engineered to survive in acidic Al3+-rich soils.

This citrate complex is crucial for not only the incorporation of Al3+ into the roots but also the constant circulation of Al3+ throughout the plant, as shown in the figure at right. The hydrangea sepals actually do not concentrate the Al3+, as all leaves on the hydrangea possess about the same concentration of Al3+ as the sepals (but only the sepals have the correct pigments to react with the ions). Because sepals are simply modified leaves, such behavior might be expected.

THANK YOU!

BY ANUKRITI KHANNA