Blue Hydrangeas – How and WHY!?

Figure 1: Yellowing tomato leaves. Leaves turning yellow or brown can be caused by numerous stresses on the plant.

This story is about hydrangeas – I promise! – but it begins with tomatoes. This week the leaves of the tomato plants in our garden began to turn yellow (Figure 1). Why does this happen? Leaves are normally green because of chlorophyll, the light-collecting compound that is highly abundant in most plant cells. So, though yellow leaves might be caused by many factors, it could mean the plant is losing its ability to produce chlorophyll.

Chlorophyll is made of carbon, hydrogen, oxygen, nitrogen, and magnesium (Figure 2). It seems unlikely that our tomatoes are suffering from a shortage of carbon, hydrogen, or oxygen, since they get these from the CO₂ in the air and water we give them. Therefore it might be that a nitrogen or magnesium shortage is leading to leaf yellowing, and that our tomatoes need these elements added to their soil.

Figure 2: Chemical structure of the chlorophyll molecule. The chlorophyll molecule has three principal components, the carbon chain (bottom left), the ring structure (right), and the magnesium metal atom (in green). Magnesium deficiency can lead to decreased production of chlorophyll in plant cells.

The elements nitrogen and magnesium are among six nutrients that plants need in substantial amounts. These macronutrients are the elements nitrogen (N) and phosphorous (P), and the metal elements potassium (K), calcium (Ca), sulfur (S), and magnesium (Mg). These are all essential for plant health and are acquired through roots. Other elements are needed in only very small amounts. These micronutrients are boron (B), chlorine (Cl), and the metals manganese (Mn), iron (Fe), zinc (Zn), copper (Cu), molybdenum (Mo), and nickel (Ni).

Not all metals are required for plant health, and some are even detrimental. For example, aluminum, the most abundant metal in the earth's crust, can inhibit root absorption of magnesium, the metal in chlorophyll, causing the plant to experience magnesium deficiency. In soils with low acidity, called basic soils, aluminum is bound to the soil and does not interfere with the plant. However, in acidic soils aluminum is released from the soil and gets stuck in root channels that normally absorb magnesium, inhibiting the absorption of Mg. In this way aluminum is toxic to many plants that grow in acidic soils.

Figure 3: Color change in Hydrangea macrophylla. Top: delphinidin in its normal state leads to blue colored hydrangea petals. Bottom: delphinidin bound with aluminum (a delphinidin-aluminum complex) leads to petals with red or pink color.

Some plant species have evolved mechanisms to reduce the toxicity of aluminum. Citric acid is released from the roots and binds to the aluminum, allowing the aluminum to be absorbed into the roots and transported up into areas of the plant where it will not interfere with the absorption of magnesium [1]. Many of us are probably familiar with one species of aluminum-tolerant plant – Hydrangeas (Hydrangea macrophylla in particular). This species has flowers that are normally red or pink because they contain a phytochemical called delphinidin (Figure 3). When there is aluminum present in the soil the Hydrangea releases citric acid to bind the aluminum and then absorbs the bound aluminum and transports it to the flowers for storage. Here the aluminum interacts with the red delphinidin pigment and transforms it into a blue pigment, and causing the flowers to turn blue [2]. Thus, when Hydrangea macrophylla is exposed to acidic, aluminum-rich soils, its flowers turn from red to blue!

It is remarkable that ingredients as simple as aluminum and acid can cause such a dramatic change in flower color. It is even more amazing that this phenomenon is no accident but is a clever strategy used by the plant to prevent aluminum from interfering with magnesium uptake. Maybe someday tomato plants will also develop a similar mechanism and prevent aluminum from inhibiting magnesium uptake, allowing them to thrive in a wider diversity of soils.

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[1] Jian Feng Ma, Syuntaro Hiradate, Kyosuke Nomoto, Takasi Iwashita, and Hideaki Matsumotol (1997). Interna1 Detoxification Mechanism of AI in Hydrangea, ldentification of AI Form in the Leaves. Plant Physiology 113: 1033-1039.
[2] Henry D. Schreiber, Amy M. Swink, Taylor D. Godsey (2010). The chemical mechanism for Al3+ complexing with delphinidin: A model for the bluing of hydrangea sepals. Journal of Inorganic Biochemistry 104: 732–739.