Question No.  19853 :
It is known that the hydrangea flower is pink if the shrub grows on acidic soil and blue if the shrub is found growing on alkaline soil. How do we explain this phenomenon and what chemical is responsible for this occurrence?

Anthocyanins are members of a class of nearly universal, water-soluble, terrestrial plant pigments that can be classified chemically as both flavonoid and phenolic. They are found in most land plants, with the exception of the cacti and the group containing the beet. They contribute colours to flowers and other plant parts ranging from shades of red through crimson and blue to purple, including yellow and colourless. (Every colour but green has been recorded). For example, there are 15 different anthocyanins in red cabbage. Most have the basic cyanidin skeleton to which is attached various groups including glucose, coumarin, ferulic acid, and sinapic acids. The colour and stability of an individual anthocyanin in solution is highly dependent on the pH. The vast majority of the anthocyanin in solution is accounted for by equilibrium between two forms of the anthocyanin. They vary from red (acidic conditions) to blue to yellow (alkaline conditions). The final tissue colour is ultimately dependent upon several factors including the type and concentration of anthocyanin pigments present, pH, and the presence of other metabolites (such as ions, sugars, hormones). Hydrangeas are fascinating in that, unlike most other plants, the colour of their flowers can change dramatically. Modifying the soil pH can change the colour of some hydrangea. Hydrangeas often change colour on their own when they are planted or transplanted. They are adjusting to the new environment. It is not unusual to see several different colours on one shrub the next year after planting. It is much easier to change a hydrangea from pink to blue than it is from blue to pink. Changing a hydrangea from pink to blue entails adding aluminium to the soil. Changing from blue to pink means subtracting aluminium from the soil or taking it out of reach of the hydrangea. The most common ways to limit aluminium availability in soils is to increase soil phosphorus levels or to lime to increase pH. Soil pH is an important factor in the availability of mineral nutrients. It is also important in regulating the solubility of compounds that may be toxic to plants. While aluminium is not usually required by plants, it is a common soil constituent and is toxic to plant roots. When soil pH is below 4.5, aluminium is solubilised (mineralised) and may be taken up with other minerals present in the soil solution. Sensitive species or cultivars may fail to develop under such conditions. In order to have colour, pigment must be present. If a cultivar possesses no pigment it will be white. If it contains very little pigment, then the colour is likely to be a pale pink or a pale blue but that depends on aluminium. The pigment involved is the same in pink and blue hydrangea infloresces. It an anthocyanin named delphinidin 3-monoglucoside. When aluminium is present in the sepals (or what we usually call hydrangea petals) it binds with the pigment and a co-pigment. The sepal colour changes to blue when aluminium is present to bind with the pigment and co-pigment. The intensity of the colour depends upon how much pigment the particular cultivar contains as well as how much aluminium is available to the plant. More pigment and more aluminium mean a deeper blue flower. Another interesting flowering plant is Hibiscus mutabilis, a shrubby or treelike hibiscus better known as the Confederate rose or the cotton rose. (mutabilis means "changing colour") Hibiscus mutabilis is downright conspicuous when in full bloom. The flowers open pure white and change colour to pink until they are deep pink and then as they die assume a dark "blue-pink" hue. The most notable characteristic of this flowering plant is that flowers of three distinct colours appear on the plant simultaneously as the blooms colour cycle independent of one another. Question Asked By:

Name: Cheng

Age Group: 30 and above

Occupation Type: Educationalist

Education Level: Post Graduate