MadSci Network: Botany

Re: Why didn't my pink flower turn green when exposed to ammonia vapors?

Date: Fri Jan 12 09:52:52 2007
Posted By: David Hershey, Faculty, Botany, NA
Area of science: Botany
ID: 1168567397.Bt

Plants have three major groups of pigments that color flowers and fruits,
anthocyanins, carotenoids and betalains. Anthocyanins are found in vacuoles and
change color as the pH changes. These are the pigments that you need for a color
change to occur when using ammonia. Ammonia produces a high pH. Students often
extract anthocyanins from red or purple cabbage and use them as a pH indicator.

Carotenoids are present in plastids and are usually yellow, orange or red.
Lycopene is the red carotenoid found in tomato fruit. Beta-carotene is the
orange carotenoid found in orange carrot roots. Betalains replace anthocyanins
in some plants, such as bougainvillea, cactus and red beet. Betalains and
carotenoids do not change color with pH as anthocyanins do. 

The purple carnation  (Dianthus caryophyllus) behaved as if it contained
anthocyanins. The pink and red carnations behaved as if they lacked
anthocyanins. You might have had white carnations that had been dyed pink or red
with an artificial dye. Florists often dye white carnation flowers to produce
other flower colors, especially the green carnations sold for St. Patrick's Day.

If you can, crush some of the pink and red carnation petals before exposing them
to ammonia vapor. Crushing would assure that the ammonia could enter the cell
and reach the pigment. If a color change occurs when crushed petals are exposed
to ammonia, it could confirm that anthocyanins were present.

Some websites indicate that colored flowers exposed to ammonia turn green
because the colored pigment is destroyed by the ammonia and the green
chlorophyll is revealed. Chlorophyll does not have to be present because
anthocyanins can change from purple or red to green as shown in the first two
webpages in the references.


Red Cabbage pH Indicator

Photos of anthocyanin solution at various pHs

Introduction to the Caryophyllids - the Betalain Bunch

Where does the color come from in purple cabbage?

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