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What's Wrong with this Poinsettia?

By Mary Welch-Keesey, Purdue University Consumer Horticulture Specialist, at White River Gardens

Take a close look at this poinsettia. Most of the leaves are light green with white edges -- but a few are dark green. Most of the bracts are pink -- but a few are deep red (the bracts are the colored leaves just below the real flowers). Look closer and you'll see that the part of the stem that gives rise to the dark green leaves and deep red flowers is red while the other stems are green. How can one part of the plant be one color and the rest a second color? What's wrong with this poinsettia?

It all started with a mutation

Wild poinsettias, native to Mexico, have dark green leaves and red bracts. Commercial producers, in an effort to create the many different colors and shapes of poinsettias available today, irradiate the poinsettia with microwaves or ultraviolet light. This causes mutations in the plant, that is, changes in the DNA of the poinsettia. Sometimes the mutation is so small that it has no effect on how the plant looks or behaves; sometimes the mutation is so great that the plant dies. Occasionally, the radiation changes the DNA in just the right way and creates a poinsettia that has the unusual variegated foliage or contorted bracts that we find so attractive.

This poinsettia, a cultivar called 'Silverstar Pink' was created by irradiation. The mutation which resulted caused the leaves to change from green to the variegated form you see here and the bracts to change from red to pink.

Chimeras and other strange beasts

So, why does this specimen of 'Silverstar Pink' have a few leaves that are dark green and a few bracts that are red? Mutations caused by irradiation are often unstable, that is they will spontaneously change back to their original form. So, when this poinsettia plant was small and growing, a few cells in one of the stems changed back to their original form (the technical term is 'reverted'). These few cells continued to grow and make new stems and leaves. From these few cells came all the green leaves and red bracts that you see next to the variegated leaves and pink bracts.

A plant which has cells that have different types of DNA is called a chimera (named after the mythical beast with a human head, a lion's body and the wings of an eagle). Our specimen of 'Silverstar Pink' has some cells with DNA that makes the leaves variegated and the bracts pink and some cells with DNA that makes the leaves green and the bracts red. These cells can exist quite peacefully next to each other. The plant may look strange -- red on one side and pink on the other -- but it is healthy and still growing well. Mutations occur naturally in plants and have given rise to some of our most popular foods -- seedless oranges, thornless blackberries and fuzzless peaches.

Last updated: 6 April, 2006
For questions on this article, please contact Mary Welch-Keesey (mwelch@indyzoo.com).
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