Q1: What is the reddish-purple pigment I see in my Fast Plants?

This pigment is known as "anthocyanin." Anthocyanin expression is controlled by both genetic and environmental factors. Purple is a simple dominant Mendelian trait, with quantitative expression. Though much is known about the anthocyanin system in other species, the complexity of this system in Brassica rapa is just now being explored.

The genetics of anthocyanin expression are complex. The placement (i.e., location on the plant) of anthocyanin expression is under genetic control. Paul Williams, director of the Fast Plants program and professor of Plant Pathology, is currently in the process of extracting one gene controlling this placement. It appears as if there is one major gene controlling the expression in many locations (hydathodes, bud tips, etc.). Many other genes control the intensity of expression at a given location, each of immeasurable and varying effect.

Expression of anthocyanin is also influenced by factors in the growing environment. Light intensity is critical: strong, high light is recommended for high levels of expression. Anthocyanin expression is controlled by a phytochrome-mediated system, so wavelengths in the red portion of the spectrum are especially influential. The plants also need to be separated from each other so that they can receive the light.

Nutrient stress is also an environmental factor, which can affect the levels of anthocyanin expression - plants grown under low-end levels of N-P-K will show higher levels of the pigment.

The anthocyaninless mutation (anl/anl) can be superimposed on this genetic background. This mutation completely shuts down anthocyanin production and expression in the plant. Having a genetically anthocyaninless plant in the same growing environment as the purple plants provides a good control for measuring expression of the pigment.

Try using a color comparator chart to distinguish between high and low levels of anthocyanin. Develop a numeric or visual scale to describe the pigmentation in a population of Fast Plants.

Some useful references on anthocyanin and other plant pigments include the following:

Chichester, C.O. 1972. The Chemistry of Plant Pigments. Academic Press (New York).

Goodwin, T.W., ed. 1976. Chemistry and Biochemistry of Plant Pigments. Academic Press (New York).

Mazza, G. 1993. Anthocyanins in Fruits, Vegetables and Grains. CRC Press (Boca Raton, FL).

Michigan State University. 1996. "Anthocyanin pigments of flowers and vegetables." Natural Science, Especially for Teachers Section (spring/summer).