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Mommy, why is my stem turning red? Maternal genetic contribution to anthocyanin production in response to light

Erica Ross, Allison Barnes, and Dana Dudle

Biology Department and Science Research Fellows Program, DePauw University, Greencastle Indiana

August 4, 2006

Abstract

Erica and Allison measuring the plants. Allison is using
the spectrometer probe to measure the anthocyanin
content while Erica is recording data in the laptop
computer. The pink flags were used to prop up
the plants .

Anthocyanins are water-soluble pigments responsible for red, blue, and purple coloring in plants.  They do not photosynthesize like chlorophyll (Jackman, Yada, and Tung 1987) but can act as antioxidants by absorbing wavelengths of light that chlorophyll cannot (Lee and Gould 2002). Unlike most plants where anthocyanins are displayed in the leaves, flowers, or fruits, Apocynum cannabinum plants produce anthocyanins in their stems.  DePauw’s Nature Park is home to a large population of A. cannabinum. The plants vary in anthocyanin expression, displaying stems from green to deep red.  We are interested in studying the causes of this variability within the population.  High light intensity along with many other environmental stresses increase the synthesis of anthocyanins in some species (Oberbauer and Starr 2002), indicating that there may be adaptive value to this trait under stressful conditions. 

We set out to confirm that increased light intensity increases the production of anthocyanins in A. cannabinum, and to investigate whether there is genetic variation in the trait within the population.

The gator is the main form of transportation
for research students at the Nature Park.

We planted seeds from 13 different maternal families in a common garden, where we could manipulate light intensity while holding other environmental variables constant. At the start of the experiment, we collected initial data on several morphological traits and light reflectance of all plants. We transplanted our plants into the common garden; 218 plants were subjected to a full sun environment and 100 were placed in 60% shade under shade boxes. In order to acquire data on pigment production from our plants we used a spectrometer with a light source and an attachable fiber optic probe. We focused on reflectance at a small subset of wavelengths to assess anthocyanin and chlorophyll content.  The pigment content was estimated using indices developed by other researchers.  We monitored reflectance of the full-sun plants at 10 dates during an 8 week period, and measured the morphological traits at the end of 8 weeks.

Our results showed that both maternal family and light intensity affected the rate of growth and morphology of the plants. Sun plants produced more branches and more nodes on the main axis than shade plants. Anthocyanin production was also higher among plants in full sun than plants in the shade. There was significant variation among maternal families in overall pigment production, indicating that some genetic variation exists in the population. In addition, families differed in the magnitude of their response to changes in light intensity. 

Future work should address the mechanisms by which families vary, and whether the genetic variation for pigment production and sensitivity to light intensity is related to increased fitness in variable environments.

Allison using the spectrophotometer in the lab.