Download Examining the Photoprotective Role of Anthocyanins in Coleus spp

Examining the Photoprotective Role of Anthocyanins in Coleus spp.
William Stafstrom, Class of 2012
Although sunlight is an essential requirement for photosynthesis, an excess of sunlight causes
severe problems for a plant as high energy wavelengths of light damage vital photosynthetic machinery
and lead to the rise of harmful oxygen free radicals.
This process is known as photoinhibition and plants handle this threat of excess light in a variety
of fashions. Some plants produce more of the green pigment chlorophyll so that they can absorb the
sunlight and use it to power photosynthesis. Plants can also use a set of light-activated pigments that
make up the xanthophyll cycle to dissipate excess light as heat. Increasing chlorophyll content and using
the xanthophyll cycle are the primary modes a plant uses to deal with this issue. However, many species
have a distinctive red or purple color in their leaves caused by pigments called anthocyanins that
intuitively seem as if they may protect the plant from high-energy visible light as well. This is because
we see them reflecting red light, meaning that they are absorbing higher energy blue and green light, and
perhaps protecting the plant from photoinhibition by acting like a shield.
Despite this intuition, literature on the subject remains divided on whether anthocyanins protect
plants from excess visible light. A paper by Burger and Edwards in 1996 is often cited as evidence for
their lack of a photoprotective effect. This paper found that anthocyanins protect the plants from some
ultraviolet light, but not from any visible light. To examine the question at hand the authors tested a
common horticultural plant, Coleus spp., whose varieties can be anthocyanin rich (red or purple leaves) or
anthocyanin poor (green leaves). The authors assessed the rates of photosynthesis of both varieties under
excess visible light and did not find a difference between them, meaning anthocyanins did not appear to
be protecting the photosynthetic machinery from excess visible light. Though their experimental set up
was solid, newer techniques that have arisen in the past 15 years warrant a reevaluation of their findings.
Burger and Edwards used an invasive method of evaluating rates of photosynthesis, and a newer
technique, known as chlorophyll fluorescence, is able to assess the percentage of light directed toward
photosynthesis with no destruction of plant matter as well as telling the photosynthetic efficiency of the
leaf. Additionally chlorophyll fluorescence is able to gauge the percentage of light consumed by the
xanthophyll cycle. My project attempts to recreate the experiment carried out by Burger and Edwards
fifteen years ago with this contemporary technique.
Using chlorophyll fluorescence, I will examine how much light absorbed by a leaf is used in
photosynthesis and by the xanthophyll cycle under ideal conditions and under light stress. I will look at
how efficient a leaf is at performing photosynthesis before and after light stress as well. These
measurements will be carried out on both varieties of Coleus. If the purported photoprotective effect is
taking place then after light stress the anthocyanin rich (red-leafed) varieties will be able to absorb more
light for use in photosynthesis than anthocyanin poor (green-leafed) varieties, as there would be less
photo-damage. Also, in red leaves less of the light should be directed towards the xanthophyll cycle
compared to the green leaves, as anthocyanins provide an alternative avenue for photoprotection.
My research focuses on just one of the many challenges faced by plants: the damage caused by
excess light and how plants respond to this obstacle. By revisiting an older work on the subject with
modern techniques, I hope to demonstrate whether or not anthocyanins serve as a photoprotector for
Coleus plants.
Faculty Mentor: Barry Logan
Funded by: Howard Hughes Summer Fellowship
Burger, J. and G.E. Edwards. 1996. Photosynthetic efficiency, and photodamage by UV and visible
radiation, in red versus green leaf Coleus varieties. Plant Cell Physiology 37 (3): 395-399.