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Detection of the UV-Blocking Layer in Plants and Snakes How DO Plants Prevent Damage due to Ultraviolet Light?

| basic experiments on this "sunscreen" topic |

PLANTS

The hurdle that must be overcome in locating the layer, if any, that blocks damaging ultraviolet (UV) light from reaching sensitive tissues and organelles in plants (e.g.: anything containing DNA) is that the only acellular layer on a leaf or flower petal is the microscopically thin cuticle layer. Being tightly adhering to the underlying epidermal cells, separation of the cuticle from those cells is very difficult. Thus study of the transparency of the cuticle alone is not easily possible.

However there is an indirect means for locating the UV-opaque layer. Just as florists put flower stems in colored water to change the colors of flowers, it was reasoned that were a stem of leaves and flowers similarly perfused with a fluorescent dye, any penetration of the UV irradiation into that stem's leaves and flower petals would produce fluorescence (the light penetrated deeply enough to stimulate the dye) of a wavelength that can penetrate the leaf as a whole. The dye used was inexpensive fluorescein free acid (Sigma-Aldrich, Cat #46955). Under short-wave UV, this dye fluoresces a brilliant chartreuse, which is a wave-length that is able to penetrate all layers of the leaf.

This was tried using short-wave UV (the wavelength responsible for DNA damage and mutation) with the result that only the water/dye solution in which the stem sat fluoresced.

In order to determine whether the dye actually had perfused the leaves and flowers, bits of leaves and petals were taken, and squashed resulting in a bright fluorescent glow. (It must be mentioned that squashes must be made within 24 hrs or less of placing the stems into the dye, as the fluorescein disappears (metabolized?) from long-standing perfusions.)

NOTES:

• Plants vary greatly in their abilities to transport dyes
• A given type of plant may transport some dyes and not others
• Fastest transporter of fluorescein dye so far found is a small, common weed called "Henbit" (Lamium amplexicaule)
  • See National Audubon Society's Field Guide to Wild Flowers [Eastern Region of North America]; page number 577; item number 527.)
  • See picture and description from the Univ of California at Davis.
• English ivy leaves work fairly well (are these "woody" plants?)
• Try rose stems, leaves and flowers. They are definitely in the family of "woody" plants (versus was are called herbaceous plants).
• Fresh-cut plants take up fluorescein much better than ones cut days before.
• Plants left standing in fluorescein solution for several days seem to metabolize the dye into something non-fluorescent. Thus you will need to see how soon you can do the testing - it may be within hours of your setting the plant in the dye
• Setting rooted plants in the dye will not likely result in uptake of the dye.
• Photography
  • It must be remembered that most films and digital cameras can "see" UV light. Thus without some means of filtering out the incoming UV, the area around the UV lamp will appear well illuminated to the camera although not to your eyes.
  • Normal window glass is a very good UV filter. Thus for the least expensive UV filter for your camera, you can tape a small piece of glass over the lens. (Make sure that the light meter's lens is also covered, if it is separate from the main lens.)

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