It is easy to make a print. Fresh wild parsnip
leaves (handle parsnips with latex gloves to avoid getting dermatitis)
are placed between two layers of phase separation filter paper (Whatman 1PS). The
sandwich then is placed between two layers of regular filter paper, and that in turn is
placed between two sheets of pyrex glass (only Pyrex or Kimax glass is suitable,
sandwiching between two glass petri dish bottoms should also work). The
finished sandwich is placed in a microwave oven and cooked for 3 to 3.5 minutes (may
have to be adjusted for different ovens). When the time is up, wearing cloth gloves.
The furanocoumarins in the leaf have now been
transferred to the phase separation papers. One can visualize the furanocoumarins by
shining black light (longwave ultra violet) onto the prints in a dark room (be sure not to
look directly at the UV bulb).
The prints will fluoresce more brightly under
UV if you first "develop" the prints in base. This causes the lactone rings of
the furanocoumarins to open and make them fluoresce even more strongly (the bottom half of
this print was treated with base).
We use 4M KOH
(potassium hydroxide) to develop our prints (be sure to wear disposable latex or plastic
gloves to prevent contact with the strong base). The surface of the prints (the side
originally against the leaf) is rolled into a shallow pool of KOH. The paper will
not get wet, as it is water repellant. This takes only a couple of seconds,
then blot the paper on towel papers to remove any beads of base. In ten minutes the
prints are ready to look at under UV.
Nice, huh? But can this trick be used to
learn something more about the defense of parsnips? The answer is
"yes". Furanocoumarin production in parsnip foliage is inducible by damage. In the prints below you can see the
results of induction in the damaged leaflets.