Investigating UV fluorescence in carnivorous plants at grad student prices

Back in February, many of you may remember the distinctive paper in Plant Biology titled “Fluorescent prey traps in carnivorous plants” and the subsequent popular science reportage. As can be expected, this opened up a whole new series of questions as how carnivores attract insect prey, with the biggest limitation being the ability to study the phenomenon. The situation is aggravated by the wild variability of consumer-grade ultraviolet light sources, particularly ones that produce the correct frequency of UV to fluoresce carnivore structures. While many UV LED arrangements, such as the flashlights used for viewing UV ink stamps at nightclubs, will fluoresce these structures, they also tend to emit enough visible light to wash out the effect.

In trying to study this further, the problem lay with finding a UV source that produced the correct wavelength, cut back on the amount of visible light being emitted, and kept the cost of the final arrangement to a reasonable amount. The last immediately removed shortwave UV lamps, used for decades for viewing fluorescent minerals, from consideration, as these can run well outside of a typical underclass or grad student’s budget. Thankfully, it’s possible, with a little modification, to make a perfectly suitable and very effective arrangement that, while not necessarily precise, allows researchers to experience carnivorous plant fluorescence in the field.

AS&S violet laser

The core of this apparatus is a violet laser, which emits enough UV for any number of fluorescence effects. (As can be expected, violet lasers are now the go-to item at raves and music festivals for precisely this reason.) While available from many sources, this one came from American Science & Surplus. One limitation, due to US regulations, is that it uses a momentary switch to turn on and off, requiring the user to keep it held down in order to use it. Other than that, it has exceptional range, which means that it has enough power for more long-range field observation, such as seeking fluorescing carnivores at night.

DISCLAIMER: Since a violet laser produces a significant amount of UV, neither the Texas Triffid Ranch nor anyone involved with it takes any responsibility or accepts any liability for damages or injuries caused or abetted by the misuse of said laser. Keep this thing out of your eyes and the eyes of innocent bystanders, and wear protective eyewear when using it. Likewise, keep it away from exposed skin whenever possible.

Laser beam (unmodified)

The other limitation to using a violet laser is tied to the basic concept of a laser. Namely, it emits a beam of coherent light in a pinpoint. As the photo above shows, this means that the light from the laser scatters in air (the reason, by the way, why the visible lasers in science fiction movies and television are impossible, unless someone fires one into a cloud of gas or vapor), but not quite enough for our purposes. What’s needed is a coherent light that also spreads out laterally, just enough to cover a larger area and to view fluorescence effects without the visible light component washing it out. For that, we’re going to need an optical diffuser.

ThorLabs diffuser

Another thing to consider when working with UV is that standard glass absorbs UV: this is the phenomenon that allows people in glass greenhouses to work in full sun all day without suffering crippling sunburn. (Take this from an authority on “shedding like a monitor lizard all summer long”.) Because of that, standard glass diffusers intended for coherent and incoherent light won’t work. You’ll have to pay a bit more, but Thorlabs offers a series of fused silica diffusers designed for UV, in polishes from 120 grit to 1500 grit. Since I knew precious little of what I was doing, I bought one 120 and one 1500 to compare the effects, and then tested it with the laser back from the diffuser by about a centimeter.

Laser beam with diffusor

As the photo shows, the diffuser does an exemplary job of spreading the laser beam while still keeping it reasonably coherent. The only problem right now is with keeping the diffuser perpendicular to the laser and turning on the laser with one hand. In a very quick and dirty installation, this could be fixed with judicious application of the Time Lord’s secret weapon, but the more realistic plan involves constructing a clip for the laser that allows the diffuser to be adjusted for best effect. That’s in the future.

Sarracenia under visible light

Now the acid test. Since most of my previous experiments involved Sarracenia and Nepenthes pitcher plants, the first series of experiments involve going out into the middle of a collection of Sarracenia with the newly modified laser and viewing the effects. As important as using UV on the plants was recording their appearance under visible light, if only to see if the plant had any correlation between its markings under visible light and any fluorescence in UV. Hence, a quick photo of the pitcher is necessary before moving on.

Sarracenia under UV, first attempt

The first test of the newly modified laser was an unqualified success, at least to the naked eye. The beam stimulated fluorescence in most carnivores, including hints in sundews (particularly Drosera filliformis), as well as reddish chlorophyll fluorescence in Venus flytraps. In fact, the extreme fluorescence in Sarracenia of all species helps explain why Sarracenia seem to capture so many moths, and the next big project is to capture similar fluorescence, if any, in the genus’s relatives Darlingtonia and Heliamphora. The only limitation lay with the camera: working without a net, the fluorescence was barely visible in final photos, even if it was nearly blinding in person.

Sarracenia pitcher under UV

Contrary to popular opinion, this is not the cover to the latest Hawkwind album. While the fluorescence can be seen in the throat of the pitcher (on the right) and the edges of the lid (left), it’s still not perfect. Time for more experimentation with shutter speed and light sensitivity.

Sarracenia under UV with blue spots

One of the more interesting phenomena that was observed while working with the laser with Sarracenia were distinctive neon blue spots on either side of the lid interior, visible here on the upper left of the lid. Not all pitchers have these, but larger pitchers do, and they almost resemble fragments of Australian fire opal or blue ammolite to the naked eye. I have no idea if these work as additional lures to insect prey, but that’s yet another experiment for the near future.

Triggerplant blooms under UV

And as an additional treat for botanists, the laser apparatus also helps bring out UV colors and patterns in flowers as well. The hot pink blooms of the triggerplant Stylidium debile already stand out to human eyes, but under UV, they’re a brilliant neon pink. Combine that with known fluorescence in the blooms of other carnivores and protocarnivores (particularly Utricularia bisquamata, which has a spot that glows a brilliant DayGlo yellow under UV), and this laser arrangement could be used to study the attractiveness of flowers to insects without requiring special camera lenses or other equipment. If further tests with sticky trap carnivores such as Drosera and Byblis work out, it may also offer a way to search for possible attractants in protocarnivorous and potentially protocarnivorous plants as Probiscidea.

In summary, with the advent of inexpensive violet lasers, carnivorous plant researchers may now view fluorescent attractors in carnivores for the cost of dinner and a movie. I hope that this encourages further experimentation with UV on carnivores, particularly among college and high school students, as well as among layperson carnivore enthusiasts. As always, please feel free to ask questions or add commentary below, particularly concerning ways to improve upon the results.

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