"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

July 10, 2016

Cuscata campestris

One of the key characteristic of plants is their ability to produce food using sunlight via the process of photosynthesis. But there are many plants that do not photosynthesise - holoparasitic plants obtain all the nutrient they need from their host. One of the most well-known group of holoparasites are the dodders. There plants have no roots, their leaves have been reduced to tiny scales, and are composed entirely of elongated tendril-like vines. Because of their appearance and what they do to the host, the dodder has also acquired many common names including "Angel Hair", "Witch's Hair" and "Devil's Hair". When the dodder latches on to an appropriate host, it extends structure call haustoria which penetrate into the host plant's tissue to suck out its nutrients. This has led to the dodder also being called "vampire plants".
Dodder seedling in the process of coiling around its host (image from Fig. 1. of this paper)
There are 200 known Cuscuta species and of those, a small handful of them (10-15 species) are considered as serious agricultural weeds. These parasitic plants wrap their vines around their hosts and literally suck the life out of them. Infestation of this plant have been known to cause massive losses in alfalfa, tomatoes, carrots, and cranberry crops, and these "vampire plants" are very difficult to get rid off; since dodder vines often completely smother their hosts, it requires a lot of manual and mechanical labour to remove them. Additionally, dodders can also produce large numbers of resilient seeds that can linger in the soil for a long time, waiting for the next year to erupt in another outbreak.

For dodders, as with most other organism, the first moments of its life are the most critical - the newly germinated dodder seedling must secure a grip on a host plant within two to three weeks of germinating, otherwise the seedling would use up its energy reserves and expire. From the moment they germinate, these parasitic plants have various ways of finding their host. Dodder vines are able to "sniff out" host plants through the chemical they give off (essentially plant BO), but they can also use other senses to find their host. A newly germinated dodder seedling can also detect the specific wavelength of light which are reflected off the surface of plants, and use it to reach a host.

The scientists in this study investigated whether exposing dodder seedling to different spectrum of light can disrupt their ability to find their hosts. They conducted their experiments on the seedlings of two dodder species - Cuscuta campestris (which parasitises tomatoes) and Cuscata gronovii (which parasitises jewelweeds), and exposed them to three light source with different spectrums - the spectrum similar to unfiltered sunlight, mostly red light, and mostly far red light. Far red is a wavelength of light which is barely visible to our eyes, but it is the wavelength which is most reflected by the surface of plants. It is also the wavelength which dodders use to home in on their host.

From the experiment, the scientists found that almost all the dodder seedlings that were exposed to unfiltered light and mostly far red light were able to attach to a host after a week or two, in fact those bathed in far red light grew faster than the other groups. However, most of those that were bathed in red light lost their ways. Only 15% of the C. campestris and 27.2% of the C. gronovii seedling that were exposed to high levels of red light had managed to wrap themselves around a host.

So will it possible to control dodder infestation simply by bathing crops in red light? No, not quite since some dodder still managed to wrap themselves around a host plant and the red light treatment is only effective during the earliest stage of the dodder's life. But at least the findings of this experiments have shown that perhaps light manipulation can be combined with other control methods to control dodder infestation.  Additionally, we have gained an insight about how these parasitic plants sense and find their way through the world.

For this particular vampire, its weakness is not against sunlight, but rather, red light.

Johnson, B. I., De Moraes, C. M., & Mescher, M. C. (2016). Manipulation of light spectral quality disrupts host location and attachment by parasitic plants in the genus Cuscuta. Journal of Applied Ecology 53: 794-803.

P.S. I recently drew some dodder-inspired art, yes, it is more Parasite Monster Girls - meet Cassandra the Dodder...

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