The researchers report that 80 percent of the dodder plants grew onto the side of the filter paper nearest the tomato, with many growing directly towards the tomato plant. Statistical analysis provided strong evidence for directed growth by dodder, but did not indicate what causes the directionality.
The Penn State researchers then challenged dodder seedlings with artificial tomato plants, pots of moist dirt, and vials of green or red water. None of these objects elicited any directional growth. Then, to narrow down the possible cues being used, they tested the seedlings' response to tomato plants slightly separated from the dodder seedlings, out of view so to speak, in a set-up designed to block possible light cues. The researchers observed a growth response toward the tomato plants similar to that in their first experiment. Finally, to firmly establish that volatile chemicals from the host plant were causing this response, the researchers used the same set-up to examine the response to extracted host volatiles, using a solvent-only sample as a control in the opposite direction. They again observed a strong growth response toward the tomato volatiles.
"This showed that host volatiles elicit a growth response in the absence of any other plant-derived clues," says Mescher. "However, while volatile chemicals might be key, our results do not rule out the possibility that other cues such as light or shade may play a role."
After establishing the role of volatiles in leading the parasite plants to their tomato hosts, the researchers looked at other potential hosts including wild impatiens, and showed that the parasites were attracted to a wide variety of plants. They even found attraction to wheat plants, a poor host on which the dodder
Contact: A'ndrea Elyse Messer