科学美国人60秒:This Plant Bleeds Nectar to Attract Help

Plants, just like animals, get injured. For plants, it’s often because some herbivore is snacking on them. And, also like animals, plants usually seal those wounds up quickly to avoid infection or the loss of important materials.

But scientists recently found a plant that does not seal up a wound. Instead, a type of nightshade called Solanum dulcamara does its version of bleeding, releasing drops of an unusual liquid at the wound site.

“What we found is that the plant is actually damaged by herbivores like most plants are in nature. And in response to that it secretes sugar secretions from the wound edges where the herbivores have damaged the plant.” Tobias Lortzing is a graduate student at Freie Universität Berlin and one of the study authors.

The researchers at first thought the flow could be just a passive bleeding, where the plant lost some of the sugar solution being transported internally.

[TL:] “But it might also be and that is what we finally could show that the plant is excreting the stuff on purpose. So it's changing the chemical composition and it also controls the amount of secretion that it produces.”

The sugary liquid is a kind of nectar. But it’s not the nectar found in flowers that attracts pollinators. It’s actually the type of nectar that’s sometimes produced to attract insects that protect the plant from herbivores. But that nectar is secreted at a specific structure called a nectary. Instead, this bleeding nectar is produced at a wound site. But it does indeed attract helpers—in this case, ants that then serve as sugar-paid mercenaries in the fight against beetle larvae.

[TL:] “And the larvae have to climb up the stems because they mine into the young stems on the plant, which are about half a meter to a meter up the stem. And on their way up to the stems, they are attacked by the ants, which are patrolling the plant.”

Protected plants grow 10 to 13 percent better than the plants without the ant army. The finding is in the journal Nature Plants. [Tobias Lortzing et al, Extrafloral nectar secretion from wounds of Solanum dulcamara]

The scientists say this is the first documented instance of nectar that attracts a particular animal assistant being produced outside of a nectary. In fact, the observed nectar bleeding might actually be an early system that evolved to include the nectaries found in other plants today. A process that, though not short, was certainly sweet.

—Cynthia Graber