Jewelweeds make nice with their leaves when their roots share roots.
In lab experiments, seedlings of Impatiens wildflowers react mildly when planted in pots with other offspring from the same mother plant, says ecologist Guillermo P. Murphy of McMaster University in Hamilton, Canada.
The work suggests these sibling seedlings tend to grow a little differently in shape, with more branches, than do solitary seedlings sprouting in their own pots. But what they don’t engage in is outright leaf warfare, report Murphy and Susan Dudley, also of McMaster, in the November American Journal of Botany. Seedlings planted among non-sibs of the same species shift extra resources into growing leaves, a plant version of elbowing out the competition in the struggle to capture light.
That sibling/stranger difference showed up only when the plants shared soil, the researchers say. Just tucking sibs or non-sibs beside each other, each in a separate pot, didn’t produce the same effect.
“This is the first paper that shows that plants are responding above ground to sibling roots,” Murphy says. That’s a contrast to the other plant species the Dudley lab has tested for kin recognition, Great Lakes sea rocket (Cakile edentula).It too showed special behavior among its kin, but only in its roots. Seedlings apparently tolerated the presence of kin but nearby strangers inspired a shift of resources to roots, as if battling to snatch water and nutrients from the enemy, the Dudley lab reported in 2007.
That the two species react differently makes ecological sense, Murphy says. Sea rockets sprout on beaches. Grabbing enough light there should be easy, but roots struggle for water and nutrients. Jewelweeds thrive along shady watersides, where moisture abounds but light is scarce. Easing the competition among siblings then takes the form most appropriate for the habitat, he says.
What’s the same between the two, though, is that it’s the roots that seem to tell kin from strangers, Murphy says, whether the reaction is expressed above or below ground.
The new results, along with previous work, suggest “the phenomenon is quite common,” says plant ecologist Hans de Kroonof Radboud University Nijmegen in the Netherlands. He notes, however, that research has not shown whether kin interactions in the plant world play out evolutionarily as they do among animals.
To test for kin recognition in jewelweeds, Murphy and Dudley set up pots with varying mixes of sibling and non-sib Impatiens seedlings of the same species. Arrays included foursomes of siblings or non-sibs as well as four-packs in narrower pots so the seedlings had the same spacing but didn’t share soil.
Soil connections appear important. Dudley and her colleagues are publishing another study reporting that Arabidopsis seedlings grow differently in response to root secretions from their siblings versus secretions from strangers.
What’s missing in these studies is information about whether clusters of siblings produce more seeds than clusters of jostling strangers do, says Rubén Milla of Universidad Rey Juan Carlos in Madrid. In July he and his colleagues reported that in clusters of lupines, siblings produced fewer seeds than did plants growing among strangers. In such a case, kin recognition does not seem to give a plant an advantage, and Milla speculates that the power to recognize sibs might rather be a by-product of an ability to recognize self from non-self in plants.