"But sometimes those plants are related, and there are benefits to not wasting resources on being competitive," Dudley said.
"And there is not really a cost to not being competitive as long as your neighbor is also not being competitive."
The researchers' new findings are reported in the latest issue of the journal Biology Letters.
More recent, still unpublished research by Dudley's team suggests other plants besides sea rocket show similar behavior.
In addition to restraining root growth, such plants may also develop different stem lengths in the presence of siblings, she said.
But how the plants determine which of their neighbors are siblings remains a mystery, Dudley said.
Learning and memory appear to be important for kin recognition in animals, but this isn't an option for plants, she noted.
Some researchers speculate that plants communicate through their roots, identifying themselves using tiny chemical signatures specific to each plant's family.
The new study may have important implications for agriculture, Dudley added, since competition between plants can reduce crop yields.
Planting more cooperative siblings together instead of strangers could theoretically mean better harvests.
The research adds to other recent studies that suggest plants are much better communicators than are generally thought.
For example, some species react to attacks by leaf-munching insects by producing chemicals that attract wasps that prey on the unwanted bugs.
(Read "Genetically Altered Plant Attracts Bug 'Bodyguards'" [September 22, 2005].)
But scientists have been puzzled to find that neighboring plants not being eaten by the insects send out similar calls for help.
In a separate recent study from Kyoto University in Japan, researchers have found that this may be evidence of an additional SOS sent out by insect-ridden plants.
Plants being eaten signaled their nearby siblings, which responded by putting out their own distress "messages," the Kyoto research suggested.
"We hypothesized that plants have evolved to emit a secondary signal to help nearby relatives by promoting the recruitment of natural enemies [of the pest insects]," wrote Yutaka Kobayashi and Norio Yamamura in the latest issue of the journal Evolutionary Ecology.
Plants are also known to be able to identify close relatives to guard against inbreeding, Dudley, the Canadian scientist, added.
"They have self-incompatibility mechanisms where they recognize pollen," she said. "This stops them from being fertilized by their own pollen or by a plant that shares its genes."
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