Study: Some Plants Recognize Their Siblings

Don't look now, but that weed you just pulled out of your garden is a lot more sophisticated than you might think. It may even recognize its brothers and sisters, and it may change its own lifestyle just to give its kinfolk a better chance to survive.

New research shows that at least one species recognizes its kin, and becomes much more aggressive in soaking up resources if the guy growing next to it is a member of the same species, but a total stranger. But if it's a sibling, it backs off.

"We tend to think of plants as fairly passive, but in fact they are actively sensing other plants and responding," said biologist Susan A. Dudley of McMaster University in Hamilton, Ontario, lead author of a study in the current issue of the Royal Society journal, Biology Letters.

Dudley and a student, Amanda File, collected seeds from Cakile edentula, an inconspicuous little plant that is common on the shores of the Great Lakes, and then carried out an elaborate experiment to see if plants grown from those seeds could actually recognize their siblings.

The answer, they say, is clearly yes -- at least as far as the "sea rocket" is concerned. If the work is confirmed by others, Cakile edentula could rewrite the textbooks on plant behavior.

For the record, no one is saying here that plants are clever creatures with lots of brainpower, but they apparently are a lot more complex than most of us think.

How the "sea rocket" responds to other plants, be they siblings or strangers, shows up mainly in the roots, Dudley told ABC News. If the neighbor is a stranger, the sea rocket puts much more of its energy into growing its roots, "snatching water and nutrients before the other plant can get them," she said. But if the other plant is a sibling, the sea rocket doesn't invest so much in its roots, leaving plenty of room for bro or sis to grow.

Pretty amazing, eh?

"Siblings were less competitive than strangers, which is consistent with kin selection," the study reports.

The animal world has many examples of critters that interact socially with others, even recognizing their own kin but not the world of plants.

"To our knowledge, no studies have yet tested directly for kin recognition in plants," the study continues. And that's what these researchers set out to change.

Their target was a "scrubby, weedy thing that grows on the beaches," Dudley said. The "sea rocket" is a member of the mustard family, and it's not much to look at, with purple and white flowers that are "not very striking," she added.

But it has an interesting recent history. Dudley's former colleague, Kathleen Donohue, now at Harvard University, found that the plants grew better in groups of siblings than groups of strangers, but her study didn't determine why.

Some other studies got "mixed results," Dudley said.

So Dudley and File decided to concentrate their efforts on the roots of the plant, because that's where competition so often shows up among plants. Tougher competition usually leads to bigger root systems, which the plant pays for in reduced production above the ground.

File collected hundreds of seeds from plants on a nearby beach, carefully labeling each seed with its ancestry. Then the two planted all the seeds in six different trays, sometimes planting only siblings nearby, and sometimes only strangers, and sometimes both. They started out with 384 plants (you've got to have fairly high numbers for credibility) but ended up with 332 after some of them died.

The trays were kept in a carefully controlled environment, and the plants were fed and watered exactly the same.

"We tried to treat everybody the same," Dudley said.

Just as the plants were about to flower, they were sacrificed in the name of science. They were dug up, their roots were analyzed, measured and weighed, allowing the scientists to see if there was any difference in the roots from plants growing with siblings, and plants growing with strangers.

"We found that kin groups allocated less to their fine root mass than did stranger groups when they competed below ground, indicating that these plants could discriminate relatives," the study concludes.

But what does it all mean? Should gardeners separate their seeds according to family history?

Maybe, but maybe not.

"We don't really know yet," Dudley said. If you want your plants to live peacefully side by side, maybe it's better to collect all the seeds from one "mom," as she puts it. On the other hand, there's lots to be said for biodiversity, and if the seeds come from different moms, the plants might have different growing habits, some sending their roots deeper than others, for example, and thus collecting nutrients from a different area.

So at this point, no one really knows what all this means. Except for one thing. Some plants, but probably not all, are a bit altruistic when it comes to taking care of their families. And they're much more interesting today than they were yesterday. And that's one of the really neat things about science.