From Nature:
Telling apart relatives from strangers is crucial in many animal species, helping them to share precious resources or avoid inbreeding. Now it seems that plants can perform the same trick.
Plants have already been shown to compete with others — of their own kind or of another species — when sharing space. For example, they sometimes choose to invest more energy in sprouting roots when they have nearby competition for water and nutrients. Now, Susan Dudley and Amanda File of McMaster University in Ontario, Canada, have shown that plants grown alongside unrelated neighbours are more competitive than those growing with their siblings — ploughing more energy into growing roots when their neighbours don’t share their genetic stock.Plants ‘know’ more about their environment than they are often given credit for: they can sense the presence of neighbouring plants through changes in water or nutrients available to them or through chemical cues in the soil, and can adjust their own growth accordingly. “That plants have a secret social life is something well known to plant ecologists,” says Dudley.
But the ability to recognize kin has not been demonstrated before.
More here.