Apparently some plants can recognize their relatives...
The plant world is highly competitive. Since they can't move around, plant have gotten quite creative in terms of defense and competition. From brute force to chemical warfare, plants are not the static entities that most write them off as. And, while most of what we see is going on above ground, underground, things get even more crazy.
Recent evidence shows that the sea rocket (Cakile edentula) seems to be able to distinguish between plants that it shares DNA with and plants that it doesn't. According to a study done by researchers at McMaster University in Hamilton, Canada, plants that grow around genetic relatives allocated less to root growth than those around non-relatives. Basically, when planted near a non-relative, the sea rocket will expand its root system to try and get the most out of its surroundings. When planted near a relative, the plant limits this expansion. So what does this mean? Well, they believe that the plants are recognizing their relation to other plants and attempting to limit the amount of competition for nutrients and water to genetically related individuals.
So, is this altruism? Not exactly. According to evolutionary geneticist John Kelly, its more along the lines of reduced antagonism. Sea rockets tend to grow in high disturbance beach habitats and because of their short lifespan they frequently self-pollinate. Their seed capsules also tend to stay on the mother plant and because of this, groups of clones tend to be found within close proximity to each other.
If they were to be as aggressive to their relatives as they would be with non-relatives, then they would be essentially competing with copies of their own DNA. From an evolutionary standpoint, preserving copies of your DNA, even in individuals other than yourself, is a boost to overall fitness. The researchers make it a point to note that, in this study, they were not looking at overall lifetime fitness of the plants in question. They do not know if reduced root mass, in this situation, incurs any positive or negative fitness to individuals overall. It should be noted that studies have shown that, at least in some plant species, reduced root mass seems to incur greater reproductive efforts. It is possible that sea rocket, in the presence of related individuals, can produce more seed.
How do the plants recognize their relation to their neighbors? The mechanism is not known at this point. My guess is that there is some form of chemical signature that the plants can recognize. How this information is processed is another story entirely. More and more we are discovering how complex the botanical world really is. According to the researchers, they feel that this type of relationship is not unique to this species alone. Research like this is opening new doors into uncharted and exciting territory.