Root competition appears to effect sex allocation in plants. Åsa Lankinen and her colleagues have studied this in the Early View paper “Allocation to pollen competitive ability versus seed production in Viola tricolor as an effect of plant size, soil nutrients and presence of a root competitor”. Here is a short summary by Åsa:
Even though plants lack brains, there is clear evidence that they can perceive and respond to their neighbours. For example, in some species plants can sense airborne chemicals transmitted from the leaves of another plant attacked by herbivorous insects, acting as a cue to start the induced defence system. Another example of plant communication is the possibility to detect the presence of self vs. non-self roots in the soil. Presence of unrelated root neighbours can even cause plants to allocate relatively more resources to their roots than to their shoots, thereby allowing more effective root-uptake when competitors are present. But can plants also use these kinds of cues to optimize their mating success, such as altering relative allocation to male versus female function in hermaphroditic plants depending on the presence or absence of competitors? In hermaphrodite animals, the social context (e.g. group size) can clearly influence male-female allocation.
In this study on violets, a hermaphrodite annual, our results indicate that sex allocation may not only be size dependent and influenced by soil nutrients, but also affected by presence of a root competitor. Taking the additional aspect of social environment into account also in studies on sex allocation in plants has the potential to increase our understanding of sex allocation across taxa. This knowledge might help us answer difficult questions such as how evolutionary transitions can occur between breeding systems.