Above- and below-ground competition cues elicit independent responses

Plants sharing pots have been observed to show more allocation to roots than do solitary plants, even when resources per plant are constant, indicating plasticity to root neighbours. Lowered red to far-red ratio (R : FR), a cue of above-ground competition, is known to cause increased stem elongation and decreased allocation to roots. In nature, however, cues for above- and below-ground competition are likely to be experienced simultaneously, and may elicit contradictory responses. We investigated whether the presence of the above-ground competition cue (low R : FR) affected the responses to the below-ground competition cue (presence of neighbouring roots) in soybean and whether these responses depended on the presence or absence of symbiotic bacteria or on the availability of nutrients. In a fully factorial study in a growth room, light quality, root neighbours, nutrient availability and Bradyrhizobium inoculation were manipulated. Stem elongation and biomass allocation were measured. As predicted, plants allocated more to roots in the presence of root neighbours, regardless of R : FR, Bradyrhizobium and nutrient treatment. Plants elongated under low R : FR, although the degree of elongation was affected by Bradyrhizobium and nutrients. R : FR did not affect allocation to roots. Surprisingly, we found no reduction in reproductive allocation and therefore no 'tragedy of the commons' cost; instead, growth was increased in the presence of root neighbours. We conclude that soybean responds to both competition cues independently, but that below-ground resources such as Bradyrhizobium and nutrient availability moderate the morphological responses to above- and below-ground competition. To our knowledge, this is the first study that tests responses to above- and below-ground competition cues simultaneously and that shows the independence of these responses. This study provides supporting evidence that plants can sense neighbouring roots as a cue of impending competition and demonstrates clearly that the root allocation response is not dependent on the presence of Bradyrhizobium or an artefact of inadvertent R : FR manipulation.