Allelopathy is the release of biologically active chemicals by one plant that inhibits the germination or growth of other plants. Traditionally, allelopathy has been viewed as a form of competition, where toxic plant chemicals disrupt the metabolism of recipient plants, thereby reducing their growth or germination. However, there is mounting evidence that phytochemical-induced germination inhibition could be adaptive and may not be a consequence of toxins. That is, there are benefits for a seed to chemically recognize a potential competitor and defer germination until better conditions for establishment exist. This Biochemical Recognition Hypothesis (BRH) challenges the allelopathy paradigm but remains untested. A key requisite for the biochemical recognition of neighboring plants by seeds is frequent ecological interactions among historically co-occurring species. For this greenhouse study, we grew flats of six native grassland species from Argentina and from central North America and collected their leachates when watered. This leachate, which contained any water-soluble phytochemicals exuded from the roots, was then be used to water the seeds of the species grown for phytochemical collection. Our primary objective was to assess whether seeds exposed to phytochemicals from sympatric species exhibited greater germination reduction relative to phytochemicals from allopatric species with no historic association.
Of the 12 focal test species, two North American species had a 10% reduction in final germination percentage when exposed to the leachates of North American relative to South American species (P=0.016 and P=0.003, respectively; Mann-Whitney rank sum test). Whereas this reduction in germination is modest, it suggests that some species possess biochemical recognition systems capable of detecting specific competitors in their immediate environment. The significance of biochemical recognition among plants in multifold. First, debate continues on whether plant communities represent loose or tightly linked associations. Our findings support the latter and suggest that evolutionary interactions among sympatric species are important to the development of plant communities. Coupling our work with a growing list of others indicates that biochemical recognition may be an important mechanism structuring plant communities. Second, lack of biochemical recognition of non-native, invasive species by native residents suggests the latter may experience seed bank depletion by naïvely germinating, and consequently suffering fitness reductions, near these novel, strong competitors. Lastly, our work indicates that it is insufficient to invoke classic allelopathy when phytochemical-induced germination reduction occurs but when a mechanism of toxicity remains unidentified.