Elemental allelopathy: processes, progress, and pitfalls

Allelopathic interference between plants has generally been discussed in terms of the production of toxic complex biochemicals; however, complex biochemicals may not be the only substances plants use to interfere with one another. It has also been suggested that inorganic elements may be used in an allelopathic manner. If, through phytoenrichment or root exudates, a plant is able to increase the bioavailable levels of a particular element and tolerate the levels better than its neighbors, it can produce an allelopathic effect. Elemental allelopathy has been implicated as the cause for the success of a number of invasive weeds, including Acroptilon repens, Tamarix spp., Halogeton glomeratus, Salsola iberica, and Mesambryenthemum crystallinum. Phytoenrichment of elements can occur through hyperaccumulation and litter deposition and by altering rhizosphere chemistry. Reported cases of elemental allelopathy have involved three types of elements: heavy metals and soluble salts in terrestrial systems and elemental S in aquatic systems. For the most part, studies that have reported elemental allelopathy have been inconclusive. In order to prevent overreaching conclusions in the study of biochemical allelopathy, criteria were set that can be adapted to the study of elemental allelopathy. Of the studies reviewed, the most common criteria left uninvestigated were whether the plant was actually responsible for changing the concentration of the element and whether the increased levels of an element negatively affected other species. If the study of elemental allelopathy is to avoid the same problems often associated with the study of biochemical allelopathy, these criteria should be included in investigations of elemental allelopathy.