**3. Conclusions**

The potential of the Asteraceae family as a repository of species with phytotoxic or allelopathic activity is evident. Although the number of studies that associate allelopathy with metabolites present in the crude extract, essential oil, or in plant biomass residues is increasing and presents promising results, studies are still needed to prove the interaction of these compounds in the soil. According to Zhang [144] among the four pathways by which allelochemicals are released into the environment, plant residues exert the most negative effect. Still, the importance of allelopathy in nature requires further investigation, as allelopathic effects were smaller in studies where soil microorganisms were likely to be abundant and when the study duration was longer. For this, two points should be considered in future studies to confirm allelopathy. The first one is the identification of one or more phytotoxins produced by the plant under investigation, or the identification of a compound(s) that can be converted into a phytotoxin in the soil, after its release. The second refers to the quantitative determination of the compound(s), that is, if they are found in sufficient quantity (in time and/or space) in the soil in which the plant grows or grew, and if in these natural conditions can affect other species. This stage is especially difficult and challenging because, in the natural environment, plants are exposed to different interactions, effects of antagonism and synergism with other compounds in the soil, growth stages and physiological factors, state of the recipient plant, soil microbiota (especially with

*Potential Allelopathic Effect of Species of the Asteraceae Family and Its Use in Agriculture DOI: http://dx.doi.org/10.5772/intechopen.108709*

rhizosphere microorganisms), soil moisture, temperature, among others. Zhang [144] after a systematic review found that the effects of allelopathic were weaker between closely related species (or individuals of the same species) than between distant species, suggesting that allelopathy would favor the coexistence of closely related species, the opposite of the predicted effect in competition for resources. These notes further reinforce the need to investigate allelopathy and competition for resources together in order to explain the results of species coexistence.
