Allelopathic interference of the exotic naturalized Paspalum dilatatum Poir. threatens diversity of native plants in urban gardens

Abstract Urban greening may enhance the distribution of exotic species that represent a threat for the native species in new urban systems. Paspalum dilatatum Poir. is one of the exotic naturalized species dominating in gardens of new urban ecosystems in Egypt. This study was conducted to evaluate the allelopathic interference of such species on the native plants in these habitats. The field study showed that the plots infested with Paspalum attained lower coverage, diversity and biomass of native plants than those free from it. Amaranthus viridis and Dactyloctenium aegyptium were completely absent from the sites involving P. dilatatum. Moreover, Paspalum-infested soils were of more water content, available nitrogen and extremely higher nitrogenase activity. In a greenhouse experiment, the rhizosphere soil under Paspalum reduced germination and growth of some native species, namely, Amaranthus viridis, Medicago lupulina, Melilotus indicus and Plantago amplexicaulis. Moreover, growth inhibition was inversely related to the seed size of these tested species. The spectrophotometric analysis revealed that higher contents of phenolics and flavonoids were detected in the Paspalum-infested soils in comparison with free ones. Besides, the high performance liquid chromatographic (HPLC) analysis indicated the presence of some free phenolics and flavonoids with higher concentrations in Paspalum-infested soils. The results indicate that the exotic naturalized P. dilatatum had negative effects on coverage, biomass and diversity of the native species in urban gardens. The obtained inhibition in these criteria was not attributed to soil characteristics. Furthermore, the significant reduction in germination and growth of the target species under greenhouse conditions could be attributed to the higher phenolic and flavonoid contents in the Paspalum-infested soils. These findings suggest that allelopathy had, at least, partial role in the negative impact caused by P. dilatatum. This species may also represent a threat for the native plant communities in new urban ecosystems.