Phytotoxic effect of invasive species Amorpha fruticosa L. on germination and the early growth of forage and agricultural crop plants

Invasive plant species can inhibit the growth of native plants by competing for resources as well as by secreting allelochemicals. Indigobush ( Amorpha fruticosa L.), a deciduous shrub adaptable to a wide range of environmental conditions, was introduced into Europe as a decorative and valuable honey plant. In this study, we assayed the phytotoxic effect of aqueous leaf extracts (1, 3 and 5%) obtained from A . fruticosa leaves on seed germination and seedling growth of four important agricultural species ( Trifolium pratense L. [cultivar “VIVA”], Medicago sativa L. [cultivar OS‐100], Helianthus annuus L. [hybrid “Matej”] and Triticum aestivum L. [cultivar OS‐Olimpija]). Five parameters were analyzed: germination percentage, mean germination time, germination index, fresh weight of seedling root and shoot. The phytotoxicity of A . fruticosa leaf extract was compared to that of juglone, a strong allelochemical that is known to inhibit growth of numerous plant species. The results demonstrate that A . fruticosa phytotoxicity is dependent on both the plant species tested and leaf extract concentration, with higher concentration extracts having stronger phytotoxicity. The highest concentration (5%) leaf extract caused similar inhibition of seed germination and seedling growth to that of juglone. The results of this study suggest that the rapid invasion of A . fruticosa in agricultural areas may have a negative impact on growth and yield of valuable forage and agricultural crops.