Role of allelopathy as a possible factor associated with the rising dominance of Bunias orientalis L (Brassicaceae) in some native plant assemblages

Leaf extracts of Bunias orientalis were shown to inhibit seed germination of a variety of cultivar plant species and of species cooccurring with B. orientalis in the field. Root exudate solutions and leaf litter leachates of B. orientalis were tested for their allelopathic activity using seedling growth assays. Additionally, in comparative seedling growth assays soil cores removed from dense B. orientalis stands were tested bimonthly for elevated allelopathic effects. The impact of root exudates on seedling growth was generally weak and varied between species. Similar results were obtained for the effect of B. orientalis leaf litter leachates on seedlings grown in sand culture relative to the effect of leaf litter leachates of a plant species mixture. When soil as a growth substrate was used, no consistent differences in seedling growth were obtained between the two litter leachate treatments. In the soil core experiment seedlings grown in soil cores collected from a dense B. orientalis stand unexpectedly showed better performance than seedlings grown in soil cores collected from a nearby mixed plant stand without B. orientalis, at least in early spring and late autumn. Predominating nutrient effects are, therefore, assumed to conceal a potentially increased allelopathic effect of soil beneath dense B. orientalis stands. It is concluded that other factors than allelopathy must be investigated to explain the rapid establishment of dense stands of this alien plant species.