PREMISE OF THE STUDY

The novel-weapons hypothesis predicts that some plants are successful invaders because they release allelopathic compounds that are highly suppressive to naïve competitors in invaded ranges but are relatively ineffective against competitors in the native range. For its part, the evolution of enhanced weaponry hypothesis predicts that invasive populations may evolve increased expression of the allelopathic compounds. However, these predictions have rarely been tested empirically.

METHODS

Here we made aqueous extracts of roots and shoots of invasive (North American) and native (European) Brassica nigra plants. Seeds of nine species from North America and nine species from Europe were exposed to these extracts. As control solutions, we used pure distilled water and distilled water with the osmotic potential adjusted with polyethylene glycol (PEG) to match that of root and shoot extracts of B. nigra.

KEY RESULTS

The extracts had a strong negative effect on germination rates and seedling root lengths of target species compared to the water-control. Compared to the osmolality-adjusted controls, the extracts had a negative effect on seedling root length. We found no differences between the effects of B. nigra plant extracts from the invasive vs. native populations on germination rates and seedling root growth of target plant species. Responses were largely independent of whether the target plant species were from the invaded or native range of B. nigra.

CONCLUSIONS

The results show that B. nigra can interfere with other species through allelochemical interactions, but do not support predictions of the novel weapons hypothesis and evolution of increased allelopathy.

To test the effect of B. nigra shoot and root extracts on seed germination and early seedling growth of the 18 target species, in May 2015, we placed 10 seeds per species in a circular formation at equal distances from each other in Petri dishes (diameter = 5 cm) lined with a sterilized Whatman  filter paper (No. 1) moistened with 2.0 mL plant extracts or control liquids. Prior to sowing, the seeds were sterilized for two minutes in a 1 % sodium hypochlorite solution and rinsed well with distilled water. We re-applied 1 mL of the liquids to the filter paper at an interval of seven days until ending the experiment after three weeks. Corresponding to the 32 B. nigra extracts (aboveground and belowground for each of the 16 populations), the water-control and the two osmolality-adjusted controls (aboveground and belowground), our experiment included 35 Petri dishes for each of the 18 target species. Each Petri dish was sealed with parafilm to keep moisture levels and concentrations of the extracts at approximately constant levels. The 630 Petri dishes were positioned randomly within trays that were kept in a phytochamber (12h day/night cycle at 21°C/17°C and 90% relative humidity). We recorded the proportion of seeds that germinated in each Petri dish on the 14^th day from the start of the experiment and again on the 21^st day. As there were no new seedlings at that point, we stopped the experiment, and measured seedling root lengths (in mm).