DOI: 10.5061/dryad.9cnp5hqfd
title: Allelopathic effects of Brassica nigra in both its native and invaded ranges do not support the novel weapons hypothesis

description: 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.
description: To
 test the effect of <i>B. nigra</i> 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 <i>B. nigra </i>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<sup>th</sup> day from the start of the experiment and
 again on the 21<sup>st</sup> day. As there were no new
 seedlings at that point, we stopped the experiment, and measured seedling
 root lengths (in mm).