Allelopathic effects of Brassica nigra in both its native and invaded ranges do not support the novel weapons hypothesis

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.