Agriculture Reference
In-Depth Information
There is evidence that environmental factors are important in mediating the
activity of scrub allelochemicals. The toxicity of hydrocinnamic acid, the
breakdown product of ceratiolin, to the sandhill grass little bluestem
( Schizachyrium scoparium ) was greater in low-N and low-K treatments during
a greenhouse bioassay lasting over four months [18].
Noting the data that support increased allelopathic effects in resource-poor
environments, Hierro and Callaway [19] predicted that “the invasibility of
plant communities should increase as resource availability decreases.” This
prediction contrasts with models which hold that invasive species should be
more successful in communities with fluctuating levels of resources [20]. In
the Florida scrub, however, it appears that environmental factors that intensify
allelopathic effects, such as low nutrients and high temperatures, play a role in
the defense of this community against invasion. The general point is that
allelopathy may also be a more common mechanism in structuring natural
plant communities in resource-poor environments. While limited resources
may increase the success of some invasive species with allelopathic potential,
data from the Florida scrub suggest that limited resources may also work
against invasive species in a community already defended by allelopathic
mechanisms.
Role of plant density
Weidenhamer et al. [12] showed that allelopathic effects are increased at low
plant densities. Bahiagrass ( Paspalum notatum ) was grown in soil treated with
hydroquinone and gallic acid, the putative inhibitors from P. myriophylla . At
eight weeks, shoot biomass of bahiagrass grown in soil treated with 400 µg/g
of each compound was 63% of the corresponding control at a density of two
seedlings per pot, but there was no inhibition in plants with 16 seedlings per
pot. Stimulation of bahiagrass growth was observed at lower concentrations,
and was also density-dependent. In communities like the Florida scrub, where
plant densities are constrained by limited resources and harsh environmental
conditions, it is likely that low plant densities contribute to the increased effec-
tiveness of phytotoxins, and hence that there has been selective pressure for
their production [12].
The role of allelopathy in plant invasions
The demonstration that allelopathy appears to play a primary role in prevent-
ing colonization by invasive grasses and herbs in the Florida scrub suggests
that the hypothesis that allelopathy is a mechanism of primary importance in
exotic plant invasions may not hold up in every community. A number of
explanations have been posed to explain the success of weedy invasive species.
The predominant one has been the “natural enemies hypothesis”, which holds
Search WWH ::




Custom Search