The amounts of the phenolic allelochemicals exuded by
the two plants were not proportional to those in the plant tissues,
and a large
part of compounds were not released into
the
surrounding environment but retained in plant tissues.
The
plant culture density of H.
verticillata
and V. spiralis in
the present study is 10 g FW/L, which approaches to a moderate
level in the natural environment [23-25]. The
phenolic
contents released by E.
mutallii
increased along with the
plant culture density increasing (unpublished data).
However, the release is limited. Even if the culture density
is beyond 100 g FW/L, the release amount of the phenolic
allelochemicals of submerged macrophytes is lower than
100 µg/L [12, 26]. TPCs exuded from M. spicatum during
two weeks contribute only 1.0% to that in the plant tissues
[27]. The similar phenomenon was also observed for wheat
when investigating the phenolic allelochemicals in wheat
root, shoot, and exudate samples [28]. Some phenolic compounds
such as PA
and CA
are easily oxidized by dissolved
oxygen
in water environment [26], which may partially
reduce
the determined phenolic contents after three days of
cultivation.
Microorganisms
are another possible cause for
the
big differences
between production and exudation as
they
always coexist with the donor plants. Even in the laboratory
experiment, it is impossible to exclude the interference
of symbiotic and epiphyted microorganisms
absolutely
[29]. The
quick degradation of the exuded phenolic com-