Abstract:
Heavy metal pollution in aquatic
environments has led to devastating long-term effects in
nature. Therefore, immediate action is required to
develop ecotoxicological monitoring strategies to combat
this environmental issue. This study evaluated the
response of two aquatic plants, Azolla pinnata (Mosquito
fern) and Lemna minor (Duckweed), to a concentration
series (0.1-10.0 mg L-1
) of two heavy metals, Cadmium
(Cd) and Lead (Pb), over a 3-day and 7-day microcosm
exposure. A multiparameter approach was used where
vegetative growth (frond number, wet weight, root length
and plant width), biochemical changes related to
oxidative stress (glutathione (GSH), peroxidase (POD),
superoxide dismutase (T-SOD) activity), as well as
chlorophyll a content were measured. Both plants showed
an increase in growth at lower metal concentrations (up
to 1 mg L-1
), but growth decreased dose-dependently with
Cd being more toxic than Pb (p<0.05), shown by severe
necrosis and browning in plants. Results of the
biochemical tests revealed more complex defense
mechanisms at play. Elevated POD activity (112-119 U
mg-1
) and T-SOD activity (2.4-14.24 U mg-1
) at higher
concentrations of Cd exposure, coupled with a significant
decrease in chlorophyll a (8.92 to 3.93 units, p<0.05),
indicating targeting of the photosynthetic apparatus,
were some of the trends observed. Both plants
demonstrated consistent responses to heavy metal
exposure within relevant environmental ranges,
indicating their suitability to be used as ethical plant
models for ecotoxicity testing and environmental
monitoring.
