Air Quality Criteria for Lead Volume II of II - (NEPIS)(EPA) - US ...

Aquatic Plants
The toxicity of Pb to aquatic plant growth has been studied using Spirodela polyrhiza,
Azolla pinnata, and Lemna gibba (Gaur et al., 1994; Gupta and Chandra, 1994; Miranda and
Ilangovan, 1996). Test durations ranged from 4 to 25 days and test concentrations ranged
between 49.7 and 500,000 µg Pb/L (Gaur et al., 1994; Miranda and Ilangovan, 1996). Research
on aquatic plants has focused on the effects of Pb on aquatic plant growth and chlorophyll and
protein content.
Of the species reviewed here, the effects of Pb on aquatic plant growth are most
pronounced in Azolla pinnata (Gaur et al., 1994). An EC50 of 1100 µg/L was reported for
A. pinnata exposed to Pb-nitrate for 4 days. S. polyrhiza exposed to Pb-nitrate under the same
test conditions had a reported EC50 for growth of 3730 µg/L (Gaur et al., 1994). Lemna gibba
was shown to be the least sensitive plant species to Pb: significant growth inhibition was
reported at concentrations of 200,000 µg/L or greater after 25 days of exposure to concentrations
of 30,000, 50,000, 100,000, 200,000, 300,000, or 500,000 µg/L (Miranda and Ilangovan, 1996).
The maximum growth rate for L. gibba was observed at 10 days of exposure. After this point,
the growth rate declined in controls and test concentrations (Miranda and Ilangovan, 1996).
Clinical signs of Pb toxicity include yellowing and disintegration of fronds, reduced frond size,
and chlorosis (Gaur et al., 1994; Miranda and Ilangovan, 1996). Toxicity results suggest that
effects to growth from Pb exposure occur in a dose-dependent manner (Gaur et al., 1994).
Effects of Lead on Metabolic Processes
Algal and aquatic plant metabolic processes are variously affected by exposure to Pb, both
singularly and in combination with other metals. Lead adversely affects the metabolic processes
of nitrate uptake, nitrogen fixation, ammonium uptake, and carbon fixation at concentrations of
20,000 µg Pb/L or greater (Rai and Raizada, 1989). Lead in combination with nickel has an
antagonistic effect on nitrogen fixation and ammonium uptake, but a synergistic effect on nitrate
uptake and carbon fixation (Rai and Raizada, 1989). Lead in combination with chromium has an
antagonistic effect on nitrate uptake, but it has a synergistic effect on nitrogen fixation,
ammonium uptake, and carbon fixation (Rai and Raizada, 1989).
Lead effects on nitrate uptake in Nostoc muscorum (µg NO3/µg Chl a) were greatest after
24 h, when exposure to 20,000 µg/L reduced nitrate uptake by 64.3% compared to controls.
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