Metabolic responses of Azolla pinnata to cadmium stress: photosynthesis, antioxidative system and phytoremediation

Abstract

This present study deals with the growth, photosynthesis, oxidative stress and phytoremediation character of Azolla pinnata L. exposed to different levels (0.05, 0.1, 0.5, 1.0, 1.5 and 2.0 mg·L⁻¹) of cadmium (Cd). Significant accumulation of Cd in Azolla fronds was noticed after 24 and 96 h of exposure and the accumulation rate was dose and time dependent. Growth of A. pinnata increased significantly after both exposure times with and without metal. At lower Cd doses (0.05 and 0.1 mg·L⁻¹), growth and photosynthesis of A. pinnata showed a marginal increase over the respective control, however, at higher Cd doses (0.5, 1.0, 1.5 and 2.0 mg·L⁻¹), a decreasing trend was noticed. At lower doses, Azolla fronds could counterbalance the negative effect of enhanced levels of superoxide radicals (SOR) and hydrogen peroxide (H₂O₂) through the greater activity of antioxidative enzymes. The decaresing trends in catalase and peroxidase activity at higher Cd doses suggest that Azolla fronds were not able to mitigate the negative effects of H₂O₂, hence an increase in malondialdehyde content was noticed. The study concludes that up to 0.1 ,mg·L⁻¹ Cd, A. pinnata can flourish and be used as biofertiliser and for phytoremedial purposes in Cd-contaminated fields; beyond this concentration poor growth may restrict its application.

Keywords:

• Azolla
• heavy metal
• growth
• photosynthesis
• oxidative stress
• antioxidants

Acknowledgements

We gratefully acknowledge Mr Vikas Kumar and Mr Vijay Kumar for their assistance during experimental work in the laboratory. The author A. Singh is thankful to CSIR, New Delhi for providing Senior Research Fellowships.