Differential physiological and antioxidant responses in roots and fronds of Spirodela polyrhiza (L.) Schleiden under pulp and paper mill effluent stress.

Spirodela polyrhiza, a free-floating aquatic macrophyte, is widely used in toxicity and effluent studies because of its rapid growth and tolerance to adverse conditions. This study compared the responses of roots and fronds of S. polyrhiza exposed to 50% and 100% pulp and paper mill effluent over five days. Effluent exposure caused concentration-dependent reductions in growth and photosynthetic pigments, accompanied by increased oxidative stress and membrane injury. Roots exhibited greater electrolyte leakage and lipid peroxidation, indicating higher physiological stress due to direct effluent contact, whereas fronds showed pronounced pigment reductions while retaining comparatively higher protein and soluble sugar contents. Increased activities of catalase, ascorbate peroxidase, and guaiacol peroxidase, together with elevated proline, hydrogen peroxide, and malondialdehyde, indicated activation of antioxidant and stress-response mechanisms. However, these responses were insufficient to prevent severe membrane destabilization and metabolic disturbance at higher effluent concentrations. Principal component analysis distinguished growth-related variables from stress-associated parameters, with root observations showing a stronger association with oxidative injury. The root-frond comparison revealed greater oxidative and membrane injury in roots and stronger impairment of photosynthetic and metabolic functioning in fronds, providing a clearer basis for interpreting plant stress and selecting indicators for future duckweed-based wastewater treatment studies.