Role of certain environmental factors on cadmium uptake and toxicity in Spirodela polyrhiza (L.) Schleid. and Azolla pinnata R. Br.

Abstract

Effects of pH, temperature, EDTA and photosynthetically available radiation on the uptake and toxicity of Cd was investigated in Spirodela polyrhiza (L.) Schleid. and Azolla pinnata R. Br. In general, Cd toxicity was accentuated in conditions which favoured enhanced intracellular Cd uptake. Extracellular binding and intracellular uptake of Cd were lowered at pH values > 7 due to reduced availability of the aquo ion; consequently, toxicity was markedly reduced. At pH value < 7, extracellular binding remained unaffected, although intracellular uptake and toxicity of Cd were enhanced. This perhaps resulted from changes in membrane permeability as extracellular Cd binding was not affected by the decline in pH from 7 to 4. Moreover, speciation of Cd is known to remain unchanged throughout this pH range as most of the Cd (> 99%) remains available as the aquo ion. The presence of EDTA in the medium decreased extracellular binding, intracellular uptake and toxicity of Cd in test plants. This was due to formation of Cd-EDTA complex which was obviously not available to test plants. Elevation of temperature increased intracellular Cd uptake and this resulted in enhanced toxic effects. Similarly, increase in photosynthetically available radiation caused a slight increase in Cd uptake and toxicity in test plants. Obviously, rise in temperature or PAR increased metabolic activities of test plants thereby leading to increased Cd transport and toxicity.