Zinc protects chloroplasts and associated photochemical functions in cadmium exposed Ceratophyllum demersum L., a freshwater macrophyte

Cadmium (Cd) is a non-essential and toxic element, without any metabolic significance whereas, zinc (Zn) is an essential element required by many vital enzymes, and plays a significant structural role as stabilizer of proteins, membranes and DNA-binding proteins (Zn-fingers). Therefore, the interactive functions of Zn and Cd on photosynthetic apparatus in Ceratophyllum demersum were investigated. C. demersum was treated with Cd 10 muM alone, and Cd along with Zn (10, 50, 100 and 200 muM). Treatments with Zn only (10, 50, 100 and 200 muM) were also given for comparison. Cd not only exhibited pronounced toxicity on the over all photosynthetic machinery, but also the pigment biosynthesis. Chlorophyll-a, chlorophyll-b, and carotenoids reduced due to Cd toxicity. Cd-induced severe destruction of chloroplast membrane structure as estimated by the intactness of isolated chloroplasts. As a result, the rate of photosynthesis, electron transport processes, and activity of photosystems (PS I and PS II) altered. Zn supplementation showed complete protection of chloroplasts and associated photochemical functions. Treatments with Zn alone did not show significant differences in the chosen parameters of investigation indicating, the non-toxic nature of the chosen Zn concentrations. These findings indicate that Zn alleviates Cd-induced toxicity in upholding the normal photochemical processes in C. demersum. (C) 2004 Elsevier Ireland Ltd. All rights reserved.