Possible impact of zinc on cadmium-induced stress in Lemna minor

Cadmium is one of the most toxic metals causing growth inhibition as well as reduction of enzyme activity, photosynthesis, respiration, transpiration and nutrient uptake (Sanita di Toppi and Gabbrielli 1999). It can also induce oxidative stress by eliciting the generation of reactive oxygen species (ROS), which if not removed can damage cell structure and function (Prasad 2002). Contrary to cadmium, zinc is an essential micronutrient for plants, being involved in several cellular functions. Due to its role as a co-factor of enzymes it could have a stabilizing and protective effect against ROS-mediated oxidative and peroxidative damage (Aravind and Prasad 2005). Since cadmium is often associated with zinc as a contaminant it is important to investigate their interactions. Duckweed is very sensitive to cadmium, so in present study it was used to investigate the influence of zinc on cadmium toxicity. Duckweed (Lemna minor) was grown axenicaly on the modified Pirson-Seidel's nutrient solution (Pirson and Seidel 1950) supplemented with 10 µ ; ; M CdCl2, 100 µ ; ; M ZnSO4 or their combination. Growth was estimated during 14 days according to Ensley et al. (1994). The cadmium content was determined by atomic absorption spectrometry. Pigments were measured spectrophotometricaly (Lichtenthaler 1987). Maximum efficiency of photosystem II (Fv/Fm) was determined by measuring in vivo chlorophyll fluorescence (Schreiber et al. 1994). The level of lipid peroxidation was determined according to Heath and Packer (1968). Plant extracts were made in potassium phosphate buffer (pH 7.0) containing 1 mM EDTA, 5 mM ascorbate and PVPP for pyrogallol (Chance and Maehly 1955) and ascorbate peroxidase (Nakano and Asada 1981) as well as catalase activity determination (Aebi 1984). The growth of Cd-exposed Lemna plants progressively decreased during two weeks while the growth of Zn-exposed plants significantly decreased only at the end of experiment. Combination of Cd and Zn reduced growth significantly but reduction was not so prominent during second week of experiment like in Cd-exposed plants. Lemna minor accumulated high amounts of both metals (522 μ gCd/gDW and 2183 μ gZn/gDW). Zn replaced Cd to a certain degree since plants grown on medium with combination of Zn and Cd accumulated ~30% Cd less (356 μ gCd/gDW) and ~20% Zn (2589 μ gZn/gDW) more. The chlorophyll content decreased progressively during experiment and on 12th day it was 48% of control value on medium with Cd, 72% on medium with Zn and 47% on medium with their combination. Carotenoids content was significantly decreased only on 12th day (Cd 69%, Cd-Zn 70% and Zn 80% of control value). Fv/Fm decreased in all treatments but on 12th day the most prominent decrease was on medium with Zn. Cd induced lipid peoxidation, Zn not, but their combination diminished Cd effect on 12th day. In Cd-exposed plants, the activity of antioxidative enzymes significantly increased while Zn did not change but even decreased it. In plants grown on medium with both metals, the activity of catalase and ascorbate peroxidase was more similar to control values especially on 12th day while the activity of pyrogallol peroxidase increased. In conclusion, Zn alleviated Cd-induced growth reduction as well as Cd accumulation in duckweed. Among oxidative stress parameters only catalase activity suggests that Zn could diminish Cd-induced ROS formation. Since Zn alone affected both, pigments content and PSII efficiency chosen concentration of Zn (100 µ ; ; M) was probably too high. References: Aebi H. (1984). Methods in Enzimology 105, 121-126. Aravind P. and Prasad, M. N. V. (2005). Plant Science 169, 245-254. Chance B. and Maehly A. C. (1955). Methods in Enzimology 2, 764– 775. Ensley H. E., Barber J. T., Polito M. A. and Oliver A. I. (1994). Environmental Toxicology and Chemistry 13(2), 325-331. Heath R. L. and Packer L. (1968). Archives of Biochemistry and Biophysics 125, 189-198. Lichtenthaler H. K. (1987). Methods in Enzymology 148, 350-382. Nakano Y. and Asada K. (1981). Plant Cell Physiology 22, 867-880. Pirson A. and Seidel, F. (1950). Planta 38, 431-473. Prasad M. N. V. (2002). Physiology and Biochemistry of Metal Toxicity and Tolerance in Plants. Kluwer Academic, Dordrecht, Netherlands. Sanita di Toppi L. A. and Gabbrielli, R. (1999). Environmental and Experimental Botany 41, 105– 130. Schreiber U., Bilger W. and Neubauer C. (1994) In: Schulze E-D and Caldwell MM (Eds) Ecophysiology of Photosynthesis. Springer, Berlin Heidelberg New York: 49-70.