The effect of salinity and osmotic stress on duckweed Lemna minor L

The response of duckweed (Lemna minor L.) to salinity and osmotic stress was evaluated by monitoring growth and peroxidase activity every second day, for a period of 17 days. The nutrient medium was supplemented with isoosmolar concentrations of NaCl and mannitol (50 mM and 100 mM NaCl, 100 mM and 200 mM mannitol). Growth decreased markedly with increasing concentrations of NaCl and mannitol. The lower concentrations of NaCl (50 mM) and mannitol (100 mM) started to reduce growth significantly in the second week of the experiment, while the higher concentrations (100 and 200 mM) reduced growth from the beginning. Protein concentrations and peroxidase activity fluctuated during the growth period but, most of the time, they were increased in plants treated with NaCl and mannitol. These results suggest that Lemna minor is sensitive to both salinity and osmotic stress. However, the continued growth, and also the increase in peroxidase activity and protein concentration, during the NaCl and mannitol treatments, demonstrate its potential for adaptation to long-term stress. Although salinity and osmotic stress have similar overall effects on Lemna minor, consistent minor differences in growth, protein concentration and peroxidase activity between plants grown on NaCl and those grown on mannitol suggest overlapping, rather than identical, mechanisms of adaptation to salinity and osmotic stress.