Salinity tolerance in Nitellopsis obtusa

Nitellopsis obtusa (Desv.) J. Groves collected from an oligohaline lake was subjected to long-term salinity treatments in the range of 1.1-17.6 psu (2 6-520 mosmol kg(-1)) by adding artificial sea salt to the lake water. The e xtent of turgor regulation and the solutes involved were estimated by exami nation of the vacuolar sap. Under salinity stress, N. obtusa did not show t he capacity to accumulate K+ which enables euryhaline characeans to restore turgor pressure perfectly and brackish water species at least in part. The K+ concentration of the vacuolar sap remained constant at lower salinities but decreased with increasing salinity and time of exposure. An increase i n pi(i) by Na+ and Cl- could be considered an inefficient turgor response, but it is better explained as a failure to regulate osmotic potential and t o inhibit influx of Na+. Sucrose concentrations did not show clear relation s to external salinity, but contributed 24% of the vacuolar pi(i) in whorl cells and 16% in internodes. Provided that ionic ratios of Na+, K+ and Ca2 in the water approximately correspond to seawater, N. obtusa can survive s alinity fluctuations up to 17 psu for at least a week. For permanent growth , however, the distribution range of the species is restricted to oligohali ne waters with salinities not exceeding 5 psu.