Salinity Tolerance in Nitellopsis obtusa

Abstract

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 (26–520 mosmol kg^–1) by adding artificial sea salt to the lake water. The extent of turgor regulation and the solutes involved were estimated by examination of the vacuolar sap. Under salinity stress, N. obtusa did not show the 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 in π_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 to inhibit influx of Na⁺ . Sucrose concentrations did not show clear relations to external salinity, but contributed 24% of the vacuolar π_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 salinity fluctuations up to 17 psu for at least a week. For permanent growth, however, the distribution range of the species is restricted to oligohaline waters with salinities not exceeding 5 psu.