Response of Carex rigescens to different NaCl concentrations and its salinity threshold calculation

The effect of different NaCl concentration (0, 100, 200, 300, 400, 500 mmol·L-1) treatments on the growth and physiological parameters of Carex rigescens were tested to determine its growth and salinity threshold under salt stress. The results showed that NaCl treatment inhibited the growth of C. rigescens. With increasing salt concentrations, plant height, leaf length, and leaf width of C. rigescens all decreased and the rate of withered leaves increased significantly. Leaf relative water content decreased and leaf membrane permeability increased under the same treatment condition and these two parameters had no significant change under the lower NaCl treatment (100 and 200 mmol·L-1). However, the change was significant when the NaCl concentration exceeded 300 mmol·L-1. The activity of leaf SOD (superoxide dismutase) and POD (peroxidase) increased at first and then decreased and the maximum activity occurred in the 300 mmol·L-1 treatment. This implied that 300 mmol·L-1 could be the critical NaCl concentration for salt tolerance of C. rigescens. Different NaCl concentration treatments had different effects on the chlorophyll content. With the increase in NaCl concentration, chlorophyll a and chlorophyll a+b content increased at first and decreased subsequently. Chlorophyll b tended to decrease and chlorophyll a/b showed no significant change. When C. rigescens biomass decreased to 50%, we considered the NaCl concentration as the threshold to evaluate the salt tolerance of C. duriuscula subsp. rigescens. The calculated salinity threshold value of C. rigescens in our study was 263 mmol·L-1. In conclusion, C. rigescens had a high salt tolerance and it could grow normally in a saline environment. The results of this study provide an important theoretical basis for further research on the salt-tolerance mechanism of C. rigescens.