Abstract:The seedlings of Leymus cliinensis were Na2CO3-stressed for 6 days by planting in various culture solutions containing 1.25–30mmol/L Na2CO3. Nine strain indexes, such as the relative growth inhibition rate etc., were determined. The growth was obviously inhibited; the Na+ content was rapidly increased andK+ content was gradually decreased; great amount of proline and citric acid was accumulated; the leakage rate of electrolyte was elevated; the content of chlorophyll was reduced;the activities of both photosynthetic carboxylases RuBPCase and PEPCase were reduced and the ratio of PEPCase activity/RuBPCase activity became great eventually, as the reduction of RuBPCase activity was much more than PEPCase. Mathematic analyses of the experimental data for relationships between the strain indexes and stress (in natural logarithm of molar concentrations of Na2CO3) revealed sigmoidal or inverted sigmoidal relationships which can be expressed by the equation Y= Ym/ [1 + Ke-(ax2+bx)] + C or its varied forms (Y. strain value; Ym. maximum strain value; X. stress value viz. In molar concentration of Na2CO3; K,a,b,C. parameters).