Physiological response to salinity stress and tolerance mechanics of Populus euphratica

The salinity stress inhibits the growth of Populus euphratica (P. euphratica), and the extent of inhibition tends to increase with a rise of salt concentration while the net photosynthesis rate, stomatal conductance, transpiration rate, and internal CO₂ concentration are seen to decline with increasing salt concentration. Compared with the control group, the percentage decline is found to be about 48.50, 15.72, 42.09, and 48.33%, respectively. Although all chlorophyll fluorescence of P. euphratica exhibits a typical O-J-I-P curve in differently concentrated salt solutions, salinity stress shows a significant influence on the value of J and I step (P < 0.05). However, salinity stress was seen to induce a decrease in variable fluorescence (Fv)/maximal fluorescence value by 2.32, 8.78, 12.80, 12.93, 16.46, and 19.63% treated by 50-, 100-, 150-, 200-, 250-, and 300-mM salt solution compared with the control group, respectively. Salinity stress appeared also to induce a decrease in Fv/minimal fluorescence values by a magnitude of 5.22, 16.02, 18.06, 22.95, 26.34, and 32.19% in P. euphratica treated by 50-, 100-, 150-, 200-, 250-, and 300-mM salt solution relative to the control group, respectively. An increase in the content of malondialdehyde amounted to 4.12, 25.59, 34.60, 68.11, 70.72, and 67.68% in P. euphratica treated by 50-, 100-, 150-, 200-, 250-, and 300-mM salt solution compared to the control group, respectively. In terms of the content of proline, the salinity stress induced an increase by 4.94, 29.49, 53.20, 77.65, 82.46, and 90.68% in P. euphratica treated by 50-, 100-, 150-, 200-, 250-, and 300-mM salt solution, respectively.