Effects of salt and alkali interaction stress on chlorophyll fluorescence in leaves of Sorghum bicolor×S. sudanense

In order to identify the physiological responses and tolerance characteristics of Sorghum bicolor ×S. sudanense under salt and alkali interaction stresses, chlorophyll fluorescence parameters were measured in the leaves. Four salt stresses, 50, 100, 150, and 200 mmol·L-1, were prepared by mixing the neutral salts NaCl and Na2SO4 and the alkaline salts Na2CO3 and NaHCO3. In addition, 4 pH levels, 7.0, 8.0, 9.0, and 10.0, were used for each salt stress treatment. The analyses showed that physiological parameters in S. bicolor×S. sudanense leaves were significantly affected by high salt and ion concentrations in the solution and by the concentration of CO32- and total salt concentration. At a low salt concentration, the effects of different pH values on chlorophyll fluorescence parameters were relatively small. When the salt concentration was <100 mmol·L-1, the photochemical activity of the PSⅡ reaction center showed no obvious effect indicating a certain resistance to salt and alkali. But under high salt concentration and high pH conditions, the PSⅡ photochemical efficiency decreased. In addition, the decrease in activity of the PSⅡ reaction center in S. bicolor×S. sudanense leaves increased with increasing pH values. S. bicolor×S. sudanense can dissipate excess light energy in a restricted range of salt concentrations by improving non-photochemical quenching (NPQ). However, under high salt concentrations, the protective ability of excess light energy dissipation by NPQ decreased and was further decreased by high pH values. The effects of salt and pH values on physiological parameters in S. bicolor×S. sudanense leaves showed clear interactions. These interactions increased with the increase in salt concentration, indicating that the influence of pH value was relatively less at low salt concentrations, but the pH effect was greater with the increase in salt concentration. The photosynthetic ability of the S. bicolor×S. sudanense leaves showed some tolerance to saline and alkali, but the degree of alkalization needs to be taken into consideration when considering growing these plants in high salt concentration areas.