## 5 Conclusions  
Based on the previous field experiments, in this study, the photosynthetic physiological characteristics of three different types of common halophytes in northwest China under salt stress and their relationship with biomass were analyzed. The following conclusions were drawn: (1) With the increase in salt concentration, the plant height, stem diameter, and biomass of _Haloxylon ammodendron_ (euhalophytes), _Tamarix chinensis_ (recreteohalophytes), and _Phragmites australis_ (pseudohalophytes) all showed a decreasing trend in different degrees. (2) Chlorophyll a, chlorophyll b, and total chlorophyll contents of _Haloxylon ammodendron_ and _Tamarix chinensis_ first increased and then decreased with the increase in salt concentration. _Phragmites australis_ showed a decreasing trend. The malondialdehydeheyde content of three different types of halophytes showed an obvious increasing trend with the increase in salt concentration. (3) Under different salt concentrations, the diurnal changes in the net photosynthetic rate, transpiration rate, stomatal conductance, and water use efficiency of three different types of halophytes showed a \"double peak\" trend. The diurnal variation in intercellular carbon dioxide concentration shows a \"W\" trend. (4) With the increase in salt concentration, the daily average values of the net photosynthetic rate, transpiration rate, and stomatal conductance of three different types of halophytes showeda downward trend. With the increase in salt concentration, the daily average value of intercellular carbon dioxide concentration showed a \"V\"-shaped trend of first decreasing and then increasing. The daily average value of water use efficiency showed a \"single peak\" trend of first increasing and then decreasing. _Haloxylon ammodendron_ and _Phragmites australis_ were mainly limited by stomata at the salt concentration of 0-200 mmol\(\cdot\)L\({}^{-1}\) and were mainly limited by non- stomata at a salt concentration of 250-300 mmol\(\cdot\)L\({}^{-1}\). _Tamarix chitinosis_ is mainly limited by stomata at a salt concentration of 0-250 mmol\(\cdot\)L\({}^{-1}\) and is mainly limited by non- stomata at a salt concentration of 300 mmol\(\cdot\)L\({}^{-1}\). Compared with _Haloxylon ammodendron_ and _Phragmites australis_, _Tamarix chitinosis_ has better water use efficiency, salt tolerance, and adaptability. (5) Through the analysis of RDA and structural equation modeling, it is found that meteorological factors (atmospheric temperature and relative humidity), growth morphological factors (plant height and stem diameter), physiological factors (chlorophyll a, chlorophyll b, and malondialdehydedehyde), photosynthetic factors (net photosynthetic rate, stomatal conductance, and intercellular carbon dioxide concentration), and salt concentration have higher explanatory degrees, which have significant effects on the biomass of halophytes. Among them, salt concentration and growth morphological factors have direct core driving effects on the biomass of three different types of halophytes, while meteorological factors, photosynthetic factors, and physiological factors have different effects due to the differences in and complexity of halophytes. At present, the global salinized land area is expanding, and some soil improvement mechanisms are still unclear. Therefore, in this study, three different types of halophytes were planted in the saline-alkali land in the northwest of China, and then the change characteristics of the three different types of halophytes were monitored and analyzed through irrigation with a high-salt-concentration solution. The field ecological restoration effect of this study is significant (Figure 10). This study explored the salt resistance mechanism of three different types of halophytes in high-saline-alkali soil and high-salt-concentration solution, which provided a theoretical basis for ecological management and restoration of farmland and saline areas; it also provided a data basis for planting and screening halophytes.