## 4 Discussion
### Response of Photosynthesis of Halophytes to Salt Stress
[PERSON] and others believe that if the net photosynthetic rate, stomatal conductance, and intercellular carbon dioxide concentration change in the same direction, the main reason for the decrease in photosynthetic parameters is stomatal limitation. If the change direction is opposite (net photosynthetic rate and stomatal conductance decrease, intercellular carbon dioxide concentration increases), then the main reason for the decrease in photosynthetic parameters is non-stomatal limitation [44]. Usually, stomatal limitation and non-stomatal limitation will work together to reduce plant photosynthesis. In this study, with the increase in salt concentration, the net photosynthetic rate, transpiration rate, and stomatal conductance of three different types of halophytes showed a decreasing trend. The daily average value of intercellular carbon dioxide concentration showed a trend of first decreasing and then increasing. Different plants have different critical concentrations of photosynthesis. [PERSON] et al. found in Australia that _Avicemnia marina_ has stronger salt tolerance and a higher critical concentration of photosynthetic rate of leaves than _Aveicem's curriculum_[45]. In this study, the critical concentration of Ha and Pa is 200 mmol-L\({}^{-1}\), and the critical concentration of Tc is 250 mmol-L\({}^{-1}\). Before the critical concentration, in order to maintain cell water potential and self-growth, plants reduce stomatal conductance to prevent cells from losing water, which leads to a decrease in photosynthesis. At this time, stomatal limitation is the main limiting factor. However, when the salt concentration exceeds the critical concentration, with the increase in salt, the photosynthetic mechanism of plants is destroyed and the activity is compromised, and non-stomatal limitation becomes the main limiting factor [46]. It was found that the daily average water use efficiency of three different types of halophytes showed a trend of first increasing and then decreasing. The daily average water use efficiency of Ha and Pa is the highest at a salt concentration of 200 mmol-L\({}^{-1}\), while that of Tc is the highest at a salt concentration of 250 mmol-L\({}^{-1}\). This shows that compared with Ha and Pa, Tc has stronger water use efficiency, salt tolerance, and better adaptability. This is because the northwest of China is dry all year round, with high evaporation and scarce precipitation. Under this environment, Tc has gradually evolved and formed scalp leaves. The leaves tightly surround the stem, which greatly reduces the area of leaves exposed to the air and increases the utilization efficiency of water in the body. At the same time, Tc has a special salt-secreting halophyte structure--salt gland, which can directly excrete excess salt in cells, thus reducing the accumulation of salt in the body, and the salt discharge efficiency is high. Compared with Tc, in arid environments, Ha leaves gradually degenerate and form assimilated branches. The assimilation of vascular tissue in branches can improve the transportation efficiency of plants to organic matter. However, when the salt concentration is too high, the vascular tissue will deteriorate continuously, thus reducing the discharge efficiency of harmful ions in vacuoles. The root cortex of Pa is suberization, and there are suberin and casparian strip zones in the root cortex. Although it can prevent some harmful ions from entering plants, when the salt concentration is high, cell gap sealing of the suberin and casparian strip will also make it difficult to discharge harmful ions from plants. Therefore, compared with Ha and Pa, Tc has better salt tolerance and adaptability [47].