## 4 Discussion
### Response of Photosynthesis of Halophytes to Salt Stress  
Photosynthesis is the process of converting light energy into chemical energy, and it is also the main source of plant matter and energy, which is of great significance to plant growth and ecological adaptation. Under salt stress, there are generally four types of diurnal variation in photosynthesis in plants, namely the \"single peak\" type, the \"bimodal\" type, the \"W\" type, and the changeable type [36]. [PERSON] et al. found that in southern Portugal, with the increase in salt stress, the diurnal variation in stomatal conductance of _Atriplex portulacoides_ showed a \"single peak\" trend [37]. In Iran, [PERSON] et al. found that the diurnal variation in photosynthesis of _Tritcium aestivum_ showed a changeable trend [38]. In this study, under different salt concentrations, the diurnal changes in net photosynthetic rate, transpiration rate, and stomatal conductance of Ha, Tc, and Pa, as well as the water use efficiency of Tc and Pa, all showed a \"double peak\" pattern, with the peak values appearing around 11:00 and 17:00, respectively, and the lowest value appearing around 13:00. The diurnal changes in intercellular carbon dioxide concentrations of Ha, Tc, and Pa all showed a \"W\" shape, with the lowest values appearing around 11:00 and 17:00, respectively, and the maximum values appearing around 13:00. It is worth noting that compared with Tc and Pa, the first peak in daily variation in water use efficiency of Ha appears around 9:00. This is because Ha belongs to euhalophytes. At 9:00, the stomata of Ha gradually opened, and there was more water in the plant vacuole. Therefore, Ha can maintain higher photosynthesis through a lower transpiration rate, which makes the numerical ratio of net photosynthetic rate and transpiration rate quite different. Therefore, the value of water use efficiency calculated according to formula (4) is large. In this study, the first extreme values of the net photosynthetic rate, transpiration rate, stomatal conductance, and intercellular carbon dioxide concentration of Ha, Tc, and Pa all appeared at 11:00. This is because at 11:00, the northwest of China has strong evaporation, high temperature, strong solar radiation, and the most active photosynthesis, so the net photosynthetic rate, transpiration rate, stomatal conductance, and intercellular carbon dioxide concentration all appear to have extreme values. At 13:00, the temperature in the study area reached the highest, and three different types of haplotypes had different degrees of \"lunch break\" in order to prevent themselves from being harmed by excessive temperature. The phenomenon of \"lunch break\" refers to the fact that at noon, the light intensity is the highest, the temperature is the highest, and the humidity is the lowest. In order to protect themselves from external damage, plants will take the initiative to close their stomata and reduce photosynthesis. After noon, the light and temperature gradually decrease, the humidity increases, the plants will open their stomata again, and photosynthesis will pick up. And with the gradual entry into the evening, the light gradually weakens, and plants can no longer carry out photosynthesis, so photosynthesis will decrease again [39; 40]. When plants carry out photosynthesis, they will gradually consume intercellular carbon dioxide in the body, and when plant photosynthesis stops gradually, intercellular carbon dioxide will gradually increase. Therefore, the change trend in intercellular carbon dioxide concentration is the opposite of other photosynthetic parameters [41]. This is the same as [PERSON] et al. 's research conclusion on the changes in photosynthesis in _Medicago sativa_[42]. According to the theory of stomatal limitation, the reasons for the decrease in plant photosynthesis caused by salt stress are mainly divided into stomatal limitation and non-stomatal limitation [43].