Common Characteristics in Salt Tolerance and Photosynthesis Between Suaeda salsa and Salicornia bigelovii

Euhalophytes are a group of plants with extremely strong salt tolerance. This study used hydroponically grown Suaeda salsa and Salicornia bigelovii seedlings as model species to probe common features of salt tolerance and photosynthesis among euhalophytes. Exposure to NaCl (100–600 mmol·L−1) for 28 d caused no detectable damage to either biomembranes or photosystem II in the two euhalophytes; moreover, SOD and POD activities were lower than those of the control group. Furthermore, salt treatment increased tissue succulence degree and promoted growth (e.g., more branches and larger photosynthetic organs) in the two euhalophytes. For instance, compared with the control group, plasma membrane permeability (PMP) and malondialdehyde (MDA) contents of S. salsa seedlings grown under 200 mmol·L−1 NaCl decreased by 49.8% and 20.6%, respectively, and the activities of SOD and POD decreased by 75.5% and 87.6%, respectively, whereas the whole-plant fresh weight increased by 94%. These results suggest that 200 mmol·L−1 NaCl is beneficial for the growth of S. salsa. Similar results were observed in S. bigelovii, with an optimal growth salinity of 400 mM NaCl. In saline environments, betaine and Na+ served as key osmotica, promoting water absorption in both euhalophytes. Concurrently, they minimized transpirational water loss through reduced stomatal conductance (Gs) and transpiration rate (Tᵣ), thereby limiting salt influx. Although this response mechanism reduced net photosynthetic rate (Pn), the two euhalophytes compensated by increasing the biomass of photosynthetic organs to enhance carbon fixation, and simultaneously downregulated chlorophyll and protein contents to lower metabolic consumption and respiratory rate (Rr). The above shared mechanism of salt-stimulated growth in the two euhalophytes could offer a breakthrough in breeding vegetable crops with improved salt tolerance and higher yield.