The pollution load of nitrogen and phosphorus in pollutants entering the sea is high, which seriously affects the water quality of coastal water. It is necessary to strengthen land-sea coordination. However, affected by the seawater encroachment, the salt and alkali content of the water in the estuary area is high and the variation range is large, which leads to difficulty in plant growth. The ability of plants to remove nitrogen and phosphorus is reduced. How to improve the survival ability of plants is an urgent problem to be solved. In this study, the addition of appropriate concentrations of Ca2+ and K+ can improve the salinity tolerance of plants and reduce the risk of eutrophication. The results showed that under the condition of pH 8 and salinity 20‰, the removal efficiency of total nitrogen and total phosphorus was the highest in the hydroponic experiment with exogenous Ca2+ and K+ concentrations of 8mmol·L-1 and 10mmol·L-1 respectively, which were 1.53-1.87 times those in the control group. At this time, the growth rate of Suaeda salsa was the best, and the maximum relative growth rates (RGR) were 2.4 times and 2.0 times that of the control group, respectively. The remission effect of Ca2+ was greater than that of K+ at the same concentration. The mechanism by which exogenous Ca2+ and K+improve the salt-alkali tolerance ability of Suaeda salsa was mainly manifested as improving osmotic regulation ability. In the aspect of inorganic osmotic regulation, exogenous ions improved the water-holding capacity of the plant, among which the correlation coefficients of inorganic ion content and water content in leaves and roots were all above 0.966. In terms of organic osmotic regulation, exogenous Ca2+ and K+ all promoted the accumulation of small molecular organic compounds such as betaine in Suaeda salsa, and alleviated plasma membrane peroxidation in leaves. Among them, the largest reductions of malondialdehyde content in leaves were 72.9% and 61.7%, respectively. Therefore, exogenous Ca2+ and K+can enhance the osmotic adjustment ability and promote the growth of Suaeda salsa, thereby improving the removal rate of nitrogen and phosphorus. The results provide a theoretical basis for improving the water purification effect and reducing eutrophication risk in coastal waters.
Keywords: Suaeda salsa; estuary areas; inorganic ions; saline-alkali stress; small organic molecule.