Physiological and Photosynthetic Characteristics with Ecological Restoration Application of Nerium indicum Under Salt Stress

Marine salinity affects mainly the roots, stems and leaves of coastal plants through groundwater intrusion, splashing and salt spray. Two-year-old Nerium indicum L. seedlings with good growth were used to carry out the salt tolerance experiments in their roots and leaves by setting multiple concentration gradients of control 0%, 0.3%, 0.6%, 1.0%, 2.0% and 3.0%. Normal watering for 4 weeks was followed by 13 weeks of salt stress treatment. The net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs) and water use efficiency (WUE) were measured at the end of salt stress and after normal irrigation. At the same time, physical and chemical indices, such as the cell membrane permeability and water saturation deficit (WSD) of leaves, were measured. The results showed that both soil salt stress and foliar salt stress affected the morphology and growth of Nerium indicum. When the soil salt stress was ≥1.0%, the growth status of Nerium indicum decreased irreversibly to less than 2, while under foliar salt stress ≤3.0%, the growth status of Nerium indicum was generally greater than 2. Soil salt stress significantly reduced the Pn, Tr and Gs of Nerium indicum leaves. Foliar salt stress had significant effects on Pn and WUE but no significant effects on Tr and Gs. There was a significant correlation between Tr and Gs; under high-concentration soil salt stress, stomatal factors had a more significant effect on the photosynthesis of Nerium indicum. Under foliar salt stress, the cell membrane permeability and WSD of Nerium indicum did not change significantly. Under high-concentration soil salt stress, the cell membrane permeability and WSD of Nerium indicum changed irreversibly, corresponding well with the results of growth status and photosynthetic responses, indicating that the damage to the membrane system in turn affected the growth and photosynthesis of Nerium indicum. In general, Nerium indicum can tolerate less than 3.0% foliar salt stress and less than 1.0% soil salt stress. Studying the salt tolerance characteristics of roots and leaves can provide theoretical guidance for the application of Nerium indicum in coastal ecology and vegetation restoration

The Effect of Domestic Sewage Treatment on Islands Using Ecological Treatment Processes: A Case Study of Haimen Island, Fujian Province

Islands are characterized by a lack of land and freshwater resources, public finances, and technical personnel. As a result, domestic sewage may not be effectively treated, which can lead to major pollution on islands and in the surrounding sea areas. In this study, a pilot treatment of domestic sewage was conducted using an ecological treatment process (i.e., a constructed wetland and ecological pond) in an abandoned pond located on Haimen Island, Fujian Province, China. The pollutant indicators were monitored to analyze this treatment method at different treatment stages. The results showed that the combination of multiple ecological treatment processes had favorable treatment effects on various pollutants in the sewage. The treatment rates of the chemical oxygen demand (CODCr) and suspended solids (SS) surpassed 88%. The treatment rate of the biochemical oxygen demand (BOD5), ammonia nitrogen (NH3-N), total nitrogen (TN), total phosphorus (TP), and fecal coliform surpassed 93%, and all the indicators met or were close to the level I B emission standards for urban sewage treatment plants. Different treatment stages have different treatment effects on different pollutants. The constructed wetland played an important role in sewage treatment through plant absorption, substrate adsorption, sedimentation, and microbial decomposition, particularly for the TP, CODCr, and BOD5. In contrast, algal photosynthesis in the ecological pond produced a large amount of dissolved oxygen, and the treatment effect was highest for the TN and NH3-N. The treatment effects on the fecal coliform in the constructed wetland and ecological pond were very significant. Ecological treatment processes based on the combination of a constructed wetland and ecological pond have favorable treatment effects, low construction and maintenance costs, and pollution-free conditions, which are suitable for application in island areas