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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. |
