Morphophysiological and biochemical responses of Populus nigra L. under salinity stress

Background and objectives: Salinity, caused by the accumulation of water-soluble salts in soil, is a major limiting factor for plant growth and development. A significant portion of Iran's soils are saline, which can severely restrict the cultivation of various poplar species. Therefore, determining the salinity tolerance of different poplar species can provide valuable guidance for poplar plantations in saline areas. This study investigates the morphological, physiological, and biochemical responses of Populus nigra L. clone “62/154” to varying levels of salinity stress. This high-yielding clone is widely used in poplar plantations across Iran.
Methodology: A pot study was conducted under controlled conditions using a completely randomized design to assess the morphological, physiological, and biochemical responses of this poplar clone to different salinity stress levels (0, 50, 100, 150, and 200 mM). Poplar cuttings were collected from the Alborz Research Center in Karaj, Iran, and planted in pots filled with clay-loam soil. The pots were placed outdoors during the experiment. The cuttings were initially irrigated with fresh water for three months before saline water (NaCl) was applied. After six weeks of salinity treatment, we measured survival rates, growth parameters, biomass production, relative water content (RWC) of leaves, macro- and micronutrient concentrations in plant tissues (leaf, stem, and root), and several physiological parameters (proline and sugar contents, malondialdehyde concentration, and enzyme activities). Data were analyzed using one-way analysis of variance to identify significant differences due to salinity treatment, with means separated by Tukey’s HSD test.
Results: The results indicated that poplar plants subjected to low salt stress (50 mM NaCl) exhibited no significant differences compared to control plants regarding survival, growth, biomass production, or physiological and biochemical parameters. However, at higher NaCl levels (100, 150, and 200 mM), soil salinity significantly reduced survival rates, growth parameters, biomass production, leaf RWC, and concentrations of nitrogen (N) and potassium (K) in plant tissues. Additionally, calcium (Ca) concentrations in roots and leaves decreased alongside magnesium (Mg) levels in leaves. Conversely, higher salinity increased leaf physiological parameters (proline and soluble sugar content), activity of antioxidant enzymes (CAT and POD), malondialdehyde concentration, as well as sodium (Na) and chloride (Cl) concentrations in plant tissues. Notably, phosphorus (P), iron (Fe), zinc (Zn), manganese (Mn), and copper (Cu) concentrations in roots and leaves also increased compared to control and 50 mM NaCl treatments. The most pronounced changes occurred at 150 and 200 mM NaCl. Overall, high salt levels (150 and 200 mM) led to a marked decrease in survival rates, growth, and biomass production along with significant morphophysiological and biochemical changes in the poplar plants. After six weeks of exposure to these salinity levels, survival rates dropped by nearly half.
Conclusion: The observed decreases in survival rates, growth parameters, biomass production, changes in physiological parameters, nutrient absorption and accumulation in plant tissues—as well as higher accumulation of proline and sodium/chlorine elements—indicate that P. nigra clone 62/154 is salt-sensitive. Therefore, it is crucial to consider soil salinity levels and the sensitivity of this poplar clone when planning poplar plantation programs.
 
Keywords: Black poplar, growth parameter, nutrients, proline, soil salinity.