Effect of Salt Treatment on the Growth, Water Status, and Gas Exchange of Pyrus pyraster L. (Burgsd.) and Tilia cordata Mill. Seedlings

Two-year-old seedlings of T. cordata and P. pyraster were exposed to salinity for 50 days, whereby each plant was subject to regular applications of a substrate solution containing 100 mM NaCl, amounting to a cumulative volume of 365 mL per plant. The adaptive reactions of the tree species in coping with salt stress were studied. The measured parameters were the growth and distribution of mass to organs, root to shoot mass ratio (R:S), content of assimilation pigments in the leaves, gas exchange parameters (gₛ, E, Aₙ), and water use efficiency (WUE). The relative increase in biomass was reduced under salt treatment for both species. A significant decrease in the total FW and DW was observed only for T. cordata, which deposited 4.5 times more Na⁺ ions in the plant tissues compared with P. pyraster. In P. pyraster seedlings, Na⁺ ions mainly accumulated in the root (75%), and their distribution was limited to aboveground organs. Thus, a balanced content of the assimilation pigments in the leaves was maintained under salt treatment. In the initial (osmotic) phase of salt stress, P. pyraster reduced water consumption and maintained a steady rate of photosynthesis (Aₙ) per unit area. T. cordata responded to salinity by regulating stomatal conductance and increasing water use efficiency (WUE). T. cordata was not effective in blocking salt intake and transported Na⁺ ions to the leaves. Due to the high cumulative salt content in the substrate, the water potential of the leaf tissues and the rate of photosynthesis significantly decreased in salt-treated T. cordata seedlings. The results document the important role of the root system in the resistance of woody plants and in ensuring their survival in conditions of excessive salinity. The investment in root growth improved the water supply of P. pyraster seedlings and enhanced the retention of salt ions in the root system, thus limiting their transfer to leaves.