The ion homeostasis and ROS scavenging responses in 'NL895' poplar plantlet organs under in vitro salinity stress

To evaluate salt tolerance and organ-specific responses of Populus deltoides x Populus euramericana cv., 'NL895' to salinity stress, plantlets were cultured on half-strength Murashige and Skoog medium containing 0, 50, 100, 150, or 200 mM NaCl for 15-30 d. The growth, ion homeostasis, antioxidant activities, and ultrastructural responses of the roots and leaves were assessed. Sodium chloride, at 50 and 100 mM (there were mostly no statistical differences in the two levels), induced lower Na+ contents, higher K+/Na+ ratio, and lower H2O2 and malondialdehyde (MDA) levels in roots compared to those in leaves. However, salt-stressed roots exhibited greater peroxidase (POD) and glutathione reductase (GR) activities and higher ascorbic acid (AsA) contents compared to leaves. The leaves maintained higher K+ and Ca2+ levels than the roots during NaCl treatment and partly balanced the excess Na+ to decrease Na+ toxicity. Also, leaves reduced oxidative damage by increasing the activities of the antioxidation enzymes superoxide dismutase and catalase, and by increasing AsA and glutathione (GSH) levels. Additionally, an increase in the stomatal density was observed in the NaCl-treated leaves. Overall, roots and leaves displayed different adaptive mechanisms to cope with salt stress and the roots were more tolerant to salt stress than the leaves. Coordination of Na+ and K+ transport in roots and leaves, and the organ-specificity of high POD activity in roots, and high CAT and APX activities in leaves, likely contributed to the salt tolerance of the whole plant. 'NL895' poplar could survive at a moderate NaCl level of 100 mM.