Developing a salinity tolerance indicator for tree varieties at challenging sites and urban forests based on inferences of physiological responses: an example of Ulmus pumila

Elevated salinity is one of the major environmental factors limiting plant growth and development. Salinity stress compromises survival and production of commercial plantations, urban forests, and agricultural crops in arid and semi-arid regions and in intertidal zones. Ulmus pumila , a salt-tolerant tree species indigenous in Asia, is widely deployed in salt-affected areas in China and holds promise for multi-varietal forestry (MWF) and urban forests. The mechanism of salt tolerance in U. pumila remains unknown. Here, we investigated the physiological responses to salinity stress based on antioxidant enzyme activity, osmotic adjustments, and gas exchange among salt-tolerant U. pumila genotypes for 100 days. The sampled trees were treated at age 2 under five different NaCl levels (0%, 0.3%, 0.5%, 0.7%, and 0.9% w / v ) with natural surroundings and rain shades. Salt stress suppressed the growth in height (HR), ground diameter (DR), and dry weight (biomass) significantly among genotypes. The HR and performance indices were positively correlated with photosynthetic rate (Pn), apparent mesophyll conductance (AMC), and chlorophyll (CHLL) but were negatively associated with the free proline, sugar, and protein accumulation ( r = − 0.5 to − 0.7***). We found that high accumulation of sugars and higher activity of the SOD enzyme in leaf tissue contributed to the osmotic adjustment and reactive oxygen species (ROS) scavenging system under salinity treatments. Sugar content and SOD activity play key roles in U. pumila’s salinity tolerance and are promising indicators for ex vitro selection applications. Our result from ex vitro selection aligns with those of previous in vitro studies (Mu et al. in Acta Physiol Plant 38:1–16, 2016).