Genotypic variation in salt tolerance of Ulmus pumila plants obtained by shoot micropropagation

Siberian elm (Ulmus pumila L.) is a tree native to Asia and is widely cultivated in the other parts of the world due to its resistance to drought, temperature extremes, and salinity. In this study, we obtained nine clones of Siberian elm by in vitro shoot propagation of trees varying in growth, morphological characteristics, and salt tolerance in the field to examine traits linked to salt tolerance. The plantlets were subjected for 2 weeks to 0, 0.3, 0.5, 0.7, 0.8, and 0.9 % (w/v) NaCl and to mixture of salts representative of sea water (NaCl, CaCl, K₂SO₄, MgSO₄, and NaHCO₃) to examine the effects on growth, tissue concentrations of major osmolytes, proteins, and chlorophyll, as well as activities of the antioxidant enzymes. While higher salt treatment (both NaCl and mixed salt from 0.7 to 0.9 %) concentrations decreased growth rates in all of the studied clones, 0.3 % NaCl treatment resulted in greater shoot growth parameters of the treated plantlets compared with untreated control. In mixed salt treatments, growth parameters, including shoot relative elongation rate (SRGR), branching rate (BR), and length of new shoots (SL), decreased in the examined clones with the exception of clones 51, 105, and 65225, respectively, in which treatment concentrations as high as 0.3 % NaCl had no effect on shoot length. In both NaCl and mixed salt treatments, the antioxidant enzymes (SOD, POD, and CAT) in most of these clones increased at 0.3–0.7 % (or 0.8 %) treatment concentrations and decreased at 0.8–0.9 % salt treatments; malondialdehyde (MDA) content increased with the growing of the salt concentrations; soluble protein and chlorophyll content show a decreased tendency with the raising of the salt concentrations; in most elm clones, sugar and free proline (Pro) content accumulate at 0.3–0.8 % salt treatment decreased in 0.9 % salt treatment. In mixed salt treatment, the superoxide (SOD) shows more activities in most of the clones compared with NaCl treatment. In most clones, chlorophyll concentrations were greater in NaCl compared with mixed salt treatment in 0.8–0.9 % concentrations; the decrease of soluble protein concentrations in the mixed salt was lower compared with NaCl treatment. Using the membership function and cluster analyses, the clones were classified into four distinctly different groups in terms of their salt tolerance. The differences in responses of the examined physiological parameters to salt treatments between these clones with different salt-tolerance capacities point to the importance of osmoregulation and maintenance of antioxidant enzyme activities in salt tolerance of Siberian elm.