Salt tolerance in Pyrus species is linked to levels of Na and Cl translocation from roots to leaves

Salt tolerance was tested in five Asian Pyrus rootstock species: Pyrus betulaefolia Bunge (strains C and N; P. betulaefolia C and P. betulaefolia N, respectively), P. calleryana Dcne. (strain No. 8; P. calleryana 8), P. pyrifolia Nakai, P. fauriei Schneid., and P. dimorphophylla Makino (strain No. 6; P. dimorphophylla 6). Four-month-old seedlings were subjected to each of 50 mM, 100 mM, 150 mM, and 200 mM NaCl solutions, and leaf injury, shoot growth, leaf water potential, and mineral uptake were evaluated. P. betulaefolia C showed the highest salt tolerance with no visible symptoms of injury even in the 200 mM NaCl treatment. P. betulaefolia N and P. dimorphophylla 6 also exhibited higher salt tolerance. In contrast, P. calleryana 8, P. fauriei, and P. pyrifolia exhibited lower salt tolerance with severe leaf injuries leading to defoliation and death. Leaf water potential decreased via NaCl treatment in all species and there was no difference among species. Sodium and Cl contents in the roots of all species increased within 2 weeks after NaCl treatment, although the differences among species and NaCl treatment was small. On the other hand, Na and Cl contents in leaves were different among species. The salt-sensitive species P. calleryana 8 and P. pyrifolia accumulated higher concentrations of Na and Cl in the leaves over 150 mM NaCl at 6 weeks after treatment compared to the salt tolerant species P. betulaefolia C. These results imply that P. betulaefolia C might have a key mechanism such as storage exclusion and/or transport-restriction between the shoot and root to depress the transport of Na and Cl to the upper plant parts, enabling a higher tolerance to NaCl.