*2.5. Salinity Tolerance of the "YNU31-2-4" Line at Seedling Stage*

We evaluated the salinity tolerance of "Kaijin", "Yukinko-mai" (WT), and "YNU31-2-4" seedlings during three weeks at 0, 75, and 125 mM NaCl. At 0 mM NaCl, there was no phenotypic difference between "YNU31-2-4" and WT (Figure 4A). Under salt stress, however, the WT leaves were rolled, whereas those of "Kaijin" and "YNU31-2-4" remained flat and stayed green even at 125 mM NaCl. Further, 125 mM NaCl reduced the survival of WT seedlings to 52.5%, whereas all seedlings of "YNU31-2-4" survived (Figure 4B). Moreover, "Kaijin" and "YNU31-2-4" had better shoot and root growth under salinity vs. WT (Figure 4C–F); at 125 mM NaCl, "YNU31-2-4" had 30% and 38% better shoot and root dry weight, respectively than WT. In a separate experiment (Figure S4), at 0 mM NaCl, there were no significant differences in leaf relative water content or chlorophyll content between WT and "YNU31-2-4" plants. Under salt stress, the "YNU31-2-4" plants were able to maintain significantly higher relative water content and chlorophyll levels than WT (Figure S4A,B). Under the control condition, "YNU31-2-4" had significantly higher proline content than the parents. Exposure to salinity led to a considerable increase in proline levels in all genotypes, and to 1.6× the WT level in "YNU31-2-4" (Figure S4C). These results clearly indicate that "YNU31-2-4" has stronger salt tolerance than "Yukinko-mai" (WT) at the seedling stage.
