You could start with growing a standard wheat cultivar and add different concentrations of salt to the substrate. Comparing traits such as height or leaf area during the growth time, you can figure out which range of salt concentration has a well-measurable effect on the plants without completely imparing growth. When you know about a reasonable salt concentration, you can apply this to your collection of genotypes and monitor their development. Monitoring different genotypes within the salt treatment already will be informative on potential differential responses. However, it might be meaningful to compare the salt-group with a salt-free group.