Cucumber (Cucumis sativus L.), an important vegetable crop, is sensitive to NaCl. Its salinity tolerance can be improved by grafting onto pumpkin rootstocks, which restricts the uptake of Na+, but not of Cl−. Although Na+ seems to be more toxic than Cl− in cucumber, tissue tolerance to Na+ and Cl− is still unclear. In this study, a mixed-salt experiment, designed for equal osmolarity and equimolar concentrations of ions between treatments, was conducted using cucumber genotypes “Aramon” and “Line-759,” which are different in Na+ and Cl− exclusion. This combination of treatments generated various patterns of ion concentrations in leaves for deriving the response curves of photosynthesis and stomatal conductance to ion concentrations. In both cultivars, photosynthesis and stomatal conductance were sensitive to leaf Na+ concentration but insensitive to Cl− concentration. In these genotypes, tissue tolerance to Na+ varied independently of Na+ exclusion. Grafting “Aramon” onto pumpkin rootstock modified the Na+/Cl− ratio in leaves, reduced Na+ uptake, enhanced K+ transport towards the young leaves, and induced Cl− recirculation to the old leaves. These results suggest that (1) cucumber cannot restrict the Na+ accumulation in leaves but is able to avoid overaccumulation of Cl−, and (2) pumpkin rootstock regulates the recirculation of K+ and Cl−, but not Na+.