Plants in arid or semiarid areas often experience simultaneous salt and boron (B) stress. Interactive effects on stress responses have been clearly established, but results are inconsistent and variably indicate antagonistic or synergistic interactions even within the same plant species. In this study, five differently B- and salt-resistant wheat genotypes were grown hydroponically at low and high B supply. The effect of increasing NaCl salinity on plant growth, boron uptake rates, shoot B concentrations, and transpiration was determined under both B regimes. The interactive effect of salt and B was different under low and high B supply. Boron-uptake rates were reduced with increasing salt concentration only under high B supply, and reductions correlated significantly with decreases in leaf area and shoot B concentrations. Under low B supply, however, salt-induced effects on B-uptake rates were variable and not significantly correlated with leaf-area reductions. These results suggest that under high B supply, when B uptake is predominantly passive by diffusion or channel-mediated via aquaporins, transpiration-driven water flow is the dominant factor for B accumulation in arial plant parts. Under low B supply, when a significant portion of B can be taken up via active pathways, transpiration is not the decisive factor for B accumulation. Under these conditions, the salt sensitivity of a genotype is a modifying factor of salt–B interactions, because salt-induced growth inhibition can result in a concentration effect, offset the reduction of B-uptake rates, and result in increased shoot B concentrations. Contradictory reports on the nature of salt–B interactions might in part be related to low levels of B supply chosen as control treatments and concomittant differences in predominant B-uptake pathways.