Boron is essential to growth at low concentrations and limits growth and yield when in excess. Little is known about plant response to excess boron (B) and water stress occurring simultaneously. The influences of B and water supply on tomatoes (Lycopersicon esculentumMill.) were investigated in lysimeters. Water application levels were 30, 60, 100, 130 and 160% of potential evapotranspiration. Boron levels in irrigation water were 0.02, 0.37, and 0.74 mol m–3. Conditions of excess boron and of water deficits were found to decrease yield and transpiration of tomatoes. Simultaneous B and drought stresses did not result in a larger effect but rather, one or the other stress-causing factor was found to be dominant in plant response. Both irrigation water quantity and boron concentration influenced water use of the plants in the same manner as they influenced yield. A dominant-stress-factor model following the Liebig-Sprengel law of the minimum was assumed and validated. The model applies the principle that, when a plant is submitted to conditions of stress caused by B in conjunction with water and/or salinity stress, the most severe stress determines yield.