Stomatal control of transpiration was implied as the major strategies by which plants cope with water stress. Here we did investigate the genetic control of this process using the following trait: Fraction of Transpirable Soil Water threshold (FTSWt) representing the threshold of soil water content at which the stomatal control of transpiration started. We conducted a progressive water deficit experiment using recombinant inbred lines (RILs) of sunflower and we analyzed the variation of FTSWt. Quantitative trait loci (QTL) mapping was then performed to determine the loci involved and to identify the genetic control. This work has shown, for the first time, QTL mapping for FTSWt in crops. In this work QTL mapping was made in sunflower.