Water de fi cit in fl uences leaf transpiration rate and photosynthetic activity. The genotype-dependent response of the latter has not been assessed in sun fl ower ( Helianthus annuus L.), particularly during the reproductive period when grain fi lling and lipogenesis depend greatly on photosynthate availability. To evaluate genotypic responses to water de fi cit before and after fl owering, two greenhouse experiments were performed. Four genotypes two inbred lines (PSC8, XRQ) and two cultivars (Inedi, Melody) were subjected to progressive water de fi cit. Non-linear regression was used to calculate the soil water de fi cit threshold (FTSWt) at which processes (transpiration and photosynthetic activity) were affected by water de fi cit. In the vegetative growth stage, photosynthetic activity was affected at a lower mean value of FTSWt (0.39) than transpiration (0.55). However, in the reproductive stage, photosynthetic activity was more sensitive to soil water de fi cit (FTSWt = 0.45). We found a signi fi cant ( P = 0.02) effect of plant growth stage on the difference between photosynthesis and transpiration rate thresholds and, a signi fi cant ( P = 0.03) effect of leaf age on transpiration. Such results will improve phenotyping methods and provide paths for integrating genotypic variability into crop models.