Soluble carbohydrates and nitrate in the cell sap of leafy vegetables act as complementary osmotic solutes to maintain the plant's turgor pressure. The implication is that nitrate at harvest time can be kept low by manipulating the source-sink balance. Pseudo-static calculations based on a model developed earlier fairly well reproduced the seasonal variability of nitrate levels in crops grown and harvested at near-monthly intervals. This paper concentrates on the dynamic behaviour of this model by calibrating it to available data of lettuce growth, and by performing a sensitivity analysis The model contains inhibition functions which are activated when the internal assimilate carbon concentration is near an upper or lower limit, in order to restrict photosynthesis or growth, respectively. Sensitivity analysis and calibration contour plots reveal that estimates of the growth parameters have a large uncertainty when growth is limited by carbon availability. In regions that are of interest for nitrate control, i.e. in the sink-limited case, all parameters determining the source-sink balance are important. Consequently, experiments must be designed such that they cover sink-limited conditions.