Possible limitations of sink strength for fructan storage in tubers of Jerusalem artichoke (Helianthus tuberosus L. cv. Waldspindel) were studied in 2-yr field experiments. Treatments included the removing of side branches and the breaking of apical dominance in unbranched plants. Data from both treatments and the comparison of results obtained for 2 yr, which differed in temperature and total irradiance, indicated that fructan storage in tubers during the storage phase was not limited by supply of assimilate (sucrose). Moreover, fructan concentrations were very constant despite large variations in tuber yields and yield components (e.g. number of tubers per plant, tuber length). However, stimulation of cell expansion and diameter of tubers increased fructan concentrations, probably owing to a larger vacuolar volume in the storage cells. We concluded that fructan storage in tubers of Jerusalem artichoke is not source-limited as far as carbon import into tubers is concerned. However, a strong shoot control, most probably regulated via phytohormones, limits sink strength. Fructan concentrations in tubers might not only be increased by sink activity (fructan-synthesizing enzymes) but also by sink capacity (vacuolar volume). In this respect the shoot might play a decisive role not only as a temporary storage pool for carbohydrates but also as a source of native growth-regulators which control sink capacity.