FRUCTAN STORAGE IN TUBERS OF JERUSALEM-ARTICHOKE - CHARACTERIZATION OF SINK STRENGTH

Possible limitations of sink strength for fructan storage in tubers of Jerusalem artichoke (Helianthus tuberosus L. cv. Waldspindel) were st udied in 2-yr field experiments. Treatments included the removing of s ide 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 t hat fructan storage in tubers during the storage phase was not limited by supply of assimilate (sucrose). Moreover, fructan concentrations w ere very constant despite large variations in tuber yields and yield c omponents (e.g. number of tubers per plant, tuber length). However, st imulation of cell expansion and diameter of tubers increased fructan c oncentrations, probably owing to a larger vacuolar volume in the stora ge cells. We concluded that fructan storage in tubers of Jerusalem art ichoke is not source-limited as far as carbon import into tubers is co ncerned. 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 enz ymes) 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 wh ich control sink capacity.