We review four hypotheses for the control of carbon acquisition by roots, and conclude that the functional equilibrium hypothesis can offer a good description of C acquisition by roots relative to shoots, but is deficient mechanistically. The hypothesis that import into roots is solely dependent on export from the shoot, itself determined by features of the shoot alone (the ‘push’ hypothesis), is supported by some but not all the evidence. Similarly, the idea that root demand, a function of the root alone, determines import into it (the ‘pull’ hypothesis), is consonant with some of the evidence. The fourth, general, hypothesis (the ‘shared control’ hypothesis) – that acquisition of C by roots is controlled by a range of variables distributed between root and shoot – accords with both experiment and theory. Top-down metabolic control analysis quantifies the control of C flux attributable to root relative to source leaf. We demonstrate that two levels of mechanistic control, short-term regulation of phloem transport and control of gene expression by compounds such as sugars, underlie distributed control. Implications for the impact of climate change variables are briefly discussed.

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.