• Abutilon theophrasti;
  • Gossipium hirsutum;
  • Ipomoea batata;
  • Echinochloa crus-galli;
  • Paspalum plicatulum;
  • Digitaria decumbens;
  • phloem;
  • speed;
  • concentration;
  • loading

Abstract Extended square-wave tracer kinetics using 11CO2 were used to measure the speed of transport and activity level (proportional to concentration) in the phloem at high and low loading rates in six species of plants. In all cases, increased loading rates resulted in increased concentration. In most cases speed also increased, however, in two cases speed was lower and tracer activity was much higher at the higher loading rate. All the responses are consistent with the Münch Horwitz theory of phloem transport, depending upon the equation used to represent the unloading mechanism as described in a previous paper (Goeschl & Magnuson, 1986). For example, the latter two cases are consistent with the assumption that the unloading rate was limited by a process with saturable kinetics (enzyme-like).