Physiological implications of the Münch‐Horwitz theory of phloem transport: effects of loading rates*

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Abstract
Predicted effects of phloem loading rates on the five profiles of unloading rate, osmotic water flux, pressure, transport speed and concentration, in hypothetical sieve tubes with different sink properties, were calculated using the steady‐state mathematical expression of the Münch hypothesis of phloem transport. The prediction that increased loading rates always increases the concentration, and generally increase the speed of translocates through the sieve tube, is emphasized since these parameters are accessible for experimental testing. This particular prediction contrasts with a previous prediction (Tyree, Christy, & Ferrier, 1974), that where concentration was held constant at the loading end, concentration along the rest of the sieve tube would decrease, while speed would increase greatly.Where the unloading mechanism was assigned saturable (enzyme‐like) kinetics, increased loading rates (in the range well below the Vmax of the sink) caused both transport speed and concentration to increase. However, as loading rates approached the Vmax of the sinks, speed reached a maximum and then declined, and concentration increased substantially. This was particularly true at very high values of Km, e.g. > 0.1 mol cm−3.