Robustness of the gradient. (A): Robustness of the gradient within the reflux-loop mechanism, shown through a 1000-fold change in the permeability values. To facilitate the analysis on the effect of changes in the permeability rate, the net influx from the shoot is kept constant (basically assuming that the auxin biosynthesis in the whole plant is independent of the transport rates). (A1): Logarithmic auxin profiles along a longitudinal cross-section through a epidermal cell file. The first 2000 μm are shown. The inset shows a zoom-in close to the maximum, illustrating a marginal change in slope. (A2): Linear auxin profile along a vascular cell file, illustrating the marginal change in the maximum value. (B): In contrast, only a 2-fold change in the ratio between the apical or basal and the lateral permeability rate leads to dramatic changes in both the maximum and slope of the gradient. Profiles are as in (A), except that the first 2500 μm are shown. The differences in apical/basal (red) versus lateral (blue) PIN expression in the epidermal and cortex cells, which determine the slope of the gradient, are schematically shown in (B3).