Figure 6.

Opposite interests of plant and fungus maintain a balance of power. The activity of H⁺-ATPases influences the zero-current potentials (plant: ${\text{V}}_{\text{p}}^0$; fungus: ${\text{V}}_{\text{f}}^0$) of the background conductances at the plasma membranes. The proton-coupled sugar (H/C, green, lighter/bigger in gray scale) and phosphate (H/P, red, darker/smaller in gray scale) fluxes depend on the difference ΔV₀ = ${\text{V}}_{\text{f}}^0$ − ${\text{V}}_{\text{p}}^0$. The plant has the interest to drive ${\text{V}}_{\text{p}}^0$ more negative (and therefore to drive ΔV₀ more positive) to reduce the loss in sugar and to increase the gain in phosphate, while the fungus tends to reduce the loss of phosphate and to increase the gain of sugar by driving ${\text{V}}_{\text{f}}^0$ (and ΔV₀) more negative. Between equal partners the different forces balance at ΔV₀ = 0. For further details, see also Figure 5. For better comparison, fluxes were normalized, as explained in Figure 4, to the H/C-flux across the plant membrane in equilibrium of the condition shown in Figure 3 and displayed as relative changes. Circle (H/C) and square (H/P) indicate this reference condition.