Schematic representation of the model of vacuolar malate storage proposed by Lobit et al . (2006) []. State variables: [Malfruit] = concentration of malate in the pulp; [Malvac] = concentration of malate in the vacuole; pHvac = vacuolar pH; ΔΨ = electric potential gradient across the tonoplast; n = coupling ratio of the proton pump ATPase. Model parameters: pHcyt = cytosolic pH; ΔGATP = free energy of ATP hydrolysis; α, β, and n0 = fitted parameters of the coupling ratio equation (Eq. 5); (Mal2−cyt) = cytosolic activity of the di-anion malate.