P uptake by plant Some are strongly upregulated when phosphate supply is inadequate. Other phosphate systems are constitutive which are not affected by changes in phosphate concentration in the nutrient medium The ph in the apoplast controls the H2PO4 the uptake rate• Some are strongly upregulated when phosphate supply is inadequate. Other phosphate systems are constitutive which are not affected by changes in phosphate concentration in the nutrient medium. • The pH in the apoplast controls the H2PO4 - the uptake rate. P uptake by plant