Root secreted acid phosphatase kinetics as a physiological marker for phosphorus deficiency

The release of acid phosphatase by roots can potentially liberate phosphorus (P) from the soil organic pool. As the degree of enzyme activity has been shown to vary among species and with the severity of P starvation, with the objective of identifying physiological markers for P deficiency in the present study, the kinetic properties for the enzyme secreted in vivo by the roots of three contrasting species [Ruellia tuberosa (L.), Euphorbia heterophylla (L.) and Cajanus cajan (L) Millsp], were examined. Plants were grown from seeds in a highly ventilated greenhouse and sand culture experiments were performed using 950-mL pots every other day, irrigated with a nutrient solution containing 1.0 or 0.01 mM P or with water. Plants were harvested at time points when moderate to severe P deficiency in the low P treatment as indicated by growth inhibition were shown. Root surface sodium (Na)-soluble AP-ase was extracted in vivo for 6 h at 4 degrees C and the secreted filtrate assayed with the substrate p-nitrophenylphosphate without further purification at 34 degrees C. Results showed considerable kinetic diversity as indicated by the apparent Km (Michaelis constant) and Vmax (maximum enzyme velocities) and by the shape of the plots. The implication of the Km and Vmax values are analyzed in terms of the potential for P liberation. Visualization of AP-ase activity in rhizoboxes filled with a P deficient soil confirmed the presence of the enzyme in situ.