Phosphorus Deficiency Impairs Early Nodule Functioning and Enhances Proton Release in Roots of Medicago truncatula L.

The eect of phosphorus supply (1±15 mM) on proton release and the role of P in symbiotic nitrogen ®xation in medic (Medicago truncatula L. `Jemalong') was investigated.As P concentration in the nutrient solution increased, shoot and root growth increased by 19 and 15 %, respectively by day 35, with maximal growth at 4 mM P. A P concentration of 15 mM appeared to be toxic to plants.Phosphorus supply had no in¯uence on nodule formation by day 12 but increased nodule number by day 35.Nitrogenase activity was estimated by in situ measurement of acetylene reduction activity (ARA) in an open-¯ow system.During the assay, a C 2 H 2 -induced decline of ARA was observed under all P concentrations except 4 mM.Speci®c ARA ( per unit nodule weight) doubled when P supply was increased from 1 to 8 mM.This eect of P was much greater than the eects of P on nodulation and host plant growth.Concentrations of excess cations in plants decreased with increasing P concentration in the nutrient solution.Phosphorus de®ciency stimulated uptake of excess cations over anions by the plants and hence enhanced proton release.The results suggest that P plays a direct role in nodule functioning in medic and that P de®ciency increases acidi®cation which may facilitate P acquisition.