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

The eect of phosphorus supply (115 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 inuence 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. Specic 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 deciency 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 deciency increases acidication which may facilitate P acquisition.