Phosphorus deficiency impairs early nodule functioning and enhances proton release in roots of Medicago truncatula L.

The effect of phosphorus supply (1-15 muM) on proton release and the role of P in symbiotic nitrogen fixation 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 muM P. 4 P concentration of 15 muM appeared to be toxic to plants. Phosphorus supply had no influence on nodule formation bq 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-flow system. During the assay, a C2H2-induced decline of ARA was observed under all P concentrations except 4 muM. Specific ARA (per unit nodule weight) doubled when P supply was increased from 1 to 8 muM. This effect of P was much greater than the effects of P on nodulation and host plant growth. Concentrations of excess cations in plants decreased with increasing P concentration in the nutrient solution. Phosphorus deficiency-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 deficiency increases acidification which may facilitate P acquisition. (C) 2001 Annals of Botany Company.