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

The effect of phosphorus supply (1-15 muM) on proton release and the role o f P in symbiotic nitrogen fixation in medic (Medicago truncatula L. 'Jemalo ng') was investigated. As P concentration in the nutrient solution increase d, 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 esti mated by in situ measurement of acetylene reduction activity (ARA) in an op en-flow system. During the assay, a C2H2-induced decline of ARA was observe d under all P concentrations except 4 muM. Specific ARA (per unit nodule we ight) 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 co ncentration in the nutrient solution. Phosphorus deficiency-stimulated upta ke of excess cations over anions by the plants and hence enhanced proton re lease. The results suggest that P plays a direct role in nodule functioning in medic and that P deficiency increases acidification which may facilitat e P acquisition. (C) 2001 Annals of Botany Company.