Nitrogen and phosphorus availability limit N-2 fixation in bean

Availability of nitrogen (N) and phosphorus (P) might significantly affect N-2 fixation in legumes. The interaction of N and P was studied in common b ean (Phaseolus vulgaris), considering their effects on nodulation and N-2 f ixation, nitrate reductase activity, and the composition of N compounds in xylem sap. The effect of N on the uptake of P by plants was estimated by an alysing rhizospheric pH and P concentration in xylem sap and in plant shoot s. Inoculated bean plants were grown in pots containing perlite/vermiculite in two experiments with different amounts of P and N. In a third experimen t, bean plants were grown on two soil types or on river sand supplied with different concentrations of N. At harvest, shoot growth, number of nodules and mass, and nitrogenase activity were determined. Xylem sap was collected for the determination of ureides, amino acids, nitrate and phosphate conce ntration. At low nitrate concentration(1 mM), increasing amounts of P promo ted both nodule formation and N-2 fixation, measured as ureide content in t he xylem sap. However, at high nitrate concentration (10 mM), nodulation an d N-2 fixation did not improve with increased P supply. Glutamine and aspar tate were the main organic N compounds transported in the xylem sap of plan ts grown in low nitrate, whereas asparagine was the dominant N compound in xylem sap front plants grown in high nitrate. Nitrate reductase activity in roots was higher than in shoots of plants grown with low P and high N. In both soils and in the sand experiment, increased application of N decreased nodule mass and number, nitrogenase activity and xylem ureides but increas ed the concentration of asparagine in xylem sap. Increasing P nutrition imp roved symbiotic N-2 fixation in bean only at low N concentrations. It did n ot alleviate the inhibitory effect of high nitrate concentration on N-2 fix ation. A decrease in plant P uptake was observed, as indicated by a lon;er concentration of P in the xylem sap and shoots, correlating with the amount of N supplied. Simultaneously with the specific inhibition of N-2 fixation , high nitrate concentrations might decrease P availability, thus inhibitin g even further the symbiotic association because of the high P requirement for nodulation and N-2 fixation.