Nitrogen and phosphorus availability limit N₂ fixation in bean

Abstract

Availability of nitrogen (N) and phosphorus (P) might significantly affect N # fixation in legumes. The interaction of N and P was studied in common bean (Phaseolus vulgaris), considering their effects on nodulation and N # fixation, 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 analysing rhizospheric pH and P concentration in xylem sap and in plant shoots. Inoculated bean plants were grown in pots containing perlite\vermiculite in two experiments with different amounts of P and N. In a third experiment, 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 concentration. At low nitrate concentration (1 mM), increasing amounts of P promoted both nodule formation and N # fixation, measured as ureide content in the xylem sap. However, at high nitrate concentration (10 mM), nodulation and N # fixation did not improve with increased P supply. Glutamine and aspartate were the main organic N compounds transported in the xylem sap of plants grown in low nitrate, whereas asparagine was the dominant N compound in xylem sap from 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 increased the concentration of asparagine in xylem sap. Increasing P nutrition improved symbiotic N # fixation in bean only at low N concentrations. It did not alleviate the inhibitory effect of high nitrate concentration on N # fixation. A decrease in plant P uptake was observed, as indicated by a lower concentration of P in the xylem sap and shoots, correlating with the amount of N supplied. Simultaneously with the specific inhibition of N # fixation, high nitrate concentrations might decrease P availability, thus inhibiting even further the symbiotic association because of the high P requirement for nodulation and N # fixation.

Seeds enriched with phosphorus and molybdenum improve the contribution of biological nitrogen fixation to common bean as estimated by 15n isotope dilution

“…Even control plants showed a strong nodulation (Figure 1b), which reinforces the argument that the common bean is a promiscuous host regarding native soil rhizobia (Hungria et al, 2003). The addition of mineral N strongly reduced nodulation at both soil P levels (Figure 1b) since the Phaseolus-Rhizobium symbiosis is quite sensitive to soil nitrate (Westermann et al, 1981;Leidi & Rodríguez-Navarro, 2000). However, at the highest soil P level nodulation was less affected by the mineral N applied.…”

“…Indeed, at the highest soil P level plants under mineral N had fewer but larger nodules, with an individual nodule mass of 2.79 mg/ nodule, as compared to 2.32 and 2.07 mg/nodule in the control and inoculated plants, respectively, averaged across seed treatments. Leidi & Rodríguez-Navarro (2000) observed that the increased P supplies improved nodulation of bean plants at low N concentrations in nutrient solutions, whereas the effect of P in high N concentrations was negligible.…”

“…Kubota et al (2008) also observed that bean plants originating from seeds with high Mo concentration showed reduced nodulation, although nitrogenase activity of root systems was not affected by seed Mo concentration. Since Mo supply is usually associated with increased nitrate reductase activity in bean plants (Pessoa et al, 2001), it is hypothesized that the additional Mo provided by enriched seeds would stimulate the assimilation of the N taken up from the soil, hence impairing nodule growth by a possible feedback mechanism induced by high concentration of reduced N compounds in roots (Leidi & Rodríguez-Navarro, 2000;Hogh-Jensen et al, 2002).…”

“…The common bean (Phaseolus vulgaris L.) has been considered of low biological N 2 fixation capacity as compared to other grain legumes, which is attributed to the poor soil conditions where the crop is usually grown, the susceptibility of the species to nutritional and environmental stresses, the short vegetative period available for N 2 fixation, the sensitivity of the symbiosis to soil nitrate, and the competition from indigenous but often ineffective soil rhizobia (Piha & Munns, 1987;Leidi & Rodríguez-Navarro, 2000;Graham et al, 2003;Hungria et al, 2003). Nevertheless, ranges of biologically fixed N 2 in fieldgrown bean lines were reported as from 25 to 65 kg ha -1 (Ruschel et al, 1982) or 40 to 125 kg ha -1 (Rennie & Kemp, 1983) in shoots at physiological maturity, or from 21 to 44 kg ha -1 in grains (Pereira et al, 1989).…”

Routes of pyruvate synthesis in phosphorus‐deficient lupin roots and nodules

“…The influence of P i on symbiotic dinitrogen (N 2 ) fixation in leguminous plants has received considerable attention (Israel, 1987) but its role in maintaining nodule metabolism remains unclear. It has been observed that soybeans grown with fertilizer nitrogen (N) have a lower phosphorus (P) requirement than when N is obtained from symbiosis (Leidi & Rodríguez-Navarro, 2000). Additionally, it was demonstrated that nodules can take up P from the host, but do not readily release its P reserves to the host root (Al-Niemi et al ., 1998) suggesting that nodules form strong sinks for P. Furthermore, this important role of P in nodules is supported by the threefold higher P concentrations compared with other tissues (Vadez et al ., 1997).…”