Investigation of the Role of Phosphorus in Symbiotic Dinitrogen Fixation

Abstract

The interactive effects of phosphorus supply and combined nitrogen (nitrate) on dry matter and nitrogen accumulation by nodulated soybean (Glycine max L. Meff.) plants, and the relative effects of phosphorus supply on nodule number, mass, and function in comparison to host plant growth were used to investigate the role of phosphorus in symbiotic dinitrogen fition. Mixed positive and negative phosphorus by nitrogen source interactions indicated that severe phosphorus deficiency markedly impaired both host plant growth and symbiotic dinitrogen fixation and that symbiotic dinitrogen fixation has a higher phosphorus requirement for optimal functioning than either host plant growth or nitrate assimilation. In the whole plant phosphorus concentration range of 0.8 to 1.5 grams per kilogram dry weight, plants supplied with 20 millimolar nitrate accumulated significantly more dry matter and nitrogen than symbiotic plants without nitrate. This suggested that the higher phosphorus requirement for symbiotic dinitrogen fixation is internal rather than being associated with differences in the ability of roots in the two nitrogen regimes to absorb phosphorus from the external solution. Increasing the phosphorus concentration in plants solely dependent on dinitrogen fixation resulted in highly significant (P = 0.0001) increases in whole plant nitrogen concentration as well as highly significant increases (P = 0.0001) in whole plant dry matter and nitrogen accumulation. This indicated a greater responsiveness of symbiotic dinitrogen fixation than of host plant growth to improvement in phosphorus nutrition. The large increases in whole plant nitrogen concentration were associated with about 3.5-fold increases in the ratio of nodule mass to whole plant mass and about 2-fold increases in specific acetylene reduction (nitrogenase) activity of the nodules. The large increase in nodule mass (>30-fold) between the 0 and 2.0 millimolar phosphorus levels resulted from 11-and 3-fold increases in nodule number per plant and average mass of individual nodules, respectively. Root mass per plant over the same concentration range increased 3.5-fold. These results indicate that phosphorus has specific roles in nodule initiation, growth, and functioning in addition to its involvement in host plant growth processes.Mineral nutrients may influence symbiotic dinitrogen fixation of leguminous plants at any of four phases of the overall process: (a) host plant growth, (b) (5-7, 9, 13). In most of these latter studies, the interactive effect of combined nitrogen and phosphorus supply on growth of symbiotic plants was not examined. Conclusions were based on the observations that increases in phosphorus supply to plants dependent on symbiotic dinitrogen fixation as the only source of usable nitrogen increased nodule number, mass, and relative growth rate more than host plant growth and increased the specific nitrogenase (acetylene reduction) activity of nodules.The objective of this study was to reconcile conflicting reports on the role of p...

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

“…Averaged across seed treatments, the highest soil P level increased shoot mass by 100 and 89 % in control and inoculated plants, respectively, whereas the highest P level increased shoot mass only by 16 % in plants with mineral N. It confirms that plants relying on symbiotic N had a larger demand for P to obtain an optimal growth than plants supplied with mineral N (Cassman et al, 1981;Israel, 1987). Plants under mineral N had greater shoot mass than the control and inoculated plants at both soil P levels, but the improved growth associated to mineral N was more intense at low soil P levels (Figure 1a).…”

“…It demonstrates the higher P demand for N acquisition of plants depending on N 2 fixation (Israel, 1987). Seeds with high P concentration increased N accumulation in shoots of inoculated plants at the lowest soil P level (Figure 2a).…”

Nitrogen and phosphorus availability limit N2 fixation in bean

“…Moreover, the P requirement of legumes is affected by the mode of N nutrition (Cassman et al, 1981). N # -fixing plants have a greater need for P than nitrate-supplied plants (Israel, 1987) and the P requirement seems particularly great in bean (Graham & Rosas, 1979) though cultivars with a lesser requirement are known (Vadez et al, 1997). Only limited research has been conducted into the effects on N # fixation of the interactions between N and P (Gates & Wilson, 1974 ;Graham, 1981).…”

“…The low concentration of P in leaves might explain the poor growth of plants in response to the increase in NO $ − supply (Table 6), since P deficiency limits growth by affecting N uptake and assimilation (Israel, 1987 ;Rufty et al, 1993).…”

Effects of mycorrhizal colonization on biomass production and nitrogen fixation of black locust (Robinia pseudoacacia) seedlings grown under elevated atmospheric carbon dioxide

“…Elevated CO # has been shown to stimulate the productivity of symbiotic N # -fixing plants (Norby, 1987 ;Arnone & Gordon, 1990 ;Thomas et al, 1991 ;Tissue et al, 1997 ;Vogel et al, 1997). However, soil P deficiency strongly limits N # fixation (Barea & Azcon-Aguilar, 1983 ;Israel, 1987) and reduces the response of N # -fixing plants to CO # enrichment (Duchein et al, 1993).…”