Effect of phosphorus on nodulation and N₂ fixation by bean (Phaseolus vulgaris)

M. Olivera, C. Iribarne, A. Ocaña and C. Lluch.

Dpto. Biología Vegetal, Facultad de Ciencias, Universidad de Granada. 18071, Granada. SPAIN

E-mail: clluch@ugr.es

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Phosphorus availability can be a major limiting factor for plant growth, particularly in tropical soils.In addition, P deficiency has a stronger effect on N-fixing legumes than on other plants because of the high energy costs of N₂ fixation, which requires a greater quantity of inorganic P (1,2). In bean, a legume of tropical origin, low P availability can seriously depress yield. Strategies to improve crop growth under P deficiency (3,4) include promoting the efficient uptake of P, or favouring effective use of endogenous P. The latter strategy involves both the appropriate distribution as well as the optimal utilization of P by the plant, resulting in greater biomass production and more N₂ fixed per unit of P taken up.

The present work examines the effect of P on growth, nutritional status, fixation and N assimilation in bean plants treated with 0, 0.1, 0.5, 1.0, 1.5 and 2.0 mM of P in the form of K₂HPO₄. The Phaseolus vulgaris variety Contender was inoculated with the Rhizobium tropici strain CIAT899. The plants were collected at 28 days of cultivation, coinciding with the flowering period.

The P availability favoured plant growth in the presence of 1.5 and 2.0 mM P, while growth was inhibited in control plant (without P). the root:shoot ratio increased with 1 mM due to maximum root growth. The P deficiency had more effect on growth than on N fixation. The nitrogenase activity ARA was inhibited in plants grown without P. On the contrary, this activity was stimulated by different rates of P application, N fixation increasing at the highest rates (1.5 y 2.0 mM). Phosphate nutrition increased N₂ fixation in correlation with ammonium assimilation via GS/GOGAT. The synthesis of ureides (XDH and uricase) did not change with the level of P applied, while amine synthesis (AAT) was stimulated.

The N and P contents expressed in mg/g D.W. in the shoot increased as the P level rose. In the roots, the P content showed the same behaviour as in shoot, whereas the variation of the N content in the root, at the different P levels, proved non-significant.

1. Israel, D.W. (1987) J. Plant Physiol. 84, 835-840

2. Vadez, V., Lasso, J.H. and Drevon, J.J. (1999) Euphytica 106, 231-242

3. Vadez, V., Drevon, JJ. (2001) Agronomie 21, 691-699

4. Clarksonm, D.T.(1985) Annu. Rev. Plant Physiol. 36, 77-115

Financial support was provided by grants No PB98-1276 from DGESIC of Spanish Government and Olivera M has a grant from the AECI