Full Length Research Paper

Effect of phosphorus limiting on phytase activity, proton efflux and oxygen consumption by nodulated-roots of common bean (Phaseolus vulgaris)

Saber Kouas¹, Ahmed Debez¹, Claude Plassard², Jean Jacques Drevon² and Chedly Abdelly¹*

¹Laboratoire d’Adaptation des Plantes aux Stress Abiotiques, Centre de Biotechnologie à la Technopole de Borj Cedria, BP 901, 2050, Hammam-Lif, Tunisia.

²Rhizosphere et Symbiose, Institut national de la Recherche Agronomique (UMR 1220), 1, Place Viala, 34060, Montpellier, France.

*Corresponding author. E-mail: chedly.abdelly@cbbc.rnrt.tn. Tel.: +216 79 412 848. Fax: +216 79 412 638.

Accepted 30 June, 2009

   Abstract

This work intended to measure the nodulated-roots oxygen consumption, proton efflux and phytase activity in 2 lines of common bean (Phaseolus vulgaris) (115, 147) at 2 levels of P supply. Rooted seedlings were inoculated with Rhizobium tropici CIAT 899 in hydroaeroponic cultivation under glasshouse. Phosphorus was supplied as KH₂PO₄ at 15 and 250 µmol pl^-1 week^-1 (15P and 250P, respectively). Our results showed that plant growth nodulation and symbiotic nitrogen fixation were significantly affected by P limiting (15P) for the both lines, but this adverse effect was more pronounced in 147 than in 115. For the both lines, the phytase activity, higher in roots than in nodules, was significantly increased by P limiting, but 115 maintained higher values as compared to 147 line. In cotyledons, the phytase activity was higher in 115 than in 147. Phosphorus shortage increased the cumulated proton release only in 115, whereas it was lowered for 147. In this line, the proton release was linked to symbiotic nitrogen fixation. Under 15P, the proton efflux per unit of nodulated-root biomass was 25% greater for 115 than 147, suggesting that under P limitation, proton efflux may constitute an efficient way to increase P uptake in the tolerant line (115). 15P increased significantly nodulated-root O₂ consumption per g nodule DW and nodule conductance, but to a higher extent in 147. As a whole, bean plants at P-deficient conditions increased the activity of phytases and proton efflux, thus maintaining the oxygen diffusion in nodules. This may represent an adaptive mechanism for N₂-fixing legumes to respond to P deficiency, by increasing the utilisation and the uptake of phosphorus for symbiotic nitrogen fixation.

Key words: bean, phosphorus, phytase activity, proton efflux, nodulated root oxygen consumption, phosphorus use efficiency for symbiotic nitrogen fixation.