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Osmotic constraints, nutrient deficiencies, especially phosphorus, and the lack of efficient strains of rhizobia in Mediterranean soils are major factors limiting symbiotic nitrogen fixation and yield in common bean (Phaseolus vulgaris). In order to improve yields, we investigated responses to rhizobial inoculation and P fertilization under glasshouse and field conditions with two bean cultivars, Coco blanc and BAT477. The nodulation test, using a hydroponic system in a glasshouse, revealed strong variability among 22 rhizobium strains of various origins, in their symbiotic efficiency, compared with Rhizobium tropici CIAT899. The introduced strains proved to be more efficient although some local rhizobia revealed high potential efficiencies. Glasshouse trials showed a significant effect of inoculation and P supply on nodulation, N content and shoot dry weight that varied with rhizobial strain. Field trials substantiated the glasshouse observations and emphasized the effects of cultivar-rhizobia interactions on symbiotic parameters and yields. It is concluded that inoculation with suitable rhizobia with supply of additional P is a technology that may improve symbiotic nitrogen fixation and yield in common bean in some soils under a Mediterranean climate. [ABSTRACT FROM AUTHOR]

The effects of NaCl and ABA on the respiration of N2-fixing nodules were analysed in common bean (Phaseolus vulgaris) inoculated with Rhizobium tropici the reference strain CIAT899. Shoot and nodule growth was more inhibited by NaCl than root growth. The O2 uptake by nodulated roots at 21 kPa O2 was significantly inhibited by salinity. Raising pO2 stimulated nodule respiration more under NaCl treatment than for the control, although it did not compensate totally for the inhibitory effect of NaCl. Short NaCl application was less destructive than long term application. Also, the external application of ABA inhibited nodule respiration, and this inhibition was partly compensated by raising pO2. [ABSTRACT FROM AUTHOR]

The effect of phosphorus supply (1–15 μM) on proton release and the role of P in symbiotic nitrogen fixation in medic (Medicago truncatula L. ‘Jemalong’) was investigated. As P concentration in the nutrient solution increased, shoot and root growth increased by 19 and 15%, respectively by day 35, with maximal growth at 4 μM P. A P concentration of 15 μM appeared to be toxic to plants. Phosphorus supply had no influence on nodule formation by day 12 but increased nodule number by day 35. Nitrogenase activity was estimated by in situ measurement of acetylene reduction activity (ARA) in an open-flow system. During the assay, a C2H2-induced decline of ARA was observed under all P concentrations except 4 μM. Specific ARA (per unit nodule weight) doubled when P supply was increased from 1 to 8 μM. This effect of P was much greater than the effects of P on nodulation and host plant growth. Concentrations of excess cations in plants decreased with increasing P concentration in the nutrient solution. Phosphorus deficiency stimulated uptake of excess cations over anions by the plants and hence enhanced proton release. The results suggest that P plays a direct role in nodule functioning in medic and that P deficiency increases acidification which may facilitate P acquisition. Copyright 2001 Annals of Botany Company [ABSTRACT FROM AUTHOR]