Host?rhizobia interaction for effective inoculation: evaluation of the potential use of the ureide assay to monitor the symbiotic performance of tepary bean (Phaseolus acutifolius A. Gray)

Abstract

Domesticated and wild-type tepary beans ( Phaseolus acutifolius A. Gray) were grown with or without inoculation with rhizobia in pots under bacteriologically controlled conditions in a temperature-controlled glasshouse. Seeds were inoculated with a mixture of seven strains isolated from nodules collected from domesticated field-grown tepary bean in Arizona, USA, or with a commercial inoculant strain for Phaseolus vulgaris (CC511). Different degrees of plant reliance upon N 2 fixation for growth were generated by supplying the inoculated plants throughout growth with nutrients containing a range of concentrations of 15 N-labeled NO 3 (0, 1, 2, 5 or 10 mM). An uninoculated treatment that received 10 mM 15 N-labeled NO 3 was included to provide data for plants solely dependent upon NO 3 for growth. Six weeks after sowing, shoots were harvested for dry matter determination and subsequent 15 N analysis, root-bleeding xylem sap was collected, and nodulation assessed. With regard to shoot biomass production, domesticated lines were more responsive to inoculation, but less responsive to applied N than wild types. All inoculated plants were nodulated, but the field isolates from tepary bean were more effective in N 2 fixation than strain CC511. It was concluded that tepary bean requires a specific inoculant to benefit from fixation of atmospheric N 2 . Xylem sap samples were analysed for ureides (allantoin and allantoic acid), amino acid content (α-amino-N), and NO 3 concentration. The amount of ureide-N present in xylem sap was expressed as a percentage of total solute N, described as the relative abundance of ureide-N (RUN), for each N treatment and was compared to the proportion of plant N derived from N 2 fixation (%Ndfa) calculated using a 15 N dilution technique. The RUN values ranged from 8% for saps collected from uninoculated plants provided with 10 mM NO 3 in the nutrient solution (%Ndfa=0) to 86–91% for nodulated plants grown in the absence of externally supplied NO 3 (%Ndfa=100). These data indicated that ureides were the principal product of N 2 fixation exported from the nodules to the shoot in xylem sap. Since RUN values were closely related to %Ndfa, it was proposed that N-solute analysis of xylem sap could provide a valuable analytical tool to monitor the symbiotic performance of tepary bean.