| Résumé | Rhizobium leguminosarum biovar viciae strains differing in their response to the addition of NH₄NO₃ were examined to determine if these responses were related to changes in nodule carbohydrate concentrations. Ammonium nitrate was supplied for 4 weeks from planting (long-term) or for 1 week to 3-week old plants (short-term) of Piswn satwum L. (cv. Homesteader) to differentiate between effects of combined N on nodule initiation and development in the former and nodule function in the latter case. When NH₄NO₃ was supplied from planting, 4-week-old plants nodulated by strains which had lower C₂H₂ reduction rates in the prsence of NH₄NO₃ showed greater decreases in starch, sucrose, fructose, succinate, malate and α-ketoglutarate concentrations in nodule tissue than those nodulated by strains with higher C₂H₂ reduction rates in the presence of NH₄NO₃. When NH₄NO₃ was applied to 3-week-old plants with fully-formed nodules plants were harvested 4 or 7 days later. Plants nodulated by a rhizobial strain which showed the greater decrease in C₂H₂ activity after NH₄NO₃ addition had lower concentrations of starch and sugar and higher concentrations of succinate and malate in nodule tissue, as well as higher rates of uptake of NH₄⁺ and NO₃⁻ than those nodulated by a less-sensitive strain. Nodules with lower carbohydrate reserves and larger organic acid pools showed greater inhibition of N₂ fixation in the presence of NH₄NO₃. These data are consistent with the hypothesis that photosynthate deprivation or competition for reductant may be involved in the inhibition phenomenon. They also suggest that selection of rhizobial strains with enhanced efficiency of C utilization may ensure a symbiosis with greater N₂ fixation capacity in the presence of combined N. |
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