NODULE CARBOHYDRATE-COMPOSITION AND NITROGEN-FIXATION IN PEA (PISUM-SATIVUM L) - EFFECT OF RHIZOBIUM STRAIN AND NH4NO3

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 Pisum sativum 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 presence of NH4NO3 showed greater decreases in starch, sucrose, fructose, succinate, malate and alpha-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.