Effects of N2 Deficiency on Transport and Partitioning of C and N in a Nodulated Legume

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Abstract
Nodulated root systems of white lupin (Lupinus albus L. cv Ultra: Rhizobium strain WU425) were exposed to Ar:O2 (80:20, v/v) or Ar:N2:O2 (70:10:20, v/v/v) and C and N partitioning were examined over a 9- or 10-day period in comparison with control plants with nodulated roots retained in air. Accumulation of N ceased in plants exposed to Ar:O2 or was much reduced in plants exposed to Ar:N2:O2, but net C assimilation rates and profiles of C utilization remained similar to those of control N2-fixing plants. There was, however, a proportional reduction in CO2 evolution from nodulated roots of the Ar:O2 treatment. Xylem N levels fell rapidly after application of Ar:O2. C:N ratios of phloem sap of petioles and of stem base rose during the first day of Ar:O2 treatment and then fell progressively back to levels close to that of control plants as leaf reserves of N became available for loading of phloem. Stem top phloem sap increased progressively in C:N ratio throughout Ar:O2 treatment, presumably due to increasing shortage of xylem derived N for xylem to phloem exchange. Reexposure of Ar:O2-treated nodulated root systems to air prompted a rapid recovery of N2 fixation and restoration of plant N status. Rates of N2 fixation in plants whose roots were exposed to a range of N2 concentrations indicated an apparent Km of 10% N2 for the attached intact white lupin nodule.