Effects of N2 deficiency on transport and partitioning of C and N in a nodulated legume

Nodulated root systems of white lupin (Lupinus albus L. cv Ultra: Rhizobium strain WU425) were exposed to $\text{Ar}\colon \text{O}_{2}$ (80:20, v/v) or $\text{Ar}\colon \text{N}_{2}\colon \text{O}_{2}$ (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 $\text{Ar}\colon \text{O}_{2}$ or was much reduced in plants exposed to $\text{Ar}\colon \text{N}_{2}\colon \text{O}_{2}$, 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 $\text{Ar}\colon \text{O}_{2}$ treatment. Xylem N levels fell rapidly after application of $\text{Ar}\colon \text{O}_{2}$. C:N ratios of phloem sap of petioles and of stem base rose during the first day of $\text{Ar}\colon \text{O}_{2}$ 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 $\text{Ar}\colon \text{O}_{2}$ treatment, presumably due to increasing shortage of xylem derived N for xylem to phloem exchange. Reexposure of $\text{Ar}\colon \text{O}_{2}$-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.