N-2 fixation of pea hypernodulating mutants is more tolerant to root pruning than that of wild type

As a legume, pea plant has the ability to symbiotically fix N-2. However, symbiotic N-2 fixation is very sensitive to environmental stresses that affect plant growth, and there is little knowledge on the impact of root pruning on N-2 fixation and plant growth. In this study, we removed half of the nodulated roots of pea wild-type Frisson and hypernodulating mutants P64, P118, and P121. Dinitrogen fixation was measured using N-15 labeling and carbon assimilation and partitioning between plant organs using C-13 labeling. Root pruning decreased N-2 fixation by -46 to -79 % in wild-type and mutants. Pea mutant P118 had a lower decrease of specific activity of N-2 fixation (-17 %) than both wild-type and other mutants (-36 to -62 %). For all genotypes, root pruning increased root and nodule sinks strengths for carbon. For P118 and for P121, this was associated to higher nodule growth than for control plants, as measured 8 days after root pruning. This is the first analysis of N-2-fixing plant response to root pruning. Importantly, we showed that some hypernodulating mutant pea lines (P118 and to a lesser extent P121) withstood this stress better than wild-type did.