Annals of Botany

Need more nitrogen? Nodule numbers not the answer

An obvious route to increasing yield in legumes might, at first sight, be to increase the number of nodules formed in the Rhizobium-legume symbiosis. Nodule numbers are regulated by the host (via both root-derived and shoot-derived signals). In the later stages of the growth cycle, no new nodules are formed, even on recently formed roots. Based on this, Bourion et al. (Dijon, France, pp. 589–598) have set out to establish whether extending the nodulation phase does in fact improve the N economy of field-grown pea (Pisum sativum) plants. They used three normal cultivars, ‘Athos’, ‘Austin’ and ‘Frisson’, the former two having a higher root biomass than ‘Frisson’, and two hypernodulating mutants, P118 and P121, mutated in the SYM29 (regulates nodulation via a shoot-derived signal) and NOD3 (regulates nodulation via a root-derived signal) genes, respectively. Plants were sampled throughout the growth cycle and after seed maturation. The contributions made by soil N and fixed atmospheric N were determined by the isotope dilution technique. The results were clear. Firstly, in hypernodulating mutants, nodulation started earlier and continued later than in the three other lines. The effect of this was that mutant plants possessed between 2.5- and 4-fold more nodules than wild-types and that nodules were maintained for longer. Further, P118 showed higher rates of N-fixation before the seed-filling phase. However, in both mutants, root development was less and overall N accumulation was lower than in the wild-types. Neither did the mutants exhibit a greater percentage of fixed N than the wild-types. Indeed, for P121, this percentage was significantly lower than wild-types. Overall, seed protein content was highest in ‘Athos’ and ‘Austin’, the two lines with the greatest root development and therefore the greatest capacity to absorb N from the soil during seed-filling. As the authors are aware, a more ‘multi-stranded’ approach than simply increasing nodule numbers is needed to improve N absorption and they make several good suggestions as to what that approach should include.