Clade-dependent effects of drought on nitrogen fixation and its components – Number, size, and activity of nodules in legumes.

Drought affects the growth of legumes directly, and indirectly, by reducing total nitrogen fixation. Here, we compiled published data to compare the sensitivity to water deficit on plant growth and total nitrogen fixation traits, i.e., the number of nodules per plant, average nodule mass, and nitrogen fixation per unit nodule mass. Hierarchies of phenotypic plasticity have been established for seeds and organelles, whereby variation in number associates with conserved size. By analogy, our first hypothesis is that there is a hierarchy of plasticities between nitrogen fixation traits. Our second hypothesis is that determinate nodules are more sensitive to water deficit than their indeterminate counterparts, because the latter can reactivate meristems when water becomes available. In our sample, onset of stress treatment averaged 28 d after sowing; median duration of stress was 12 d; and intensity of stress (ratio of shoot biomass between stressed and control) averaged 0.65. These drought conditions (i) reduced total nitrogen fixation and average nodule mass more severely than plant shoot mass, (ii) elicited a hierarchy of plasticities whereby number of nodules per plant varied substantially, and average nodule mass and nitrogen fixation per unit nodule mass were relatively conserved, and (iii) affected more severely Milletioids (determinate, ureide exporting nodules) than their IRLC counterparts (indeterminate, amide exporting nodules). • Total N fixation was partitioned as nodule number, average nodule mass, and nitrogen fixation per unit nodule mass. • Early-season drought reduced nodule number more than average nodule mass and N fixation per unit nodule mass. • Drought effect were more severe in Milletioid (determinate, ureide) than in IRLC (indeterminate, amide) nodules. [ABSTRACT FROM AUTHOR]