Variability of nitrogen-fixing Frankia on Alnus species

Abstract

Plants maintain mutualistic symbioses with multiple symbiont genotypes that differ in the benefits they provide. To investigate differences in the effect of nitrogen-fixing Frankia on Alnus species, spore-producing (sp+) nodules from Alnus rubra Bong. and Alnus incana subsp. rugosa (Du Roi) Clausen and non-spore-producing (sp–) nodules from Alnus viridis subsp. crispa (Ait.) Turrill, A. rubra, and A. incana subsp. rugosa were collected from each of four different populations and used to inoculate all three Alnus species. As expected, sp+ Frankia produced significantly more nodules on all three species. However, A. crispa, which normally does not have sp+ nodules in the field, was more susceptible to a high level of infection by sp+ Frankia in general, and by any source of sp+ Frankia in particular, whereas A. incana subsp. rugosa, which has the highest abundance of sp+ in the field, was less susceptible to high levels of infection. This suggests that A. incana subsp. rugosa develops resistance to high levels of infection. The infectivity of an sp+ Frankia source on A. viridis subsp. crispa and A. rubra was positively correlated with the proportion of sp+ nodules on the site it was collected from, suggesting that the variation in the abundance of sp+ in the field is caused by sp+ Frankia with different levels of infectivity. There was no effect of Frankia sources on nodule allocation. Plant growth was positively correlated with the specific nodule mass and the specific nodule activity, and negatively correlated with the nodule number per plant. Sp+ Frankia resulted in significantly smaller plants in A. rubra. While there was no overall sp+ type effect on the growth of A. viridis subsp. crispa, the largest plants always resulted when they were inoculated with sp–, and the smallest with sp+ Frankia. Neither spore type nor inoculum source had any effect on the performance of A. rugosa. These results suggest that Alnus species remain susceptible to infection by both Frankia spore types, but are able to modulate the effectiveness of these spore types when they are the common symbionts in the field.