Features of the intrageneric Alnus-Frankia specificity

Host specificity between local Frankia strains and native alders [Alnus incana (L.) Moench and A. glutinosa (L.) Gaertn.] was evaluated in inoculation experiments. Pure cultures of Frankia , whether originating from A. incana or A. glutinosa , were infective and effective on both host species. These pure cultures were isolated from spore-negative (Sp⁻) nodules. From spore-positive (Sp⁺) nodules no Frankia isolates were obtained. This strain type resisted all our isolation attempts and therefore crushed nodules had to be used for Sp⁺ type inoculations.
The Sp⁺ type Frankia populations differed in their host specificity. Sp⁺ nodules from A. glutinosa were effective on both alder species, but Sp⁺ nodules from A. incana induced effective nodules only on the original host; on A. glutinosa only small (1-3mm) prenodule-like structures were found. Such A. glutinosa plants died on N-free medium, thus showing that these nodules were ineffective. In the effective nodules the middle cortex was dominated by infected cells filled with vesicle clusters. In the ineffective nodules only a few cortical cells were infected and sporangia predominated in these cells. Surprisingly enough they also contained vesicle-like structures as demonstrated in electron micrographs.