Features of the intrageneric Alnus‐Frankia specificity

Abstract

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.