SPORE FORMATION AND ENDOPHYTE DIVERSITY IN ROOT NODULES OF ALNUS GLUTINOSA (L.) VILL.

SUMMARY Analysis of root nodules of Alnus glutinosa sampled in a natural alder vegetation revealed two types, one containing clumps of endophytic spores and one without spores. The distribution pattern of both types, but especially the latter, showed considerable clustering. Both types of nodule sometimes occurred on the same tree, and some information on the dynamics of the distribution pattern was obtained from age analysis of nodules of both types. Since cross‐inoculation experiments showed a genetic basis for the presence or absence of spores in root nodules, the existence of two endophytic strains differing in their ability to produce spores seems likely. Different concentrations of ammonium and nitrate incorporated into the nutrient solution of nodulated test plants had only a slight influence on spore‐clump abundance in root nodules, and complete inversion of the spore type was not observed. Attempts to find additional strain differences led to the comparison of isomers of diamino‐pimelic acid (DAP) from the cell wall of both types of endophyte. DAP isomer analysis of genetically more distinct ‘ Alnus ‐type’ root nodules, i.e. those of Myrica gale and Hippophae rhamnoides , were used as reference. No additional taxonomic differences were found between the nodule types in Alnus glutinosa , but spore‐free nodules of Hippophae rhamnoides and Myrica gale differed with respect to the DAP isomer composition. Test plants of Alnus glutinosa provided artificially with either spore‐rich or spore‐free nodules did not differ significantly in acetylene reduction rate.