The effectivity of Frankia for nodulation and nitrogen fixation in Alnus rubra and A. glutinosa

The effectivity of nodulation of Alnus rubra Bong, by Frankia isolates from A. rubra and Alnus glutinosa (L.) Gaertn. in Northern Britain was compared with strains from The Netherlands and North America, using plants grown in combined nitrogen-free conditions. All strains gave rise to spore (-) nodules, even when isolated from nodules from sites known to contain spore (+) nodules. Nodules of all plants evolved little hydrogen, probably due to the presence of an efficient uptake hydrogenase in the microsymbkmts. Nodule weight as a percentage of whole plant weight was higher for nodules of low specific activity (N fixed per unit weight nodules), attaining a maximum of 5.1% of plant dry weight in the least effective of the heterologous associations of A. glutinosa Frankia with A. rubra . The range of variation in nodule specific activity was much greater in heterologous than homologous associations, but nodules of high specific activity were found in both associations. However, plants that fixed most N during the growth period were not those with nodules of highest specific activity. The most effective associations were homologous symbioses, which combined good nodule growth per plant with satisfactory specific activity, fixing N at rates which would support superior plant growth under the prevailing growth conditions. Preliminary field experiments suggest that the most effective of the A. rubra isolates is suitable for use as an inoculant in nurseries. Strains isolated from A. glutinosa were more effective and showed a different order of effectivity in homologous symbioses compared with their association with A. rubra . An A. glutinosa strain was isolated, which stimulated satisfactory nodule growth and gave good nodule specific activity in both A. rubra and A. gtutinosa .