FRANKIA IN THE RHIZOSPHERE OF NONHOST PLANTS - A COMPARISON WITH ROOT-ASSOCIATED N2-FIXING ENTEROBACTER, KLEBSIELLA AND PSEUDOMONAS

Bacterial growth in the rhizosphere and resulting changes in plant gro wth parameters were studied in small aseptic seedlings of birch (Betul a pendula and B. pubescens) and grasses (Poa pratensis and Festuca rub ra). The seedlings were inoculated with three Frankia strains (Ai1a an d Ag5b isolated from native Alnus root nodules and Ai17 from a root no dule induced by soil originating from a Betula pendula stand), and thr ee associative N2-fixing bacteria (Enterobacter agglomerans, Klebsiell a pneumoniae and Pseudomonas sp., isolated from grass roots). Microsco pic observations showed that all the Frankia strains were able to colo nize and grow on the root surface of the plants tested without additio n of an exogenous carbon source. No net growth of the associative N2-f ixers was observed in the rhizosphere, although inoculum viable counts were maintained over the experimental period. Changes in both the bio mass and morphology of plant seedlings in response to bacterial inocul ation were recorded, which were more dependent on the plant species th an on the bacterial strain.