INTRAGENERIC VARIATION IN NODULATION OF ALNUS - CONSEQUENCES FOR QUANTIFYING FRANKIA NODULATION UNITS IN SOIL

Methods based on amplification and recognition of Frankia-specific DNA have recently been developed to quantify Frankia in soil. But measuring numbers of infective Frankia in soil still requires bioassays involving nodulation of appropriate host plants. We compared three alder species, Alnus glutinosa (L.) Gaertn., A. incana (L.) Moench and A. rubra Bong. grown in two nutrient solutions under controlled conditions in a climate chamber. Plants were either kept in the same cultivation tube or, to remove most of the inoculum, they were moved into a new tube 2 weeks after inoculation. Soil dilution series prepared from a forest soil served as inocula. Roots were inspected for nodules weekly for 8 weeks and the number of nodulation units (NU) g-1 soil was calculated according to the most probable number (MPN) method and according to a nodulation capacity method. Alnus glutinosa required the longest time to nodulate. The number of NU detected was in the order A. rubra > A. incana > A. glutinosa in each of the nutrient solutions used. Eight weeks after inoculation, number of NU ranged from 5 to 400 as determined by the MPN-method, and from 10 to 380 as determined by the nodulation capacity method in the different species, solution and tube combinations used. The bioassay seems appropriate for comparisons between soils when the same plant genotypes are used under the same conditions, but otherwise comparisons should be made with caution. The MPN method and the nodulation capacity method essentially gave the same results. The MPN method is somewhat less labour intensive.