Population size and diversity of Frankia in soils of Ceanothus velutinus and Douglas-fir stands

The influence of host plants on Frankia populations was investigated using soils from Ceanothus velutinus (Dougl.) and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stands. Population sizes of Ceanothus-infective Frankia in the soils were measured using plant bioassays with C. velutinus, C. sanguineus (Pursh), and C. integerrimus (H. & A.) as trap plants. The Frankia population in soil from the C. velutinus stand soil was about 10 times higher than that from the Douglas-fir stand. This result supports previous reports that, although the presence of host plants increases Frankia populations, Frankia persist without host plants. Nodulation capacities of the three trap plants were not significantly different. All nodules showed N2 fixation activity using the acetylene reduction assay. The diversity of Frankia that nodulated trap plants was examined using repetitive intergenic DNA and the polymerase chain reaction (rep-PCR). A newly designed, direct repeat sequence and a BOX sequence were used as rep-PCR primers. The results showed that infective Frankia in the two soils contained a common group of Frankia as well as some Frankia strains unique to each soil. The level of host specificity of the infective Frankia was low; however, one group of Frankia nodulated only C. integerrimus seedlings. Taken together, the results suggest that the higher populations in the soil from the C. velutinus stand may be due to preferential increases in particular groups of Frankia.