Growth responses of indigenous Frankia populations to edaphic factors in actinorhizal rhizospheres

Quantitative PCR (qPCR) was used to follow population dynamics of indigenous Frankia populations in bulk soil, in leaf-litter-amended soil and in the rhizosphere of Alnus glutinosa or Casuarina equisetifolia at 2 matric potentials representing dry and wet conditions in soil microcosms. Analyses revealed between 10- and 100-fold increases of Frankia populations within the incubation period of 12 weeks independent of treatment. Numbers were generally higher under dry conditions and in the rhizosphere, with that of C. equisetifolia supporting highest abundance. Frankiae detected at any time and treatment belonged to either subgroup I of the Alnus host infection group or the Elaeagnus host infection group, with those of the Elaeagnus host infection group largely representing the genus in all samples under wet conditions, and in bulk and leaf litter amended soil under dry conditions. Subgroup I of the Alnus host infection group was most prominent in the rhizosphere of both plant species where it represented up to 95% of the genus with higher percentages in that of C. equisetifolia.