In situ analysis of introduced Frankia populations in root nodules of Alnus glutinosa grown under different water availability

The competitive ability for nodulation of Alnus glutinosa (L.) Gaertn. plan ts by Frankia strains inoculated into soil with indigenous Frankia populati ons was studied at two matric potentials representing "dry" (-0.016 MPa) an d "wet" (-0.001 MPa) conditions. In pots kept at a matric potential of -0.0 01 MPa, nitrate concentrations decreased within 3 weeks more than 10-fold t o an average of approx. 200 mu mol.(g soil dry wt.)(-1). After 4 months, ni trate concentrations in these pots were 16 +/- 16 and 277 +/- 328 mu mol.(g soil dry wt.)(-1) (mean +/- SD) for non-inoculated and inoculated soils, r espectively. At a matric potential of -0.016 MPa, nitrate concentrations fo r non-inoculated and inoculated soils were 687 +/- 491 and 1796 +/- 1746 mu mol.(g soil dry wt.)(-1), respectively Inoculated plants always grew bette r than their non-inoculated counterparts. The largest plants were found on inoculated soil at a matric potential of -0.001 MPa, whereas the smallest p lants were found on non-inoculated soil at the same matric potential. At a matric potential of -0.016 MPa, plants grown on non-inoculated soil were no t as tall as those grown on inoculated soil and were slightly chlorotic, in dicating that the high level of nitrate in the soil was not providing optim al plant growth conditions. The number of nodule lobes formed on plants was not significantly different among treatments, though size and weight of lo bes differed. Nodules from plants grown on inoculated soils always harbored vesicle-producing Frankia populations, while nodules from plants grown on non-inoculated soils harbored only Frankia with distorted vesicles or no Fr ankia at all. All strains in nodules from plants grown on non-inoculated so il were of Alnus host infection group IIIa. Nodules from plants grown on so il inoculated with strains ArI3 (group IIIa), Ag45/Mut15 (group IV), and Ag B1.9 (group I) were also infected by Frankia strain Ag45/Mut15. These resul ts indicate that by inoculation, Frankia populations could be established u nder conditions that did not favour vesicle formation in root nodules forme d by the indigenous Frankia population. Inoculation even in soils with high nitrogen content might therefore be an appropriate strategy to enhance pla nt growth.