GROWTH INCREMENT OF ALNUS-GLUTINOSA UPON DUAL INOCULATION WITH EFFECTIVE AND INEFFECTIVE FRANKIA STRAINS

Investigations on the ecological function of ineffective Frankia strains and their behaviour in competition with effective Frankia strains indicated an enhanced plant growth upon dual inoculation with increasing amounts of effective (i.e. N2-fixing)Frankia strains and simultaneous inoculation with a constant amount of an ineffective Frankia strain. Enhanced plant growth was measured as increase in plant height and total dry weight at constant shoot/root ratio. The stimulating effect of the ineffective strain was independent of the plant clone and was obtained with both Alnus glutinosa clones W I and B II, which were resistant and susceptable, respectively, to the ineffective strain. Stimulation was also independent of the nodulation conditions. Short-term studies (7 weeks) under axenic conditions and greenhouse experiments during 3 months showed comparable results, not only in plant growth but also in nodule formation. Increment in plant growth was not necessarily correlated to higher nodule formation with the effective Frankia strains. © 1990 Kluwer Academic Publishers.