Ectomycorrhizal Effects on Nodulation, Nitrogen Fixation and Growth of Alnus Glutinosa as Affected by Glyphosate

Abstract

Actinorhizal plants such as Alnus and Casuarina offer good potential for increasing the productivity of impoverished soils because of their ability to fix atmospheric nitrogen in a symbiotic relationship with the actinomycete, Frankia. The occurrence of mycorrhizae on actinorhizal plants is seen as a hypersymbiosis which could be of added nutritional significance. To test this hypothesis, the effects of an ectomycorrhizal fungus, Paxillus involutus, on Alnus glutinosa seedlings nodulated with Frankia were investigated in the greenhouse and in the field both in the absence and presence of the herbicide, glyphosate. Under greenhouse conditions, simultaneous inoculation with Frankia and Paxillus involutus stimulated biomass production, nodulation and acetylene reduction in the absence of glyphosate. Glyphosate at 1.5 1 ha⁻¹ stimulated biomass production and acetylene reduction, but tempered the influence of the tripartite association. The 9 1 ha⁻¹ glyphosate treatment reduced shoot growth, stimulated root growth, and further reduced the impact of the tripartite association. In the field, at the end of one season, about 45 percent of the seedlings failed to survive in the non-herbicide treated plots, whereas all survived the 1.5 1 ha⁻¹ and 9 1 ha⁻¹ treatments. Trends were similar to those observed in the greenhouse. In in vitro studies, pure cultures of Frankia and Paxillus involutus were treated with different concentrations of glyphosate. At concentrations of up to 10 ppm, neither of the two organisms were affected by glyphosate but their growth was significantly reduced by the 50–1000 ppm treatments. Our results indicate significant enhancement in the performance of Alnus glutinosa seedlings when inoculated with both Frankia and Paxillus involutus. Recommended application rates of glyphosate may not have any negative effects on growth.