Effects of a Vesicular-Arbuscular Mycorrhizal Fungus on Nitrate Reductase and Nitrogenase Activities in Nodulating and Non-Nodulating Soybeans

Abstract

The root systems of soybeans can be infected by vesicular-arbuscular (VA) endomycorrhizal fungi and by N-fixing bacteria. Both microorganisms are beneficial to the plant and the possibility of a direct interaction between the fungus and bacterium was considered. Nodulating and non-nodulating soybean isolines were treated with various combinations of Glomus fasciculatus, Rhizobium japonicum and phosphate fertilizer in an attempt to evaluate this interaction. Dually-infected nodulating soybean plants showed increases in total dry weight and nodule dry weight, as well as higher levels of nitrogenase and nitrate reductase activities over singly or noninfected plants. When P was substituted for mycorrhizal infection, similar growth and enzyme activity increases were observed. This suggests that VA endomycorrhizal fungi, which can assist legumes in the uptake of P, do not interact directly with N-fixing bacteria.