The Glycine‐Glomus‐Bradyhizobium symbiosis. VIII. Phosphorususe efficiency of CO2 and N2 fixation in mycorrhizal soybean

Abstract

Soybean [Glycine max (L.) Merr. cv. Hobbit] plants nodulated by Bradyhizobium japonicum strain USDA 110 were grown in pot cultures in severely P‐ and N‐deficient soil and either colonized by the vesicular‐arbuscular mycorrhizal (VAM) fungus Glomus mosseae (Nicol. & Gerd.) Gerd. and Trappe or fertilized with a high (HP) or low (LP) level of KH2PO4 (0.6 or 0.3 mM, respectively), After 7 weeks of growth, nodule and chloroplast activities (C2H2 reduction and CO2 exchange rate) were determined. Photosynthetic P‐use efficiency of CO2 fixation was significantly higher in VAM than in HP plants, while that of nitrogenase activity was lower. The LP plants were intermediate in both respects. The ratio of nodule to chloroplast activity [mol C2H2 reduced (mol CO2 fixed)−1] was highest in HP and lowest in VAM plants. Root colonization by the VAM fungus significantly increased nodule number and dry weight and reduced nodule specific mass and activity in comparison to HP plants. In spite of lower nodule activity, VAM plants were significantly larger and had higher N concentrations than the HP plants. The results suggest nonnutritional. VAM‐elicited and host‐mediated effects on the symbiotic functions of the legume association.