The Glycine–Glomus–Bradyrhizobium symbiosis, XI. Nodule gas exchange and efficiency as a function of soil and root water status in mycorrhizal soybean

Soybean [Glycine max (L.) Merr, cv, Hobbit) plants nodulated by Bradyrhizobium 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 KH,PO4 (0,6 or 0,3 mM. respectively). After 7 weeks of growth, nodule and ehloroplast activities (C.H, reduction and CO, exchange rate) were determined, Photosynthetic P-use efficiencies were calculated as mg CO, reduced per g of leaf or nodule P per plant. The P-use efficiency of CO; 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 CH, reduced (mol CO, fixed) ' | 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 aetivity in comparison to HP plants. In spite of lower nodule activity, VAM plants were significantly larger and had higher N eoneentrations than the HP plants. The results suggest nonnutritional, VAM-elicited and host-mediated effects on the symbiotic functions of the legume association