The Glycine‐Glomus‐Bradyrhizobiun 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 were inoculated with a HUP− strain of Bradyrhizobium japonicum (Nitragin 61A118) and either colonized by the vesicular‐arbuscular mycorrhizal (VAM) fungus Glomus mosseae (Nicol & Gerd.) Gerd. and Trappe or fertilized with KH 2 PO 4 (nonVAM). They were grown for 50 days in a growth chamber and harvested over a 4‐day drought period during which available soil water decreased to 0. Nodule P concentrations and P‐use efficiency declined linearly with soil and root water content during the harvest period in both VAM and nonVAM plants. Nitrogenase activity, estimated from H 2 evolution and C 2 H 2 reduction data, was also a linear function of declining nodule P concentrations and CO 2 ‐exchange rates and showed simular patterns in both treatments. Hydrogen evolution and the relative efficiency of N 2 fixation, on the other hand, reacted differently to increasing drought in VAM and nonVAM plants. Differences in the responses of nodule activity in VAM and nonVAM plants to drought are interpreted in terms of demand for nodule P and carbohydrates and of the effects of dehydration on O 2 diffusion through nodule tissue.