Photoassimilate partitioning in nodulated soybean III. The effect of changes in nodule activity shows that carbon supply to the nodule is not linked to nodule nitrogen metabolism

The hypothesis that photoassimilate partitioning to the soybean nodule is controlled by the rate of N export (via an effect on the turgor of the unloading pathway) was tested. Cessation of N 2 fixation due to exposure of the nodulated root to Ar:O 2 for over 3 h did not affect the partitioning of photoassimilate to the nodule. In contrast, anaerobic conditions (100% N 2 ) resulted in a temporary cessation or marked slowing of carbon import into the nodule and root organs, reflecting an O 2 requirement of the unloading process. Carbon accumulation by the nodulated root was less affected by a rhizosphere treatment of 2% O 2 , although partitioning was decreased over a period of hours. Treatment with 100% O 2 also caused an immediate diversion of photoassimilate from the root to the shoot system, although the extent of this diversion was variable. Treatment with stepped increases in O 2 did not affect partitioning. It is concluded that the unloading kinetics of the nodule were not disturbed by changes in nodule N metabolism, and therefore that control of assimilate partitioning to the nodule is not influenced by a turgor mechanism involving a balance between the primary export and import solutes. However, photoassimilate import was matched to the respiratory demand of the root svstem.