PHOTOASSIMILATE PARTITIONING IN NODULATED SOYBEAN III - THE EFFECT OFCHANGES 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 e xposure of the nodulated root to Ar:O-2 for over 3 h did not affect th e partitioning of photoassimilate to the nodule. In contrast, anaerobi c conditions (100% N-2) resulted in a temporary cessation or marked sl owing of carbon import into the nodule and root organs, reflecting an O-2 requirement of the unloading process. Carbon accumulation by the n odulated 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 fr om 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 nodul e were not disturbed by changes in nodule N metabolism, and therefore that control of assimilate partitioning to the nodule is not influence d by a turgor mechanism involving a balance between the primary export and import solutes. However, photoassimilate import was matched to th e respiratory demand of the root system.