Abstract
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.
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