Photoassimilate partitioning in nodulated soybean II. The effect of changes in photoassimilate availability shows that nodule permeability to gases is not linked to the supply of solutes or water

It is concluded that the permeability of the soybean nodule to gases is notlinked to the supply of solutes or water via the phloem to the nodule.Nodule respiration and nitrogenase activity were less affected by dielvariation and shading treatments than partitioning to the nodule, asassessed using a non-invasive ¹¹C-based technique.Thus C import to the nodule was not matched to C requirement by the nodule.Transit times of tracer to, and within, the nodulated root increased underconditions of reduced photosynthetic rate. The increase in transit time wasinterpreted as a reduction in the flux of phloem sap. Thus the fluxes ofboth water and C to the nodule decreased following a reduction inphotosynthetic rate. The change in partitioning of recent photosynthate tosoybean roots and nodules in response to changes in photoassimilateavailability was also used to assess the 'priority' of these sinks.Partitioning from the leaf to the root system was greatly decreased whenphotoassimilate availability was limited, indicating that root systempriority is lower than that of the shoot, as reported for other systems.However, partitioning of tracer arriving in the root system between thenodulated and non-nodulated zones of the root was not affected by changesin photoassimilate availability, as caused by diel change, shading, orsteaming of branch roots. Thus although nodules are sinks of high sink'activity', they have 'priority' equal to that of other root sinks. It issuggested that there are similar phloem unloading kinetics, despite thevery different metabolic destiny of the carbohydrate within the twoorgans.