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 not linked to the supply of solutes or water via the phloem to the nodule. Nodule respiration and nitrogenase activity were less affected by diel variation and shading treatments than partitioning to the nodule, as assessed using a non-invasive 11 C-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 under conditions of reduced photosynthetic rate. The increase in transit time was interpreted as a reduction in the flux of phloem sap. Thus the fluxes of both water and C to the nodule decreased following a reduction in photosynthetic rate. The change in partitioning of recent photosynthate to soybean roots and nodules in response to changes in photoassimilate availability was also used to assess the 'priority' of these sinks. Partitioning from the leaf to the root system was greatly decreased when photoassimilate availability was limited, indicating that root system priority is lower than that of the shoot, as reported for other systems. However, partitioning of tracer arriving in the root system between the nodulated and non-nodulated zones of the root was not affected by changes in photoassimilate availability, as caused by diel change, shading, or steaming of branch roots. Thus although nodules are sinks of high sink 'activity', they have 'priority' equal to that of other root sinks. It is suggested that there are similar phloem unloading kinetics, despite the very different metabolic destiny of the carbohydrate within the two organs.