Journal of Experimental Botany, Vol 49, 1817-1825, Copyright © 1998 by Oxford University Press
M Thorpe, K Walsh and P Minchin
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 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 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.Key words: 11C, nitrogen
fixation, legume, nodule, photoassimilate partitioning, sink, source,
soybean
ARTICLES
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
Horticulture and Food Research Institute, Ruakura Research Centre, Private Bag 3123, Hamilton, New Zealand; Department of Biology, Central Queensland University, Rockhampton 4702, Australia; Corresponding author e-mail: k.walsh@ccqu.edu.au
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