Journal of Experimental Botany, Vol 49, 1805-1815, Copyright © 1998 by Oxford University Press
M Thorpe, K Walsh and P Minchin
An established method using 11C for the in
vivo measurement of photoassimilate partitioning within intact
plants was applied to the characterization of partitioning of
photoassimilate to soybean nodules. The method describes partitioning in
terms of the magnitude and stability of partitioning flows, i.e. sink
'activity' and 'priority', and the transit time of tracer to a given sink.
Leaflet labelling with 11CO2 was recommended over
whole shoot labelling to allow information on transport properties of the
shoot to be acquired. The assumptions inherent in the method, that labelled
and unlabelled photoassimilate in passage within the stem to the root
system were well mixed and that tracer flow is unidirectional between
source and sink (nodule), were validated. Tracer was re-exported from root
to shoot, but this re-export process did not invalidate the assumption of
unidirectional flow because the transit time of the re-export process was
long relative to the half-life of the isotope. The transit time of tracer
between entry to, and respiration from, the root system was also long
(>60 min) relative to the half-life of the isotope. However, a
significant fraction of tracer entering the root system was respired (c.
10% within 200 min), mainly by nodules (37% of tracer entering a nodule
cluster was respired with 200 min). Therefore root-respired tracer was
trapped and attributed to the nodule in partitioning calculations. A case
study is presented using the method to assess changes in partitioning to
nodules following treatment of the root system with nitrate, highlighting
the limitation to this method of ontogenetic changes in the pattern of
export from the load leaflet.Key words: C-11, nitrogen
fixation, nodule, photoassimilate, partitioning, soybean.
ARTICLES
Photoassimilate partitioning in nodulated soybean I. 11C methodology
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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