Photoassimilate partitioning in nodulated soybean: I.11C methodology

• Published in: Journal of experimental botany 1998-11, Vol.49 (328), p.1805-1815
• Main Authors: Thorpe, M.R., Walsh, K.B., Minchin, P.E.H.
• Format: Article
• Language: English
• Subjects: Internodes; Leaves; Nodules; Phloem; Plant roots; Plants; Regulation of Growth, Development and Whole Organism Physiology; Respiration; Root systems; Soybeans; Tracer bullets
• ISSN: 0022-0957; 1460-2431

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 within 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.