PHOTOASSIMILATE PARTITIONING IN NODULATED SOYBEAN I - C-11 METHODOLOGY

An established method using C-11 for the in vivo measurement of photoa ssimilate partitioning within intact plants was applied to the charact erization of partitioning of photoassimilate to soybean nodules, The m ethod describes partitioning in terms of the magnitude and stability o f partitioning flows, i.e. sink 'activity' and 'priority', and the tra nsit time of tracer to a given sink. Leaflet labelling with (CO2)-C-11 was recommended over whole shoot labelling to allow information on tr ansport properties of the shoot to be acquired. The assumptions inhere nt in the method, that labelled and unlabelled photoassimilate in pass age within the stem to the root system were well mixed and that tracer flow is unidirectional between source and sink (nodule), were validat ed. Tracer was re-exported from root to shoot, but this re-export proc ess did not invalidate the assumption of unidirectional flow because t he 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 trac er entering the root system was respired (c. 10% within 200 min), main ly by nodules (37% of tracer entering a nodule cluster was respired wi thin 200 min). Therefore root-respired tracer was trapped and attribut ed to the nodule in partitioning calculations. A case study is present ed using the method to assess changes in partitioning to nodules follo wing treatment of the root system with nitrate, highlighting the limit ation to this method of ontogenetic changes in the pattern of export f rom the load leaflet.