The Flux of14C-Labelled Photosynthate through Soyabean Root Nodules during N2Fixation

Abstract

Experiments are described which examine the flux of photosynthetic assimilates from leaves to nodules of soyabean during N2 fixation. The first part, where the respiratory efflux of 14C02 by nodules was used as a means of assessing the import of labelled photosynthate from leaves, shows that most 14C02 loss from nodulated roots is due to the metabolic activity of nodules. Much less photosynthate was imported by nodules if the metabolic activity associated with N2 fixation was inhibited by low 02 concentration. The second part describes the chemical fate of current photosynthate as it is utilized by nodules. Labelled material was detected in nodules within c.15 min of supplying 14C02 to the leaf. This rose to a maximum at c.70 min before declining by 85% within the following 4 h. Most (80%) 14carbon imported by nodules was either lost as respiratory 14C02 or re-exported as products of N2 fixation. Ten per cent of imported carbon was found as structural material and 10% as starch. Of the 14C soluble in ethanol, most was found in the neutral fraction (80% declining to 50% as sucrose) with smaller amounts as amino acids, organic acids (each category rising from 10% to 20%) and phosphate esters ( < 5%). Comparison of the distribution of 14C among amino acids, amides and ureides in the nodules with that of xylem exudates indicated that selected compounds were exported from nodules. The 14C data indicate that c.80% of the nitrogen exported from nodules was in the form of ureides (mainly allantoic acid) and only 10-12% as amides.