Integration of Carbon and Nitrogen Metabolism During Plant-Microbe Symbioses: Introductory Remarks

Abstract

The reduction of N2 to NH3 inorganically or in biological processes, requires the input of a large amount of energy which is needed to break the very stable N=N bond. The main source of reductant and energy for symbiotic nitrogen fixation in legumes is photosynthetically fixed carbon which is translocated into root nodules predominantly as sucrose. Numerous estimates have been made of the carbon cost of symbiotic nitrogen fixation, most of which are in the range between 5 and 18 g of carbon per g nitrogen fixed (eg Pate et al., 1981; Streeter, 1991; Vance and Heichel, 1991). In addition to providing energy and reductant for the nitrogenase reaction inside the bacteroids (equivalent to 16 ATP and 8 electrons per molecule of N2 reduced), carbon is utilised in the assimilation of fixed nitrogen and for nodule growth and maintenance. On the basis of a requirement of 12 g of carbon per g of nitrogen fixed, Streeter (1991) has calculated that approximately 1.5 tonnes of sucrose/ha would be consumed to support the seasonal nitrogen fixation of a typical crop of soybeans. As much as 50% of the daily photosynthetic product may be consumed in the nodules during the peak nitrogen fixing period.KeywordsNitrogen FixationRoot NoduleNitrogen MetabolismSymbiotic Nitrogen FixationCarbon CostThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.