Energy requirement for symbiotic nitrogen fixation

Abstract

COMPARING the energy required by legumes for symbiotic nitrogen fixation with that of assimilation of nitrate, Gibson¹ concluded the costs to be about the same. About 15% of net photosynthetic production by the plant may be used in meeting its nitrogen requirements. If energy for the fixation of nitrogen symbiotically and that for the assimilation of NH₄⁺ or NO₃⁻ from the soil solution are both provided by the chemical products of photosynthesis, then the CO₂ respired in supplying that energy must contribute to the total efflux of CO₂ from the plant in the dark. McCree² and Thornley³ have discussed methods of partitioning the dark CO₂ efflux into a growth (synthesis) and a maintenance component. Both nitrogen fixation and nitrogen assimilation can be expected to contribute to the CO₂ flux associated with synthesis. If Gibson is correct, nodulated plants using only symbiotically fixed nitrogen should have the same growth coefficient as non-nodulated plants supplied with exogenous mineral nitrogen when grown in the same conditions. I have examined this hypothesis using a modification of the method used by McCree with Trifolium subterraneum L. cultivar Woogenellup as test material.