Interactions between Symbiotic Nitrogen Fixation, Combined-N Application, and Photosynthesis in Pisum sativum

The effect of photosynthetic photon flux density (PPFD) on nitrogen utilization was determined in peas (Pisum sativum L. cv. Alaska) inoculated with Rhizobium leguminosarum and treated with nutrient solutions containing no combined nitrogen, 16 mM NO₃^?, or 16 mM NH₄⁺. Plants were grown under controlled conditions at three PPFD values ranging from severely limiting to nearly saturating. Carboxylation efficiencies and CO₂-exchange rates were highest in the N₂-fixing plants and lowest in plants supplied with NH₄⁺, and they generally increased with increasing PPFD. Photoefficiencies increased with PPFD but did not differ appreciably with the form of nitrogen applied. Nitrogen fixation, calculated from C₂H₂-reduction and H₂-evolution data, was inhibited more by NH₄⁺ than by NO₃^?application. Inhibition was counteracted by increasing PPFD. Percentage nitrogen decreased with increasing PPFD in plants treated with combined nitrogen and increased in the plants dependent on N₂ fixation. The data reveal that photosynthetic efficiency and the capacity to fix N₂ in peas are functions of PPFD and the availability of combined nitrogen and that these two factors are interrelated.