Different apparent CO2 compensation points in nitrate- and ammonium-grown Phaseolus vulgaris and the relationship to non-photorespiratory CO2 evolution

The classical theory of the relationship between gas fluxes and photosynthetic electron fluxes was extended by two additional terms: J_L describing flux to electron sinks other than the Calvin cycle, and R_L accounting for light-induced changes in non-photorespiratory CO₂ evolution. R_L comprises two main components, R_r resulting from light-induced decrease in tricarboxylic acid activity, and R_S related to extra CO₂ evolution resulting from citrate-to-2-oxoglutarate conversion for N-assimilation in NO₃^– grown leaves. This extended theory was applied to two experiments. First, A–C_i curves (dependence of CO₂ flux on stomatal CO₂ concentration) revealed a higher apparent CO₂ compensation point (Γ*_app) in NO₃^–-grown plants than in NH₄⁺-grown plants. Secondly, photosynthetic electron fluxes at different light intensities were determined by means of the Genty parameter of chlorophyll fluorescence and compared with those calculated from measured CO₂ uptake. Curve-fitting based on the extended theory provided a coincidence of these two measurements and resulted in higher R_S in NO₃^–-grown than in NH₄⁺-grown plants. This difference in R_S (about 15% of the CO₂ flux bound by carboxylation) is the same as that obtained from the analysis of Γ*_app. Further, the analysis suggests that J_L related to the extra electron flux used for N-assimilation in NO₃^–-grown plants is diverted to other sinks in NH₄⁺-grown plants. SHAM decreased photosynthetic electron flow and O₂ evolution in NH₄⁺-grown plants, antimycin A in NO₃^–-grown plants. The effect of oligomycin was small. The results are discussed in terms of different mechanisms of chloroplast/mitochondrion interaction in NO₃^–- and NH₄⁺-grown plants, their effects on non-photorespiratory CO₂ evolution and on Γ*_app.