Chlorophyll a Fluorescence Predicts Total Photosynthetic Electron Flow to CO2 or NO3−/NO2− under Transient Conditions

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Abstract
A model which predicts total photosynthetic electron flow from a linear regression of the relationship between corrected steady-state quantium yield and nonphotochemical quenching (E Weis, JA Berry [1987] Biochem Biophys Acta 894: 198-208) was formulated for N-limited cells of the green alga Selenastrum minutum. Unlike other models based on net CO2 fixation, our model is based on total photosynthetic electron flow measured as gross O2 evolution. This allowed for the prediction of total photosynthetic electron flow from water to both CO2 fixation and NO3-/NO2- reduction. The linear regression equation predicting electron flow is of the form: J = I.cntdot.Qq[0.4777-0.3282 QNP] (where J = gross photosynthetic electron flow, I = incident PAR, Qq = photochemical quenching, QNP = nonphotochemical quenching). During steady-state photosynthesis, over a range of irradiance, the model predicted a photosynthetic light saturation curve which was well correlated with that observed. Although developed under steady-state conditions, the model was tested during non-steady-state photosynthesis induced by transient nitrogen assimilation. The model predicted transient rates of gross O2 evolution which were in excellent agreement with the rates observed under a variety of conditions regardless of whether CO2 or NO3-/NO2- served as the physiological electron acceptor. The fluorescence transients resulting from ammonium and nitrate assimilation are discussed with respect to metabolic demands for reductant and ATP.