Using combined measurements of gas exchange and chlorophyll fluorescence to estimate parameters of a biochemical C3 photosynthesis model: a critical appraisal and a new integrated approach applied to leaves in a wheat (Triticum aestivum) canopy

We appraised the literature and described an approach to estimate the parameters of the Farquhar, von Caemmerer and Berry model using measured CO2 assimilation rate (A) and photosystem II (PSII) electron transport efficiency (Phi(2)). The approach uses curve fitting to data of A and Phi(2) at various levels of incident irradiance (I-inc), intercellular CO2 (C-i) and O-2. Estimated parameters include day respiration (R-d), conversion efficiency of I-inc into linear electron transport of PSII under limiting light [kappa(2(LL))], electron transport capacity (J(max)), curvature factor (theta) for the non-rectangular hyperbolic response of electron flux to I-inc, ribulose 1.5-bisphosphate carboxylase/oxygenase (Rubisco) CO2/O-2 specificity (S-c/o), Rubisco carboxylation capacity (V-cmax), rate of triose phosphate utilization (T-p) and mesophyll conductance (g(m)). The method is used to analyse combined gas exchange and chlorophyll fluorescence measurements on leaves of various ages and positions in wheat plants grown at two nitrogen levels. Estimated S-c/o (25 degrees C) was 3.13 mbar mu bar(-1); R-d was lower than respiration in the dark; J(max) was lower and theta was higher at 2% than at 21% O-2; kappa(2(LL)), V-cmax, J(max) and T-p correlated to leaf nitrogen content; and g(m) decreased with increasing C-i and with decreasing I-inc. Based on the parameter estimates, we surmised that there was some alternative electron transport.