Fitting light saturation curves measured using PAM fluorometry

R.J. Ritchie, School of Biological Sciences, The University of Sydney, NSW 2006, Australia.

Blue diode PAM (Pulse Amplitude Modulation) fluorometry was used to measure photosynthesis in Synechococcus (classical Cyanobacteria), Prochlorothrix (Prochlorophyta), Chlorella (Chlorophyta), Rhodomonas (Cryptophyte), Phaeodactylum (Bacillariophyta) and Acaryochloris (Chl d/a cyanobacteria). Effective quantum yield (F_v) vs. irradiance (E) curves could be described by a simple exponential decay function (F_v = F_vmaxe^-kE) although Log/Log transformation was sometimes found to be necessary to obtain the best fits. Photosynthesis (P) was measured as Electron Transport Rate (ETR) standardised on a chlorophyll basis. P vs. E curves were fitted to the waiting-in-line function (an equation of the form P = P_max·k·I·e^-kE) allowing 1/2 saturating and optimal irradiances to be estimated. At twice optimal light intensities there is 26.4% photoinhibition of P_max and is the irradiance at which all PSII would be "closed". The waiting-in-line model was found to be a very good descriptor of photosynthetic light saturation curves and superior to hyperbolic functions with an asymptotic saturation point (Michaelis-Menten, exponential saturation and hyperbolic tangent). The exponential constants (k) of the Y_v vs E and P vs. E curves should be equal because ETR is directly proportional to Y_v × E. Non-Photochemical Quenching (NPQ) in Synechococcus was not significantly different to zero but NPQ vs E curves for the other algae could be fitted to an exponential saturation model.