The Kok effect revisited

The Kok effect refers to the abrupt decrease around the light compensation point in the slope of net photosynthetic rate vs irradiance. Arguably, this switch arises from light inhibition of respiration, allowing the Kok method to estimate day respiration (R-d). Recent analysis suggests that increasing proportions of photorespiration (quantified as Gamma*/C-c, the ratio of CO2 compensation point Gamma* to chloroplast CO2 concentration, C-c) with irradiance explain much of the Kok effect. Also, the Kok method has been modified to account for the decrease in PSII photochemical efficiency (phi(2)) with irradiance. Using a model that illustrates how varying R-d, Gamma*/C-c, phi(2) and proportions of alternative electron transport could engender the Kok effect, we quantified the contribution of these parameters to the Kok effect measured in sunflower across various O-2 and CO2 concentrations and various temperatures. Overall, the decreasing phi(2) with irradiance explained c. 12%, and the varying Gamma*/C-c explained c. 25%, of the Kok effect. Maximum real light inhibition of R-d was much lower than the inhibition derived from the Kok method, but still increased with photorespiration. Photorespiration had a dual contribution to the Kok effect, one via the varying Gamma*/C-c and the other via its participation in light inhibition of R-d.