Changes in the chloroplastic CO₂ concentration explain much of the observed Kok effect: a model

Mitochondrial respiration often appears to be inhibited in the light when compared with measurements in the dark. This inhibition is inferred from the response of the net CO₂ assimilation rate (A) to absorbed irradiance (I), changing slope around the light compensation point (I_c). We suggest a model that provides a plausible mechanistic explanation of this ‘Kok effect’. The model uses the mathematical description of photosynthesis developed by Farquhar, von Caemmerer and Berry; it involves no inhibition of respiration rate in the light. We also describe a fitting technique for quantifying the Kok effect at low I. Changes in the chloroplastic CO₂ partial pressure (C_c) can explain curvature of A vs I, its diminution in C₄ plants and at low oxygen concentrations or high carbon dioxide concentrations in C₃ plants, and effects of dark respiration rate and of temperature. It also explains the apparent inhibition of respiration in the light as inferred by the Laisk approach. While there are probably other sources of curvature in A vs I, variation in C_c can largely explain the curvature at low irradiance, and suggests that interpretation of day respiration compared with dark respiration of leaves on the basis of the Kok effect needs reassessment.