Using a reaction‐diffusion model to estimate day respiration and reassimilation of (photo)respired CO (2) in leaves

Methods using gas exchange measurements to estimate respiration in the light (day respiration [Formula: see text]) make implicit assumptions about reassimilation of (photo)respired CO (2); however, this reassimilation depends on the positions of mitochondria. We used a reaction‐diffusion model without making these assumptions to analyse datasets on gas exchange, chlorophyll fluorescence and anatomy for tomato leaves. We investigated how [Formula: see text] values obtained by the Kok and the Yin methods are affected by these assumptions and how those by the Laisk method are affected by the positions of mitochondria. The Kok method always underestimated [Formula: see text]. Estimates of [Formula: see text] by the Yin method and by the reaction‐diffusion model agreed only for nonphotorespiratory conditions. Both the Yin and Kok methods ignore reassimilation of (photo)respired CO (2), and thus underestimated [Formula: see text] for photorespiratory conditions, but this was less so in the Yin than in the Kok method. Estimates by the Laisk method were affected by assumed positions of mitochondria. It did not work if mitochondria were in the cytosol between the plasmamembrane and the chloroplast envelope. However, mitochondria were found to be most likely between the tonoplast and chloroplasts. Our reaction‐diffusion model effectively estimates [Formula: see text] , enlightens the dependence of [Formula: see text] estimates on reassimilation and clarifies (dis)advantages of existing methods.