Localization of (photo)respiration and CO₂ re-assimilation in tomato leaves investigated with a reaction-diffusion model

The rate of photosynthesis depends on the CO₂ partial pressure near Rubisco, C_c, which is commonly calculated by models using the overall mesophyll resistance. Such models do not explain the difference between the CO₂ level in the intercellular air space and C_c mechanistically. This problem can be overcome by reaction-diffusion models for CO₂ transport, production and fixation in leaves. However, most reaction-diffusion models are complex and unattractive for procedures that require a large number of runs, like parameter optimisation. This study provides a simpler reaction-diffusion model. It is parameterized by both leaf physiological and leaf anatomical data. The anatomical data consisted of the thickness of the cell wall, cytosol and stroma, and the area ratios of mesophyll exposed to the intercellular air space to leaf surfaces and exposed chloroplast to exposed mesophyll surfaces. The model was used directly to estimate photosynthetic parameters from a subset of the measured light and CO₂ response curves; the remaining data were used for validation. The model predicted light and CO₂ response curves reasonably well for 15 days old tomato (cv. Admiro) leaves, if (photo)respiratory CO₂ release was assumed to take place in the inner cytosol or in the gaps between the chloroplasts. The model was also used to calculate the fraction of CO₂ produced by (photo)respiration that is re-assimilated in the stroma, and this fraction ranged from 56 to 76%. In future research, the model should be further validated to better understand how the re-assimilation of (photo)respired CO₂ is affected by environmental conditions and physiological parameters.