Using a reaction‐diffusion model to estimate day respiration and reassimilation of (photo)respiredCO2in leaves

peer-reviewedMethods using gas exchange measurements to estimate respiration in the light (day respiration Rd) make implicit assumptions about reassimilation of (photo)respired CO2; 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 Rd 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 Rd. Estimates of Rd 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 CO2, and thus underestimated Rd 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 Rd, enlightens the dependence of Rd estimates on reassimilation and clarifies (dis)advantages of existing methods.KU Leuve

CO2 concentration profiles for (photo)respiratory CO2 release in the cytosol gaps.

<p>CO₂ partial pressure profile within half the computational domain at <i>C</i>_i = 25 Pa levels and saturating light (<i>I</i>_inc = 1500 μmol m^-2 s^-1). The color bar displays CO₂ partial pressures (Pa). (Photo)respired CO₂ is produced in the cytosol gaps.</p

Overview of surface to volume ratios and parameterizations.

<p>Overview of surface to volume ratios and parameterizations.</p

Estimated values of parameters of the FvCB model and their standard error for each scenario for (photo)respired CO₂ release (it takes place in the inner cytosol, or in the outer cytosol, or in the cytosol gaps).

<p>Estimated values of parameters of the FvCB model and their standard error for each scenario for (photo)respired CO₂ release (it takes place in the inner cytosol, or in the outer cytosol, or in the cytosol gaps).</p

Values of parameters and variable input for the model.

<p>Values of parameters and variable input for the model.</p

Differences between predicted and measured net CO₂ assimilation rates.

<p>Differences between the predicted net CO₂ assimilation rate and the average measured net CO₂ assimilation rate for different ambient CO₂ partial pressures (A) and irradiances (B). In both figures, it is assumed in models that (photo)respired CO₂ is released in the inner cytosol, or in the outer cytosol, or in the gaps between the inner and the outer cytosol.</p

Schematic representation for the different types of models for the resistance of CO₂ transport in the mesophyll.

<p>The <i>C</i>_i, <i>C</i>_cyt and <i>C</i>_<i>c</i> represent the CO₂ partial pressure in the intercellular air space, the cytosol and the CO₂ binding cites of Rubisco in the chloroplast stroma, respectively. In the model in Panel A), all structural barriers of the mesophyll for CO₂ transport are lumped in a single resistance, called mesophyll resistance <i>r</i>_m. The intracellular sinks and sources for CO₂ are assumed to be at the same location, i.e. in the chloroplast stroma. The net flux of CO₂ from the chloroplast stroma equals the net CO₂ assimilation rate <i>A</i>_N. In the model in Panel B) an additional cytosol compartment is added. The resistance components for CO₂ transport between the air spaces and this compartment is the sum of resistances of the of the cell wall (<i>r</i>_w), of the plasma membrane (<i>r</i>_mem) and half the resistance of the cytosol (<i>r</i>_cyt). The resistance components for CO₂ transport between the cytosol compartment and Rubisco consists of the resistance of the chloroplast envelope (<i>r</i>_env) and the CO₂ diffusion path in the stroma (<i>r</i>_str). The rate of carboxylation by Rubisco (<i>W</i>) in the chloroplast stroma is the sink for CO₂. The intracellular sources of CO₂ are the rate of respiration in the light (<i>R</i>_d) and the rate of photorespiration (<i>R</i>_p). Both sources are located in the cytosol. This model places the source for CO₂ between two cytosol resistance components and can, therefore, only be used to study C₃ leaf photosynthesis if the mitochondrion are located in the outer cytosol layer. The model in Panel c) is largely similar to the model in Panel C), with the exception that the resistance of the cytosol is negligible. Consequently, the CO₂ partial pressure is equal in any part of the cytosol and <i>C</i>_cyt is not affected by the location of the mitochondrion relative to the chloroplast. Therefore, this model cannot be used to study how the position of the mitochondria relative to the chloroplast affects C₃ leaf photosynthesis.</p

CO2 concentration profiles for (photo)respiratory CO2 release in the inner cytosol.

<p>CO₂ partial pressure profile within half the computational domain at <i>C</i>_i = 25 Pa levels and saturating light (<i>I</i>_inc = 1500 μmol m^-2 s^-1). The color bar displays CO₂ partial pressures (Pa). (Photo)respired CO₂ is produced in the inner cytosol.</p

Schematic drawing of the computational domain and its position relative to the intercellular air space and the vacuole.

<p>Schematic drawing of the computational domain and its position relative to the intercellular air space and the vacuole.</p