Unravelling the CO diffusion pathway in C...
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Unravelling the CO 2 diffusion pathway in C 3 plants Herman Berghuijs 1,2 , Xinyou Yin 1 , Bart Nicolaï 2 , Paul Struik 1 1. Wageningen University and Research Centre, Centre for Crop Systems Analysis, Droevendaalsesteeg 1, 6700 PB Wageningen, The Netherlands; 2. Katholieke Universiteit Leuven, Flanders Centre of Postharvest technology/BIOSYSTS-MeBioS, Willem de Croylaan 42, B-3001 Leuven, Belgium. Introduction Photosynthesis can be defined as the conversion of solar energy into chemical energy. In green plants, this applies to the conversion of CO 2 into organic compounds. The energy stored in these compounds can later be used to supply energy to run physical and chemical processes in plant cells. Since photosynthesis allows crops to maintain themselves and to grow , it is of great importance for agriculture to understand this process. The efficiency of CO 2 transport from the atmosphere to the sites where CO 2 is fixed depends on various CO 2 sources (normal respiration, photorespiration), CO 2 sinks (RuBP carboxylation), and physical intercellular (figure 1) and intracellular barriers (figure 2) for CO 2 diffusion along the diffusion pathway in mesophyll cells to the sites of fixation. Commonly, these constraints are lumped in a single, apparent parameter, called mesophyll conductance. However, this approach does not provide a mechanistic explanation on how various structures and processes affect CO 2 transport in the mesophyll. Therefore, we moved beyond these resistance models. In this study, we investigated how the location of photorespiration and normal respiration affects the leaf photosynthetic efficiency in C 3 plants. Computational Methods Figure 3 shows the computational domain. The geometry consists of a gas phase (intercellular air) and a liquid phase (mesophyll cell) compartment. The liquid phase compartment is further subdivided into loose chloroplasts, surrounded by a cytosol layer. This cytosol layer is further subdivided into outer cytosol (facing gas phase), cytosol gap (between two chloroplasts), and inner cytosol. We used the COMSOL physics interface “Transport of Diluted Species” to solve a reaction-diffusion model over this geometry. Sources for CO 2 consisted of normal respiration and photorespiration in either the inner cytosol, the outer cytosol or both of these compartments. The CO 2 sinks consisted of RuBP carboxylation in the chloroplasts. Results: We solved the model for three scenarios (release of (photo)respired CO 2 in either the inner cytosol, the outer cytosol or both, figure 4). We up-scaled the local rates of CO 2 production and consumption to calculate net CO 2 assimilation rate of the whole leaf. The simulated net CO 2 assimilation rates described measured rates reasonably well (figure 5). We suspected that differences between the scenarios can be explained by the re-assimilation of (photo)respired CO 2 Conclusions: We developed a model to simulate CO 2 transport and assimilation in leaves by reaction-diffusion equations, rather than resistance models. Reaction-diffusion models are more flexible, which gives us additional opportunities to study the relationship between leaf anatomy and photosynthesis. As a case study, we used the model to show that the localization of (photo)respiration does affect the net CO 2 assimilation rate. In future research, we will use our model to quantify the fraction of (photo)respired CO 2 that is re-assimilated to see to what extend re- assimilation affects the photosynthetic efficiency of a leaf. We surmise that our model contributes to a better understanding of C 3 photosynthesis and, ultimately, to more efficient crop production. Figure 2: Once inside the mesophyll cell, CO 2 has to cross a number of barriers in mesophyll cells to reach the sites of fixation. Figure 4. CO 2 partial pressure within mesophyll cells at ambient CO 2 levels and saturating light. The color bar displays partial pressures (Pa). Photorespired CO 2 is produced in either the inner cytosol (left, the outer cytosol (right) or both compartments (middle). Figure 5: CO 2 response curve (left) and light response curve (right). (Photo)respiration takes either place in the inner cytosol (green dashed line), the outer cytosol (purple dotted line) or both these compartments (solid red line) Figure 1. CO 2 can only enter the mesophyll cells at the exposed mesophyll surface. Figure 3: Geometrical model for mesophyll cell tissue Net CO 2 assimilation rate (μmol m -2 s -1 ) CO 2 partial pressure (Pa) Irradiance (μmol m -2 s -1 ) A1 Excerpt from the Proceedings of the 2015 COMSOL Conference in Grenoble
Transcript of Unravelling the CO diffusion pathway in C...
Unravelling the CO2 diffusion pathway in C3 plantsHerman Berghuijs1,2, Xinyou Yin1, Bart Nicolaï2, Paul Struik1
1. Wageningen University and Research Centre, Centre for Crop Systems Analysis, Droevendaalsesteeg 1, 6700 PB Wageningen, The Netherlands;
2. Katholieke Universiteit Leuven, Flanders Centre of Postharvest technology/BIOSYSTS-MeBioS, Willem de Croylaan 42, B-3001 Leuven, Belgium.
IntroductionPhotosynthesis can be defined as the conversion of solar energy into chemical energy. In green plants, this applies to the conversion of CO2into organic compounds. The energy stored in these compounds can later be used to supply energy to run physical and chemical processes in plant cells. Since photosynthesis allows crops to maintain themselves and to grow , it is of great importance for agriculture to understand this process. The efficiency of CO2 transport from the atmosphere to the sites where CO2 is fixed depends on various CO2 sources (normal respiration, photorespiration), CO2 sinks (RuBP carboxylation), and physical intercellular (figure 1) and intracellular barriers (figure 2) for CO2 diffusion along the diffusion pathway in mesophyll cells to the sites of fixation. Commonly, these constraints are lumped in a single, apparent parameter, called mesophyll conductance. However, this approach does not provide a mechanistic explanation on how various structures and processes affect CO2 transport in the mesophyll. Therefore, we moved beyond these resistance models. In this study, we investigated how the location of photorespiration and normal respiration affects the leaf photosynthetic efficiency in C3 plants.
Computational MethodsFigure 3 shows the computational domain. The geometry consists of a gasphase (intercellular air) and a liquid phase (mesophyll cell) compartment.The liquid phase compartment is further subdivided into loose chloroplasts,surrounded by a cytosol layer. This cytosol layer is further subdivided intoouter cytosol (facing gas phase), cytosol gap (between two chloroplasts),and inner cytosol. We used the COMSOL physics interface “Transport ofDiluted Species” to solve a reaction-diffusion model over this geometry.Sources for CO2 consisted of normal respiration and photorespiration ineither the inner cytosol, the outer cytosol or both of these compartments.The CO2 sinks consisted of RuBP carboxylation in the chloroplasts.
Results:We solved the model for three scenarios (release of(photo)respired CO2 in either the inner cytosol, the outer cytosol orboth, figure 4). We up-scaled the local rates of CO2 production andconsumption to calculate net CO2 assimilation rate of the wholeleaf. The simulated net CO2 assimilation rates described measuredrates reasonably well (figure 5). We suspected that differencesbetween the scenarios can be explained by the re-assimilation of(photo)respired CO2
Conclusions: We developed a model to simulate CO2 transport and assimilation in leaves by reaction-diffusion equations, rather than resistance models. Reaction-diffusion models are more flexible, which gives us additional opportunities to study the relationship between leaf anatomy and photosynthesis. As a case study, we used the model to show that the localization of (photo)respiration does affect the net CO2 assimilation rate. In future research, we will use our model to quantify the fraction of (photo)respired CO2 that is re-assimilated to see to what extend re-assimilation affects the photosynthetic efficiency of a leaf. We surmise that our model contributes to a better understanding of C3photosynthesis and, ultimately, to more efficient crop production.
Figure 2: Once inside the mesophyll cell, CO2
has to cross a number of barriers in
mesophyll cells to reach the sites of fixation.
Figure 4. CO2 partial pressure within mesophyll cells at ambient CO2 levels and saturating
light. The color bar displays partial pressures (Pa). Photorespired CO2 is produced in
either the inner cytosol (left, the outer cytosol (right) or both compartments (middle).
Figure 5: CO2 response curve (left) and light response curve (right). (Photo)respiration takes
either place in the inner cytosol (green dashed line), the outer cytosol (purple dotted line) or
both these compartments (solid red line)
Figure 1. CO2 can only enter the mesophyll cells at
the exposed mesophyll surface.
Figure 3: Geometrical model for mesophyll cell tissue
Net
CO
2as
sim
ilati
on
rat
e (μ
mo
l m
-2s-1
)
CO2 partial pressure (Pa) Irradiance (μmol m-2 s-1)
A1
Excerpt from the Proceedings of the 2015 COMSOL Conference in Grenoble
