Article Figures & Data
Figures
- Figure 1.
Modeling Water-Saving Flux Modes in an Environment-Coupled Model of Leaf Metabolism.
(Top left) A diel (24-h) leaf model was constructed by concatenating copies of a core model of plant metabolism (Shameer et al., 2018). The individual models were connected via linker reactions that allowed the transfer of storage compounds in the vacuole and the plastid between successive models. Light uptake was constrained by the diel light curve. The day:night ratios of phloem output and maintenance were set to 3:1 for each hour of the diel cycle, and N uptake was constrained to a ratio of 3:2 based on previous estimates (Cheung et al., 2014).
(Top right) The effect of T and RH on stomatal water loss was modeled by a simplified gas-diffusion equation. T and RH data determined the relationship between CO2 uptake and water loss. The four stomata pores illustrate the water-saving mechanism of nocturnal CO2 uptake. While respiration occurs in all four scenarios, dominant carbon fixation leads to a net uptake of CO2 during the day in C3 plants and at night in CAM plants.
(Bottom) Combining metabolic and gas-exchange models allowed us to study the trade-off between productivity and water loss as competing objectives on a Pareto frontier (i.e., the line that denotes combinations of productivity and water-loss values where one objective cannot be improved without compromising the other) and revealed alternative water-saving carbon-fixation mechanisms.
- Figure 2.
Metabolic Fluxes and Water Loss for Different Modeling Scenarios.
(A) Example of the T and RH data used throughout the simulations.
(B) Metabolic flux profiles in a C3 leaf optimized toward phloem output (100% phloem output). The diel light curve is indicated in yellow and peaks at a maximum intensity of 250 μmol m−2 s−1.
(C) Pareto analysis of phloem output versus water loss in a C3 leaf (top) and a CAM leaf (bottom). The CAM leaf enabled a better trade-off between the two competing objectives.
(D) Metabolic flux profiles for a C3 leaf (left), a CAM leaf (middle), and a leaf with unlimited vacuolar storage capacity for different Pareto steps (right; 80, 60, 40, and 20% of the maximum phloem output [shown for a C3 leaf in (B)]). Note the different flux scales on the right plot axis for the C3 and CAM leaves and the leaf with unlimited storage capacity.
- Figure 3.
Different Flux Distributions in a Water-Saving CAM Leaf at 80% Productivity with (Model ICDHrev) and without (Model ICDHirrev) Reversible Mitochondrial ICDH.
(A) CO2 budget for the two models reveals different CO2 turnover fluxes over the course of the day. Shown are all reactions with flux > 0.5 μmol m−2 s−1 for at least one time point. I to IV indicate the four phases of the CAM cycle. The values for the cumulative contribution are given next to the reaction name for either model ICDHrev or both models (model ICDHrev and model ICDHirrev). c, cytosolic; m, mitochondrial; p, plastidial.
(B) Significant linker fluxes for both models. Model ICDHrev accumulated (iso)citrate as carboxylic acid and additionally Pro and Asp. Model ICDHirrev accumulated both malate and (iso)citrate but no amino acids. Starch levels in model ICDHirrev were almost threefold higher than in model ICDHrev.
- Figure 4.
Major Flux Routes Involved in the CAM-Like Temporally Separated Carbon-Fixation Mechanism in a Water-Saving CAM Leaf at 80% Productivity.
Analysis with reversible mitochondrial ICDH (model ICDHrev; [A]) and analysis without reversible mitochondrial ICDH (model ICDHirrev; [B]) are shown. The two models used different pathways to fix and release CO2. A-CoA, acetyl-CoA; CS, citrate synthase; (Iso-)Cit, (iso)citrate; Mal, malate; P5C, 1-pyrroline-5-carboxylic acid. The gray area in (B) highlights those reactions that are active in phase II. Roman numerals indicate the sequences of reactions described in the text.
- Figure 5.
Water Saving of a Leaf with CAM-Like Nocturnal Carbon Fixation by ICDH at 80% Productivity for Different Environments.
(Top) Overview of the environmental conditions analyzed. The Tmax-RHmin space was analyzed for different combinations of light intensity and daylength. Conditions A, B, and C are shown below. Conditions with an x are shown in Supplemental Results, Section 2.3.
(Bottom) Shown are heat maps for the absolute water saving of model ICDHrev with respect to the C3 scenario (orange), the absolute water-saving contribution of ICDH (i.e., the difference in water saving between model ICDHrev and model ICDHirrev; blue), and the relative water-saving contribution of ICDH with respect to the C3 scenario (green; note the different scaling of the color bar) for combinations of Tmax and RHmin. Also shown are representative fluxes involved in carbon fixation and the shared nocturnal carbon fixation by PEPC and ICDH for the different environments at Tmax = 30°C and RHmin = 0.4 (right column).
Tables
Phase Description I Stomata open during the dark period and nocturnal CO2 assimilation by PEPC II Transition phase between dark and light periods with a peak in CO2 uptake and fixation of CO2 by Rubisco III Stomata closed during the light period and fixation of CO2 that is released by decarboxylation of carboxylic acids IV Transition phase between light and dark periods with direct Rubisco-mediated fixation of CO2
Supplemental Data
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