Fig 7. C₄ photosynthesis at limited CO₂ in the IAS.

(a): The net assimilation rate as a function of the IAS CO₂ pressure and PEPC concentration in the cytoplasm, for the default parameter choice (Table 1; specifically, the envelope permeability is 600 μm/s). No light utilisation cap is imposed, but the utilisation thresholds are marked in green. The vertical dotted line marks the CO₂ pressure used as default in other figures. (b): Assimilation rate vs PEPC-to-Rubisco carboxylation capacity ratio for several CO₂ pressures (marked with arrows in a). (c): The relative gain in the assimilation rate (compared to C₃ photosynthesis) at the PEPC activity levels where the light usage reaches 40 mol m^-3s^-1 and 80 mol m^-3s^-1 (dotted green lines) and the maximal assimilation gains when the corresponding light limits are imposed (blue lines). (d): the photon costs corresponding to assimilation gains in (c); the black line marks the cost of C₃ photosynthesis (below 50 μbar C₃ photosynthesis cannot reach the compensation point), the blue and green lines as in (c). (e): the respective PEPC concentrations at which the optimal gains are achieved in (c) and (d).