Fig 5. Chloroplast size and C₄ photosynthesis.

(a) and (b): The photon cost and the net assimilation rate as functions of the chloroplast radius and PEPC-to-RubisCO carboxylation capacity ratio, for the default parameter choice (Table 1). Level-lines are in white. The blue lines in (b) mark the light-utilisation thresholds (in mol m^-3s^-1). The carboxylation capacity ratio is used instead of the PEPC concentration to quantify the C₄ cycle activity because the stromal volume per cell-surface area changes with chloroplast radius. The vertical dotted line marks the chloroplast size used as default in other figures. (c): Dependence of the photon cost on the PEPC-vs-Rubisco carboxylation capacity ratio for several evenly-spaced chloroplast sizes (marked with arrows in (a) and (b)). (d): The corresponding dependence of the assimilation rate. The lines turn dashed (dotted) where the required light-harvesting capacity exceeds 40 mol m^-3s^-1 (80 mol m^-3s^-1).