  Chloroplasts regulate their growth to optimize photosynthesis. Quantitative
data shows that the ratio of total chloroplast area to mesophyll cell area is
constant across different cells within a single species, and also across
species. Wild-type chloroplasts exhibit little scatter around this trend;
highly irregularly-shaped mutant chloroplasts exhibit more scatter. Here we
propose a model motivated by a bacterial quorum-sensing model consisting of a
switch-like signalling network that turns off chloroplast growth. We calculated
the dependence of the location of the relevant saddle-node bifurcation on the
geometry of the chloroplasts. Our model exhibits a linear trend, with linearly
growing scatter dependent on chloroplast shape, consistent with the data. When
modelled chloroplasts are of a shape that grows with a constant area to volume
ratio (disks, cylinders) we find a linear trend with minimal scatter.
Chloroplasts with area and volume that do not grow proportionally (spheres)
exhibit a linear trend with additional scatter.
