  Recently, interest in crassulacean acid metabolism (CAM) photosynthesis has
risen and new, physiologically based CAM models have emerged. These models show
promise, yet unlike the more widely used physiological models of C3 and C4
photosynthesis, their complexity has thus far inhibited their adoption in the
general community. Indeed, most efforts to assess the potential of CAM still
rely on empirically based environmental productivity indices, which makes
uniform comparisons between CAM and non-CAM species difficult. In order to
represent C3, C4, and CAM photosynthesis in a consistent, physiologically based
manner, we introduce the Photo3 model. This work builds on a common
photosynthetic and hydraulic core and adds additional components to depict the
circadian rhythm of CAM photosynthesis and the carbon-concentrating mechanism
of C4 photosynthesis. This allows consistent comparisons of the three
photosynthetic types for the first time. It also allows the representation of
intermediate C3-CAM behavior through the adjustment of a single model
parameter. Model simulations of *Opuntia ficus-indica* (CAM), *Sorghum bicolor*
(C4), and *Triticum aestivum* (C3) capture the diurnal behavior of each species
as well as the cumulative effects of long-term water limitation. The results
show potential for use in understanding CAM productivity, ecology, and climate
feedbacks and in evaluating the tradeoffs between C3, C4, and CAM
photosynthesis.
