Determining photosynthetic limitations under saturated and non-saturated light conditions

Photosynthesis is limited by CO2 if chloroplastic CO2 concentration (Cc) is lower than the intersection point (Cctr) in the Farquhar-von Caemmerer-Berry (FvCB) model. This range is defined as light-saturated conditions. If Cc > Cctr, photosynthesis is limited mainly by light (non-light-saturated conditions). Therefore, the relationship between Cc and Cctr can reveal whether photosynthesis is limited by light or CO2. In this work, a process to identify the photosynthetic limitation was illustrated based on the relationship between Cc and Cctr. We described Cctr as a function of light and biochemical parameters (Vcmax) in the FvCB model and simulated the Cctr-light response with different sets of FvCB parameters. Results of simulations indicated that photosynthesis is limited by light when Vcmax is above 120 (µmol CO2 m⁻²s⁻¹), and a decrease of Vcmax increases the limitations from CO2 diffusion pathways. We show data that photosynthesis of young cucumber leaves (10–14 days) was limited by light, and limitation caused by CO2 started on day 14 after leaf appearance.