Using multirate rapid A/Ci curves as a tool to explore new questions in the photosynthetic physiology of plants.

Steady-state photosynthetic CO2 responses (A/Ci curves) are used to assess environmental responses of photosynthetic traits and to predict future vegetative carbon uptake through modeling. The recent development of rapid A/Ci curves (RACiRs) permits faster assessment of these traits by continuously changing [CO2 ] around the leaf, and may reveal additional photosynthetic properties beyond what is practical or possible with steady-state methods. Gas exchange necessarily incorporates photosynthesis and (photo)respiration. Each process was expected to respond on different timescales due to differences in metabolite compartmentation, biochemistry and diffusive pathways. We hypothesized that metabolic lags in photorespiration relative to photosynthesis/respiration and CO2 diffusional limitations can be detected by varying the rate of change in [CO2 ] during RACiR assays. We tested these hypotheses through modeling and experiments at ambient and 2% oxygen. Our data show that photorespiratory delays cause offsets in predicted CO2 compensation points that are dependent on the rate of change in [CO2 ]. Diffusional limitations may reduce the rate of change in chloroplastic [CO2 ], causing a reduction in apparent RACiR slopes under high CO2 ramp rates. Multirate RACiRs may prove useful in assessing diffusional limitations to gas exchange and photorespiratory rates.