A mechanistic model for submerged aquatic macrophyte photosynthesis: Hydrilla in ambient and elevated CO2

There are significant knowledge gaps about the responses of submerged aquatic macrophytes to CO2 enrichment and global warming. A mechanistic steady-state photosynthesis model for submerged aquatic macrophytes was developed to provide an analysis tool to investigate the responses of plant photosynthesis to CO2, temperature and light. The model was based upon a general simplified scheme for inorganic carbon assimilation of submerged aquatic macrophytes which integrated the knowledge about aquatic plant photosynthesis from previous research, mainly on Hydrilla. The model includes: (1) diffusion and/or active transfer of inorganic carbon (CO2 and/or HCO3-) in the bathing medium into the leaf mesophyll and cytosol; (2) diffusion and/or 'pumping' of CO2 through the PEPcase-related C-4 pathway into the chloroplast; (3) inter-conversions between CO2 and HCO3- inside cells; (4) photosynthetic carbon reduction cycle (PCR) in the chloroplast. In the model, the PCR processes in the chloroplast were described using the widely accepted C-3 photosynthesis model. The activity of the C-4 cycle was related to environmental CO2 'stress'. In this way, the model can simulate the shift between C-3-like and C-4-like photosynthesis under different environmental conditions. The model was validated using gas exchange data from Hydrilla plants grown in ambient and elevated CO2. The model predicted quite well photosynthetic responses to incident PAR, temperature and ambient CO2 for both ambient and elevated atmospheric CO2 treatments. Model predictions agreed well with measured Hydrilla gas exchange data. The simulated and measured CO compensation points of Hydrilla leaf photosynthesis were about 100 ppm. The light compensation point of photosynthesis was about 25 mu mol m(-2)s(-1) (PAR), and photosynthesis rate was saturated at about 100 mu mol m(-2)s(-1) (PAR). Higher pH slightly increased photosynthesis rates at ambient CO2 (similar to 350 ppm). There was no significant acclimation of Hydrilla photosynthesis to elevated CO2 within the experimental period. Simulated CO2 compensation point decreased with increasing activity of C-4-cycle processes.