Simulation of Water Use Efficiency for Maize At Elevated CO.

Maize uses the C4 pathway for photosynthesis and therefore there is little evidence that maize dry matter and yield is increased when grown at high levels of CO2 (~800 ppm). Stomatal closure does occur at elevated CO2, however resulting in decreased water uptake per unit CO2 assimilated. The objective of this study was to evaluate the ability of the maize model MaizSim to simulate the effects of water stress and CO2 on water use and photosynthesis. MaizSim uses a Farquhar type photosynthesis model coupled a Ball-Berry stomatal control model. Non-linear beta functions are used to estimate the effects of temperature on growth and development processes. Data to test the model come from experiments in outdoor, sunlit growth chambers at the USDA-ARS  Beltsville Agricultural Research Center. The eight treatments comprised two levels of carbon dioxide concentrations (400 and 800 ppm) and four levels of water stress (well-watered control, mild, moderate, and severe). The water stress treatments were applied at both CO2 levels. Water contents were monitored hourly by a Time Domain Reflectometry (TDR) system. The model simulated higher water contents at the same time after applying water stress at the high CO2 treatment than for the low CO2 treatment as was found in the measured data. Problems simulating intense water stress will be addressed.