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Atmospheric carbon dioxide concentration [CO2] will increase in the future and will affect global climate and ecosystem productivity. Crop models used in past assessments of climate change effect on ecosystem productivity have not been adequately tested for the ability to simulate ecosystem responses to [CO2]. Our objective was to evaluate the ability of the default CROPGRO–Soybean model to predict the responses of net leaf photosynthesis (A) and canopy photosynthesis (Acan) to photosynthetic photon flux (PPF) at different [CO2]. We also compared the default leaf photosynthesis equations in CROPGRO with the full Farquhar equations for ability to predict the response of A to [CO2]. Simulated and observed A and Acan were light saturated at 800 mmol m s PPF at ambient [CO2] but did not light saturate at PPF .1100 mmol m s at elevated [CO2]. Observed and simulated A responded asymptotically to increasing intercellular [CO2]. The CROPGRO default photosynthesis equations and the Farquhar equations simulated A equally well at all [CO2]. Doubled [CO2] increased simulated A by 52% and Acan by 42%; these values are close to the increases of 39 to 48% for A and 59% for Acan reported in the literature. Root mean square errors for simulated A and Acan were low, and Willmott’s index of agreement ranged from 0.86 to 0.99, confirming that the CROPGRO model with default photosynthesis equations can be used to evaluate potential effects of [CO2] on soybean photosynthesis and productivity. INCREASE OF carbon dioxide concentration [CO2] in the atmosphere will change future global climate via increased temperature and altered precipitation patterns, which affect ecosystem productivity. Carbon dioxide concentration steadily increased from 280 mmol CO2 mol during the preindustrial period (around the year 1750) to about 375 mmol CO2 mol in the year 2000. Projections of [CO2] in the year 2100 will range from 540 to 970 mmol CO2 mol of air, depending on emission scenarios (Houghton et al., 2001). The projected rise in [CO2] will influence plant processes at various hierarchic levels from short-term effects on net leaf photosynthesis (A) to long-term effects on primary productivity of terrestrial ecosystems. Approaches to predict the effects of climate change on primary productivity often involve the use of mechanistic ecosystem simulation models coupled to climate change predictions from atmospheric general circulation models. Crop simulation models have been used to assess the productivity responses of various crops to anticipated future changes in [CO2] and temperature (Adams et al., 1990; Wang et al., 1992; Easterling et al., 1993; Wall et al., 1994; Matthews et al., 1995). Using the SOYGRO model V5.42 (Jones et al., 1989) with empiric adjustments to simulate effects of [CO2] on biomass accumulation, Peart et al. (1989) evaluated the impact of [CO2] on potential soybean production. The CROPGRO model (Boote et al., 1998) is more mechanistic than the SOYGRO model. Leaf photosynthesis simulation in CROPGRO is an adaptation of the equations of Farquhar et al. (1980) in an hourly leaf-level to canopy assimilation scaling approach with hedge-row light interception. The goal of this paper is to evaluate the CROPGROmodel for its ability to simulate soybean leaf and canopy assimilation response to [CO2]. The leaf photosynthesis model of Farquhar et al. (1980) has been widely used to simulate response of A to [CO2]. This model assumes that A is limited by the slower of two processes, namely the maximum rate of Rubisco-catalyzed carboxylation (Rubisco-limited) and the Ribulose 1,5 bisphosphate (RuBP) regeneration rate controlled by electron transport rate (RuBP-limited). The Farquhar model requires Rubisco enzyme kinetic parameters. Some of the kinetic parameters of the Rubisco enzyme, such as Michaelis constants for oxygen (Ko) and for CO2 (Kc) andCO2 compensation point in the absence of dark respiration (G*), are constant across species of C3 plants. However, other required parameters that depend on the Rubisco enzyme concentration, such as maximum RuBP-saturated rate of carboxylation (Vc,max), maximum RuBP-saturated rate of oxygenation (Vo,max), and dark respiration rate (Rd), vary even within individual plants because they are conditioned by growing conditions. This makes application of the Farquhar model in mechanistic crop simulation models difficult. The CROPGRO model uses a modified Farquhar and von Caemmerer (1982) approach in which only the RuBP-limited part is used to simulate responses of A to [CO2]. Unlike the leaf photosynthesis model of Farquhar et al. (1980) and Farquhar and von Caemmerer (1982), CROPGRO’s default leaf photosynthesis equations do not require Kc, Ko, Vc,max, or the maximum rate of electron transport (Jmax) to simulate A. Rather, the approach in CROPGRO defines light-saturated leaf photosynthetic rate (Amax) at reference values of CO2, O2, temperature, reference specific leaf weight (SLWREF), and leaf nitrogen (N) concentration. It uses an Abbreviations: LAI, leaf area index; MSE, mean squared error; MSEs, systematic mean squared error; MSEu, unsystematic mean squared error; PPF, photosynthetic photon flux; QE, quantum efficiency; RuBP, ribulose 1,5 bisphosphate; RMSE, root mean squared error. G. Alagarswamy and K.J. Boote, Dep. of Agronomy, Univ. of Florida, Gainesville, FL 32611-0500; L.H. Allen, Jr., USDA-ARS and Dep. of Agronomy, Univ. of Florida, Gainesville, FL 32611-0965; and J.W. Jones, Dep. of Agricultural and Biological Engineering, Univ. of Florida, Gainesville, FL 32611-0570. Florida Agric. Exp. Stn. Journal Ser. no. R-10598. Received 6 Dec. 2004. *Corresponding author (ala-