1 National Aeronautics and Space Administration Specialized Center of Research and Training in Bioregenerative Life Support, Purdue University, 1165 Horticulture Building, West Lafayette, IN 47907-1165
An artificial neural network (NN) and a statistical regression model were developed to predict canopy photosynthetic rates (Pn) for `Waldman's Green' leaf lettuce (Latuca sativa L.). All data used to develop and test the models were collected for crop stands grown hydroponically and under controlled-environment conditions. In the NN and regression models, canopy Pn was predicted as a function of three independent variables: shootzone CO2 concentration (600 to 1500 mmol·mol-1), photosynthetic photon flux (PPF) (600 to 1100 μmol·m-2·s-1), and canopy age (10 to 20 days after planting). The models were used to determine the combinations of CO2 and PPF setpoints required each day to maintain maximum canopy Pn. The statistical model (a third-order polynomial) predicted Pn more accurately than the simple NN (a three-layer, fully connected net). Over an 11-day validation period, average percent difference between predicted and actual Pn was 12.3% and 24.6% for the statistical and NN models, respectively. Both models lost considerable accuracy when used to determine relatively long-range Pn predictions (≥6 days into the future).