DYNAMIC MODEL FOR GREENHOUSE CROP PHOTOSYNTHESIS: VALIDATION BY MEASUREMENTS AND APPLICATION FOR CO2 OPTIMIZATION

Measurements on photosynthesis were performed in fullgrown cucumber crops in three experimental greenhouse compartments (9.6 x 6 m) in autumn 1987 and in two compartments (12.8 x 15 m) in spring 1988. The employed method is based on measurements of the carbon dioxide concentration and calculation of the carbon dioxide balance of the greenhouse air, combined with on-line measurement of the ventilation rate with a tracer gas N2O. Environmental data, measured in the experiments, were given as input data into a model. The simulated rates of photosynthesis generally showed a strong similarity with the measured photosynthetic rates.

Optimization of the CO2 concentration was tested in two experiments with cucumber. Costs and predicted yield of CO2 enrichment were calculated. Economic data and on-line measured radiation, wind speed and window aperture were used as input data. The CO2 concentration which came out to be economically optimum, was passed as set poin to a CO2 control programme.

With optimization, CO2 is supplied selectively: at more ventilation the calculated optimal CO2 set point is lower and at better light conditions the set point is higher. In both experiments optimization resulted in a high production of cucumber and a relatively low expenditure of CO2, as compared to common CO2 strategies.

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