Design and performance of a measuring system for CO2 exchange of a greenhouse crop at different light levels

At low light levels measured and in a dynamic greenhouse climate, crop photosynthesis does not match prediction by photosynthesis models. In order to address this problem, a greenhouse scale crop photosynthesis measuring system was designed to generate data that can be used to validate crop photosynthesis models. The system is based on net CO2 exchange of a crop and was tested with a full scale cut chrysanthemum crop including the canopy bed structure. The system was used to measure crop photosynthesis at different CO2 concentration levels (350 and 970 μmol mol−1), different temperatures, various low-photosynthetic photon-flux densities (PPFD; 85 and 160 μmol m−2 s−1) and natural light conditions (0-600 μmol m−2 s−1). The system consisted of two identical closed greenhouses (44 m2 floor area each) that were sealed against air infiltration and tested on the occurrence of air leakage and unwanted CO2 sources with the help of tracer gases. The CO2 was supplied by computer-controlled flow proportionally to the difference between set point and measured concentration. An error analysis was applied successfully to find the required system characteristics needed to meet the requirement of a maximum system error of 5% for net CO2 exchange measurements even at low photosynthesis levels. The description of the arrangement, calibration and testing of this system supports the design process of accurate large-scale crop photosynthesis measuring systems for use in crop photosynthesis research.