Design and performance of a measuring system for CO 2 exchange of a greenhouse crop at different light levels

Abstract

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 CO 2 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 CO 2 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 m 2 floor area each) that were sealed against air infiltration and tested on the occurrence of air leakage and unwanted CO 2 sources with the help of tracer gases. The CO 2 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 CO 2 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.