However, all of these methods have severe limitations in terms of providing accurate measurements of the performance of a canopy in a large greenhouse. The methods employed will usually change the climate conditions in relation to those found in the greenhouse, and it therefore becomes difficult to interpret the measurements. In spite of these limitations, however, the different methods have provided much valuable knowledge on optimal climate conditions for different crops. So far, steady-state photosynthetic responses to environmental factors have been extensively studied, but knowledge of the effect of continuous variation in climate on photosynthesis remains scarce (Marcelis et al., 2014).With the introduction of semi-closed greenhouses, new possibilities have opened with respect to climate control and energy efficiency of greenhouses (Dannehl et al., 2014;Marcelis et al., 2014;Qian, 2017). With this type of greenhouse, the air exchange rate between the greenhouse and the outside environment can be measured, since the ventilation is controlled by fans in the greenhouse walls and not by roof vents. This gives rise to new possibilities with respect to continuously recording crop photosynthesis. Photosynthesis can be accurately measured by precisely measuring airflow, and the inlet and outlet CO 2 concentration in the air. Our objective was therefore to evaluate the potential of measuring canopy photosynthesis in a large semi-closed greenhouse. Although numerous semi-closed greenhouses have been built worldwide, this method of continuously monitoring crop respons-