Development and validation of two crop photosynthesis monitoring systems to support greenhouse climate control

Over the last years, energy consumption in greenhouse horticulture has been reduced considerably by using (multiple) screens, controlled dehumidification and more efficient use of assimilation light. All these actions affect greenhouse climate and thereby plant processes. Measuring crop photosynthesis, a process that responds very rapidly to changes in climate, would be suited to monitor whether the climate is set according to the needs of the plant. Two monitoring systems have been developed, the crop photosynthesis monitor and the CropObserver. The crop photosynthesis monitor is a soft sensor that calculates the CO₂ uptake of the entire greenhouse, based on the mass balance of a greenhouse for CO₂. At known CO₂ supply rates, measuring the CO₂ concentrations inside and outside the greenhouse, and determining the ventilation loss allows the calculation of the crop photosynthesis. The CropObserver is a fluorescence sensor that provides laser light pulses from above in a surface of 3×3 m. Since the measurements are fast, the sensor provides a good image of the electron transport rate (ETR) of a large crop area in a short period of time. Evaluation of the sensors in a tomato cultivation showed that the data of the crop photosynthesis monitor matched the daily pattern of manual photosynthesis data up-scaled to crop photosynthesis by a crop growth model reasonably well. The CropObserver data were very comparable to local reference fluorescence measurements, and closely followed irradiance patterns over the day. Both systems have attracted the attention of Dutch growers, and will be further evaluated in commercial greenhouses.