Model-based control of CO2 concentration in greenhouses at ambient levels increases cucumber yield

In the nearly airtight energy-saving greenhouses of the Northern countries, CO2 concentrations may drop to very low levels in autumn, winter and spring due to CO2 uptake by the plants, resulting in considerable yield decreases. Exhaust gas from the heating system is therefore added to the greenhouse air when natural gas is burned. In other cases, however, CO2 is quite expensive and should be applied very efficiently. For these purposes, strategies were developed and tested to maintain the CO2 concentration inside the greenhouse at the same level as found outside. In two experiments on cucumber, CO2 was added to the greenhouse air according to the uptake by the plants, which was estimated by two simple photosynthesis models. Thus, CO2 concentration in the greenhouses was maintained at around the outside CO2 concentration of about 380 μmol mol−1, while it dropped significantly in the greenhouses not supplied with CO2. The CO2 supply strategies resulted in an increase in yield of about 35% compared to the unsupplied standard, while the CO2 input was on average 400 g per 1 kg of yield increment. The differences between supplied and not supplied greenhouses in CO2 concentration and photosynthesis, and thus the CO2-supply efficiency, were maximal at moderate radiation and decreased with increasing outside air temperature due to required ventilation at high radiation and high outside air temperature.