Greenhouse cooling in summer in Finland -: Preliminary results of climate control and plant response

Despite the country's Northern location, greenhouse production in Finland suffers from high temperatures in summertime and the plant production could be improved by cooling greenhouses. To investigate the effects of a novel direct airwater heat exchanger cooling system on greenhouse climate conditions and plant growth, a trial was carried out with cucumber (Cucumis sativus L. cv. Cumuli) cultivated from June to September. In the cooling compartment, the cooling system was capable of maintaining air temperature below the maximum set point without any need for shading or ventilation, when the global radiation was less than 600 to 700 Wm(-2). In the control compartment, temperature was controlled by means of ventilation alone. The cooling system allowed the vents to be kept closed and a high CO2 concentration maintained, except for a few midday hours on the hottest summer days, while in the control compartment the vents were open for at least 11 hours daily. During the experiment, the mean CO2 concentration and vapour pressure deficit (VPD) were 910 ppm and 0.495 kPa in cooling and 565 ppm and 0.706 kPa in control, respectively. The cooling system increased cucumber yield both in qantity and quality. Total cumulative yield over 12 harvesting weeks was 44.4 kg m(2) in cooling and 35.8 kg m(-2) in control. Cooling also affected plant structure and dry matter partitioning: the dry weight of fruits, leaves, total above-ground biomass, and harvest index increased, whereas the leaf weight ratio decreased.