Altered Plant Structure and Greater Yield of Cucumber Grown at Elevated CO2 in a Semi-Closed Greenhouse

High temperature decreases summertime yields of greenhouse vegetables in Finland. A cooling and drying system of greenhouse air (described in Sarkka et al., 2006) was tested on cucumber (Cucumis sativus L. 'Cumuli'). Reduced need for ventilation in the cooled compartment (semi-closed greenhouse) allowed maintaining of constantly higher CO2 concentration than in the conventional roof-ventilated control compartment. In the cooled compartment, the first class yield was 41%, greater than in the control compartment. In addition, cooling improved the quality of yield, as the proportion of the first class fruits increased from 79% in the control compartment to 88% in the cooled compartment. Higher harvest index in the cooling showed that a greater proportion of assimilates was allocated to the fruits than in the control. Gas exchange and chlorophyll fluorescence measurements made on cucumber leaves revealed that the response of photosynthesis to CO2 concentration and to irradiance was not changed during growth at higher CO2 concentration, i.e. there were no signs of down-regulation of photosynthetic capacity. Growth at higher CO2 concentration led, however, to reduced N concentration and higher C:N-ratio of leaves. Because of higher CO2 concentration, photosynthesis operated at a higher level in the cooled compartment than in the control most of the time. It is concluded that the increase in yield obtained from the cooled compartment was brought about by an increased net photosynthesis because of higher CO2 concentration and in addition, by an increase in the allocation of biomass to the fruits. The relatively small differences in mean temperature and vapour pressure deficit between the two compartments had probably smaller effect on plant growth and physiology than CO2 concentration had.