Leaf photosynthetic and morphological responses to elevated CO2 concentration and altered fruit number in the semi-closed greenhouse

Semi-closed greenhouses have been developed to reduce the energy consumption in horticulture. In these greenhouses, CO2 concentration is higher than in the conventional modern greenhouses due to the reduction of window ventilation. Photosynthetic and morphological acclimation to elevated CO2 has been found in many plant species with feedback inhibition being the main mechanism to explain this. The aim of this study was to investigate the occurrence of photosynthetic and morphological acclimation to elevated CO2 concentration in the semi-closed greenhouse. Our hypothesis was that photosynthetic and morphological acclimation to elevated CO2 concentration only occurred in plants with low sink strength. Experiments were carried out with tomato plants with varying fruit loads in a semi-closed greenhouse and a conventional modern greenhouse. Our results showed that photosynthetic acclimation to elevated CO2 concentration only occurred when the number of fruits was considerably reduced. Elevated CO2 as well as fruit removal reduced specific leaf area. Reduction in photosynthesis rate was associated with, but not caused by reduced stomatal conductance. The increase of dry matter production in the semi-closed greenhouse was mainly explained by a higher CO2 concentration compared to the open greenhouse. We suggested that elevated CO2 concentrations in the semi-closed greenhouse do not cause feedback inhibition in high producing crops, because the plants have sufficient sink organs (fruits) to utilise the extra assimilate