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Transpiration rates of potted spray carnation plants Cerise Royalette decreased about 0.04% per vpm CO2 between ambient atmospheric CO2 concentration and 1500 vpm CO2 at several light flux densities and leaf temperatures. Measurements of daily water losses of potted spray carnation plants placed under high solar radiation conditions in two minigreenhouses with 300 and 5000 vpm CO2 demonstrated that elevated CO2 concentrations reduced water losses by 20–30%. The effect of the increase in global CO2 concentration on stomatal closure was calculated to have decreased the yearly transpiration rate of an outdoor crop by 1.6% in this century and is expected to cause a decrease of 10% within the next 50 years if all other factors remain unchanged. From a model of CO2 uptake of carnation plants it was calculated that the expected water use efficiency (net photosynthesis rate/transpiration rate) will increase by about 40–50% over the next 50 years due to the expected increase in global CO2 concentration.