Commonly, in greenhouses in northern Europ, as long as the houses are not venting, CO2 is added to bring ambient concentrations up to 0.1%. The difficulty with CO2 enricchment,especially in warm-winterareas,is that plants are more responsive in absolute terms to elevated CO2, when light is high (Hurd and Thornley, 1974) and temperatures (Mortensen, 1987) are high.Unfortunatery,when light and temperatures are high, and plants are potentially most responsive to high CO2, greenhouse temperatures often rise so much that the greenhouse must be vented, precluding further enrichment.This often means that only a few hours of enrichment are possible each day in the spring or in areas with mild winters (Peet and Willits, 1987).