Effects of CO2 concentration and irradiance on the stomatal behaviour of maize, barley and sunflower plants in the field
Abstract Maize, barley and sunflower plants were grown in the field, well supplied with water and nutrients. During growth, net CO2 exchange and transpiration of the crops at varying ambient CO2 concentrations and irradiance were determined by infra-red gas analysis. In maize the net photosynthetic rate (Pn) was linearly related to the irradiance (I) and independent of the ambient CO2 concentration (Ca). The transpiration rate (ET) was also linearly related to I but decreased strongly with increasing Ca. In sunflower and barley Pn increased and ET decreased with increasing Ca. A mean stomatal conductance and intercellular CO2 concentration (Ci) were calculated. In all three species the internal CO2 concentration was independent of the irradiance. In maize it was also independent of Ca, but in sunflower and barley Ci was proportional to Ca with a ratio of 0.6. 
 
It is concluded that differences in stomatal behaviour are only partly species-specific and depend mainly on growing conditions. The importance of stomatal regulation for crop growth under conditions of water shortage and CO2 depletion is discussed.