A fully automatic growth chamber system was built in order to study NH3 exchange and NH3 compensation points of plant canopies growing under controlled environmental conditions in which atmospheric NH3 concentrations corresponded to those naturally occurring over terrestrial ecosystems. The system included plant cuvettes with separate root and shoot compartments constructed of coated polycarbonate, This material did not change the spectral composition of photosynthetically active light and had a low adsorption of NH3 and water vapour, Atmospheric NH3 concentrations in the inlet of the cuvettes were controlled by mass-flow controllers, Inlet and outlet NH3 concentrations were measured on-line with a modified chemiluminescent NH3 monitor. At airflow rates per unit leaf area of about 3 dm(3) m(-2) s(-1), the system allowed accurate determinations of NH3 exchange rates down to about 0.1 nmol NH3 m(-2) s(-1). The NH3 compensation points s at anthesis for barley cultivars Laevigatum and Golf were 4.2+/-2.8 and 4.6/-+2.9 nmol mol(-1) of NH3 in air (SE, n=4), respectively, NH3 absorption in both cultivars increased linearly with atmospheric NH3 concentration in the range 0-30 nmol mol(-1) of NH3 in air. NH3 absorption was much higher in the light than in the dark, indicating a strong stomatal and/or metabolic control of NH3 exchange, Photosynthesis and transpiration were not affected by exposure to NH3 concentrations in the range 0-30 nmol mol(-1) for 7 d.
