Physiological regulation of plant-atmosphere ammonia exchange

Plants have a compensation point for NH3 which ranges from 0.1 to 20 nmol m ol(-1), and may be several-fold higher or lower than naturally occurring at mospheric NH3 concentrations. This implies that NH3 fluxes over vegetated s urfaces are bi-directional and that ammonia exchange with the atmosphere in many cases contributes significantly to the nitrogen economy of vegetation . Physiological regulation of plant-atmosphere NH3 fluxes is mediated via p rocesses involved in nitrogen uptake, transport and metabolism. A rapid tur nover of NH4+ in plant leaves leads to the establishment of a finite NH4+ c oncentration in the leaf apoplastic solution. This concentration determines , together with that of H+, the size of the NH3 compensation point. Barley and oilseed rape plants with access to NH4+ in the root medium have higher apoplastic NH4+ concentrations than plants absorbing NO3-. Furthermore, the apoplastic NH4+ concentration increases with the external NH4+ concentrati on. Inhibition of GS leads to a rapid and substantial increase in apoplasti c NH4+ and barley mutants with reduced GS activity have higher apoplastic N H4+ than wild-type plants. Increasing rates of photorespiration do not affe ct the steady-state NH4+ or H+ concentration in tissue or apoplast of oilse ed rape, indicating that the NH4+ produced is assimilated efficiently. Neve rtheless, NH3 emission increases due to a temperature-mediated displacement of the chemical equilibrium between gaseous and aqueous NH3 in the apoplas t. Sugarbeet plants grown with NO3- seem to be temporarily C-limited in the light due to a repression of respiration. As a consequence, the activity o f chloroplastic GS declines during the day causing a major part of NH4+ lib erated in photorespiration to be assimilated during darkness when 2-oxoglut arate is supplied in high rates by respiration.